User's Guide
Pollutant Load Estimation Tool (PLET)
Version 1.0
Developed for U.S. Environmental Protection Agency
By
Tetra Tech, Inc.
10306 Eaton Place, Suite 340
Fairfax, VA 22030
March 2022
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Table of Contents
PLET Quick Guide 3
Introduction 5
Overview of PLET 5
Background and Difference from STEPL 5
Program Flow 5
Getting Set Up 6
Technical Requirements 6
Accessing PLET 6
Using the Model 7
Creating a Model 7
Input Data Server 13
Inputs Module 15
Manure Application 19
Gullies and Streambanks 20
BMPs Module 23
Urban BMP Tool 25
BMP Calculator 27
Total Loads Module 30
Additional Reference Tables Module 32
APPENDIX A - BMPs in Series and Parallel 34
APPENDIX B - References Used in PLET 40
APPENDIX C - PLET Underlying Formulas Documentation 46
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PLET Quick Guide
Note: The Pollutant Load Estimation Tool (PLET) model may be accessed at the PLET web site
(https://epa.gov/nps/plet) or, for registered Grants Reporting and Tracking System (GRTS) users, by logging into
GRTS and navigating to the Tools drop-down menu and selecting PLET Models. For more detailed information,
refer to the Accessing PLET section of the User's Guide. Optionally, you may obtain the initial model input data
from the PLET Input Data Server. However, it is the user's responsibility to check and refine the initial data for
study areas. The Input Data Server can be accessed from the "Download Input Data Server Data" button at the
top of the model page, or outside of the model at https://ofmpub.epa.gov/apex/grts/f?p=109:333.
Step 1. Access the PLET model interface and click on the Create a New Model button in the upper right hand
corner.
Step 2. Name the model and select the state where the modeled area is located. Additional data entry fields will
appear.
• Specify the area to be modeled by:
o selecting the HUC12 watershed from a drop-down list,
o manually creating a watershed using the [Custom Watershed] option, or
o looking up a HUC12 watershed on a map.
• Select the appropriate weather station. This will automatically specify values for rainfall parameters and the
USLE parameters found later in the PLET Input Module.
• Click Create Model.
Step 3. Add as many watersheds or modeled areas as needed for the scenario. Click on the Add Watershed button near
the top of the model to add new rows, each representing a different watershed area. The Delete Watersheds button,
also located near the top of the model, can be used to remove any extra or erroneous rows.
Step 4. The Input Module is composed often input tables. The first several tables require local data provided by
the user or data from the Input Data Server. Remaining tables contain default values that users are encouraged
to update, based on the availability of local data.
• Manually enter land use in acres and specify the representative Soil Hydrologic Group (SHG) in the Land
Use Area table (Table 1) or review data provided by the PLET Input Data Server and update, as
appropriate.
• Enter total number of agricultural animals by type and number of months per year that manure is
applied to croplands and pastureland in the Agricultural Animals (Animal Count) table (Table 2). To
calculate the average number of months manure is applied, click on the Manure Application button at
the top of the PLET interface to open the calculator.
• Enter values for septic system parameters in the Septic and Illegal Wastewater Discharge table (Table 3).
• If the percent nutrient content in soil is known, adjust the default values in Table 4.
• If more local data are available, modify Universal Soil Loss Equation (USLE) parameters associated with the
selected county in Table 5.
Step 5. You may stop here and proceed to Step 7. If you have more detailed information on your watershed(s),
proceed with adding data to the remaining input tables.
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Step 6. Review parameter values for tables 6, 6a, 1, 7a, 8, 9, and 10 and update with more local data, if
available:
• Modify the curve number table in Table 6 and Table 6a.
• Modify the nutrient concentrations (mg/L) in surface runoff in Table 7.
• Modify the nutrient concentrations (mg/L) in shallow groundwater in Table 7a.
• Specify the detailed land use distribution in the urban area in Table 8.
• Specify cropland irrigation information in Table 9.
• Specify the wildlife density on cropland by animal type in Table 10.
Step 7. Enter representative dimensions and characteristics for gullies and streambanks, if desired. Click on the
Gullies and Streambanks button at the top of the PLET interface to open the form. This form can also be used to
represent gully and streambank restoration practice efficiencies.
Step 8. Add best management practices (BMPs) to the model scenario. Navigate to the BMP Module by clicking
on the BMP tab towards the top of the PLET model.
• Click the Add BMP button on the right side to open the BMP entry form.
• Select the appropriate watershed and land use and click Add BMP.
• Double click in the resulting BMP cell and select the desired BMP from the drop-down list.
• Enter the proportion of the land use for which the BMP will be applied.
To represent more than one BMP per land use type, click the BMP Calculator button to open the calculator and
created a combined BMP efficiency for multiple BMPs. Refer to the complete User's Guide for instructions on
how to use the BMP Calculator.
For urban land uses, BMPs are entered using the Urban BMP Tool, which can be accessed by clicking on the
Urban BMP Tool button near the top of the PLET model interface. Enter BMP information for each urban land
use and click Apply LID/BMP to add the BMP to the model.
Step 9. View the estimates of loads and load reductions in the Total Loads Module. Loads and load reductions
are automatically generated and appear in the Total Loads Module.
To generate the groundwater load information, click the box next to Groundwater load calculation to turn on
that feature.
The Treat All Subwatersheds as Part of a Single Watershed check box changes the sediment delivery ratio. This
box is only relevant if there is more than one watershed in the model. Checking the box allows the sediment
delivery ratio to be calculated using the total watershed area of all watersheds included in the model.
Important: This feature does not represent routing through the watersheds in a particular order. Unchecking
the box allows the sediment delivery ratio to be calculated independently for each watershed in the model.
Results can be downloaded as an Excel spreadsheet by clicking on Download in the upper right corner of the
Total Loads Module.
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Introduction
Overview of PLET
This document is a user's guide to the Pollutant Load Estimation Tool (PLET). PLET provides a user-friendly web
interface to create a customized model. It employs simple algorithms to calculate nutrient and sediment loads
from different land uses and the load reductions that would result from the implementation of various best
management practices (BMPs), including Low Impact Development practices (LIDs) for urban areas. It computes
surface runoff; nutrient loads, including nitrogen, phosphorus, and 5-day biological oxygen demand (BOD); and
sediment delivery based on various land uses and management practices. The land uses considered are urban
land, cropland, pastureland, feedlot, forest, and a user-defined type. The pollutant sources include major
nonpoint sources such as cropland, pastureland, farm animals, feedlots, urban runoff, and failing septic systems.
The types of animals considered in the calculation are beef cattle, dairy cattle, swine, horses, sheep, chickens,
turkeys, and ducks. For each watershed, the annual nutrient loading is calculated based on the runoff volume
and the pollutant concentrations in the runoff water as influenced by factors such as the land use distribution
and management practices. The annual sediment load (from sheet and rill erosion only) is calculated based on
the Universal Soil Loss Equation (USLE) and the sediment delivery ratio. The sediment and pollutant load
reductions that result from the implementation of BMPs are computed using the known BMP efficiencies.
PLET is applied at the user's own direction. Neither the U.S. Environmental Protection Agency nor any of its
contractors assumes responsibility for the operation, output, interpretation, or use of this tool.
Background and Difference from STEPL
The design for PLET was inspired by the need for a more accessible and functional version of the Spreadsheet
Tool for Estimating Pollutant Loads (STEPL). STEPL was initially developed over 20 years ago as a Microsoft Excel-
based spreadsheet model. Since then, additional functionality has been added to STEPL and Excel capabilities
and support for various code have changed substantially. Therefore, EPA decided to move to a web-based
model to improve the interface, efficiency, and user experience that comes with a web-hosted product and to
provide integration with EPA's Grants Reporting and Tracking System (GRTS), so state recipients of EPA grants
can directly report their project load reduction results to GRTS to more efficiently meet grant reporting
requirements. While users may experience a different look and feel using the PLET model, the underlying
assumptions, calculations, and functions are identical to the STEPL tool. Using the same inputs in either model
produces identical output results.
Unlike STEPL, users no longer need to download and install executable files or spreadsheets to their local
computer. PLET is entirely web-based. A web browser and internet connection are the only requirements to
access the tool.
Other improvements include an updated approach to BMP inputs. Rather than each land use being represented
in a separate table, the PLET model integrates all land uses and BMPs into a single table and lets the user select
the land use and watershed for each row in the table. The way in which combined BMPs are added in the model
has also been improved though the elimination of the combined BMPs worksheet, and an updated BMP
calculator, which accommodates both simple and complex configurations with an improved user interface.
Program Flow
Figure 1 shows the overall model structure of PLET. It is composed of interactive modules for input and BMPs
based on user data, and an output module that presents the results of the modeled scenario, as well as hidden
processes to handle intermediate calculations. The input data include state name, county name, weather
station, land use areas, agricultural animal numbers, manure application months, population using septic tanks,
septic tank failure rate, direct wastewater discharges, irrigation amount/frequency, and BMPs for simulated
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watersheds. When local data are available, users may choose, and are encouraged, to modify the default values
for USLE parameters, soil hydrologic group, nutrient concentrations in soil and runoff, runoff curve numbers,
and detailed urban land use distribution. Pollutant loads and load reductions are automatically calculated for
total nitrogen, total phosphorus, BOD, and sediment.
Land Uses
Animal
Precipitation
and
Irrigation
Soil and USLE
Parameters
Septic Systems
And Direct
Discharges
User Input
Runoff
Groundwater
Sheet/Rill
Erosion
Gully and
Streambank
Erosion
Pollutant
Transport
Processes
Nitrogen
Phosphorus
BOD
Sediment
Load
BMPs/LID
I
Load
Reduction
Figure 1. Model structure
Getting Set Up
Technical Requirements
To use PLET, the user must have an internet-connected computer. PLET Is compatible with all available internet
browsers except for Internet Explorer.
Accessing PLET
The PLET modeling component can be accessed in three ways:
(1) Registered GRTS users can log into GRTS with their account info (username/password) and then
navigate to PLET using the 'Tools' drop-down menu within the GRTS application, as shown in Figure 2.
Grants Reporting and Tracking System (GRTS) Grants &PrqjaEi ~
Tools * My Accountw Help ~ Admin *
Logout ( HVALLE )
EPA Home / GRTS Home
Polluted Runoff: Non point Source Home
Grant Reporting and Tracking System
GRTS Reports
Watershed Plan Tracker
~
UPDATED: 11/16/21
Success Stones
Hello GRTS Community!
Tribal Success Stories
Please note these important GRTS updates. Here is a short summary with additional information on each item below if needed.
Success Stories Days Lapsed Report
1. Action: Please use https://grts.epa.gov for access to the GRTS application from now on (feel free to bookmark it in your browser for easy access).
Program Highlights
2. Note: On Friday (11/19), the ATTAINS Waterbody Lookup feature in the "waterbody information" section of the GRTS project pages and in the Succi
XML Data Import
he query of
modernized and current ATTAINS data instead of a static snapshot of legacy ATTAINS data.
3. Action for Non EPA Staff (states, tribes, territories). You will need to register for a loqin.qov account prior to Dec. 31 to access the GRTS OBI Reports
City / CD Search
1. EPA Staff, no action necessary at the moment, but you will use the "PIV Card" (or "single sign-on") option to access GRTS OBI Reports from no*
Vocabulary Downloads
Additional Info: |
PLET Models
1. We've created a new "vanity URL" for GRTS (https://grts.epa.gov). Please use this URL for accessing the GRTS application moving forward and feel free to bookmarHn^our browser for easy access. Note: this does not
apply to the OB! environment that access will be via the existinq link with the additional information on loqin.qov access for externa! users (state, tribes, territories), see item #3 in this post
Figure 2. Location of the PLET model in the GRTS program.
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(2) Registered GRTS users can also access PLET via the "Registered User Access" link from the main PLET
webpage at https://epa.gov/nps/plet, and then entering their account info.
(3) Guest users can access PLET by using the "Guest Access" link from the main PLET webpage at
https://epa.gov/nps/plet, and then entering their email address and responding to the confirmation
email, as shown in Figure 3.
Pollutant Load Estimation Tool
Please enter your email in the field below. This information will not be used to contact you, but simply to use as an ID to save your models, user-defined BMPs, and BMP Calculator
is not required, but please note that if left empty, you will not be able to access your saved items after your session ends.
If the email entered is linked to an active GRTS account, you will instead be redirected to the GRTS login page.
Email
Submit
Figure 3. PLET guest access form.
Using the Model
PLET is primarily composed of four modules—Inputs, BMPs, Total Loads, and Additional Reference Tables—all
designed for user interaction. PLET also includes several other interactive data entry forms. Data entries in the
modules and forms are in different colors.
• Orange entries designate values or controls that should be specified (e.g., cropland area in acres) by the
user.
• Black entries on the Inputs, BMPs and Additional Reference Tables modules provide useful information
and assumptions to help users understand the spreadsheet tool. These can also be adjusted by the user,
if more accurate local data are available
• Black entries on the Total Loads module are information calculated by the spreadsheet and cannot be
changed.
Data fields can be edited by double clicking within the cell to be edited. Once a cell is edited, the upper right
corner will have blue icon. Edits will only be marked during the current session of the tool. If the user exits and
returns to the tool, edits from previous sessions will be saved, but unmarked.
All model changes are automatically saved. There is no save button in the tool.
Creating a Model
Upon logging into the PLET tool, the home page provides a list of models created by the user (or an empty list if
no models have been created yet). To create a new model, the user clicks on the Create a New Model button in
the upper right hand corner. To access an existing model, the user clicks on the edit symbol to the left of the
desired model in the table (as shown in Figure 4). Once a model is created, it is shown in the list, including the
name of the model, the state, county, and 12-digit Hydrologic Unit Code (HUC) of the model domain and the
date the model was created.
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Pollutant Loads Estimation Tool
Models
a-
v Go Actions v
Title
State
County
Date Created
Waterfr><^HUC12 ^
Test
AZ
COCONINO
01-JUN-21
140700060406
(*
Sakonnet
Rl
NEWPORT
27-MAY-21
010900040910
Uj
New beta test
AL
MARSHALL
07-MAY-21
[ Custom Watershed ]
%
Beta Test Drive AM
AL
CALHOUN
07-MAY-21
031501060513
%
Emuckfaw Creek
AL
TALLAPOOSA
06-MAY-21
031501090308
6?
Blue River Watershed
MO
JACKSON
05-MAY-21
103001010106
t?
Shoal Creek
MO
NEWTON
05-MAY-21
110702080305
Bf
North Fabius HUC 071100020405
MO
LEWIS
05-MAY-21
071100020306
Sf
WI-TEST
Wl
OUTAGAMIE
23-APR-21
040302040402
5?
Twelvemile Creek Watershed X2 TEST
MN
WRIGHT
15-APR-21
070102040605
Sf
Twelvemile Creek — North Fork Crow-Test
MN
WRIGHT
14-APR-21
070102040605
9f
Copy of Beta Demo
a
HARTFORD
13-APR-21
010802050402
Figure 4. PLETmodeling home page.
Selecting the Modeled Area
After clicking on Create a Model, a model set-up screen will open where the user is prompted to name the
model run and select the state in which the area to be modeled is located (Figure 5). Upon selecting the state,
other data entry fields will automatically appear including the watershed HUC drop-down list, the county drop-
down list, and the weather station drop-down list (Figure 6). There are three ways to specify the area to be
modeled:
1. Select a HUC12 watershed from a drop-down list,
2. Manually create a watershed, or
3. Look up a watershed on a map.
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Pollutant Loads Estimation Tool
Title
State
-Select State
Exit
2.0 Set Screen Reader Mode On
Figure 5. Initial set up page.
Pollutant Loads Estimation Tool
Example Watershed
State
Georgia
Watershed
[ Custom Watershed ]
County
-Select County-
Rainfall Correction Factor
Weather Station
-Select Station-
Raindays Correction Factor
2.0 Set Screen Reader Mode On
Figure 6. Initial set up page once the Title and State are selected.
Selecting a HUC12 watershed from a drop-down list
If the user is working at the HUC12 scale, knows the HUC12 number of the watershed of interest, and would like
to start building the model with default watershed characteristics provided by the Input Data Server, the HUC12
can be selected directly from the Watershed drop-down list.
Upon selecting a HUC, the County and Weather Station fields will be automatically populated with the closest
County and weather station, which can be adjusted by the user, if appropriate. The weather station drop-down
list will be automatically updated to show the nearest weather stations to the selected county.
Click on Create Model to generate the Input tables. Once the model is created, the rainfall correction factor and
raindays correction factor will populate. The watershed data in Tables 1-5 (land use, animal counts, etc.) will be
pre-populated for the selected watershed. The values provided should be used as a starting point and replaced
with local data when available.
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Looking up a watershed on a map
If the area to be modeled is known, but the specific HUC12 is unknown, the user may generate location specific
data by selecting the state from the drop-down list and then clicking on the Lookup button to the right of the
Watershed drop-down. This will open an interactive map of the United States. Zoom into the area of interest
and click on the map. The HUC12 boundary for watershed containing the area of interest will appear (Figure 7).
To change which HUC12 is selected, click another location on the map to display the boundary. Once the correct
HUC12 has been selected, click Submit to select the watershed. This will return the user to the model setup,
which will be populated with the HUC12 name and number, as well as the County, weather station and rainfall
and raindays correction factors. Tables 1 through 5 will be populated with initial values.
Pollutant Load Estimation Tool model & HUC12 map
Figure 7. Map view watershed selection with example watershed selected.
Manually Creating a Watershed
If the area to be modeled is not a HUC12 watershed, "Custom Watershed" can be selected from the Watershed
drop-down list after selecting the state. The Custom Watershed allows the user to fill in all information about
the watershed manually. Select the appropriate county and weather station and click Create Model. This will
populate the rainfall and raindays correction factors, the hydrologic soil group (HSG) in Table 1, and the
universal soil loss equation (USLE) values in Table 5, based on the location information specified by the user. All
other user specified data fields in Tables 1-3 will remain set to 0. The pre-populated values should be evaluated
by the user and updated if more specific local data are available. The user must also populate Tables 1-3 with
land use acreages, agricultural animal counts, and information about septics and illegal wastewater discharges.
Navigation Dashboard
Once the model is set up, the action buttons and summary information at the top of the model page remain
visible in any of the submodules in the model. Moving across from left to right and top to bottom, these include
(Figure 8):
Share Model - allows the user to share the current version of the model with another user. When clicked it
opens a pop-up window with an input field to enter the email address or username of the intended recipient of
the model. Enter an email address or username and click Submit. To leave this pop-up without sharing the
model, click cancel.
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Copy Model - creates a new version of the existing model. When clicked, the user will be brought into a new
copy of the model that is named "Copy of..." as part of the model's title. Changes to this version will not impact
the settings and inputs to the original version.
Delete Model - deletes the version of the model. Once deleted, the model cannot be recovered. Prior to
deleting a pop-up will ask the user to confirm the model should be deleted. To delete, click ok. To keep the
model, click cancel.
Download Input Data Server Data - provides a screen with the Input Server Data for the selected watershed(s)
that have been added to the model. These data are specific to the initial data provided by the Input Data Server
and does not incorporate any edits that may have been made since populating the model initially.
Exit - brings the user back to the home page with the list of existing models.
Rainfall Correction Factor- automatically populated based on the selected weather station. Provides the
percentage of rainfall events that exceed 5 mm per event.
Raindays Correction Factor- automatically populated based on the selected weather station. Provides the
percentage of rain days (events) that generate runoff.
Rainfall Initial Abstraction - determines initial rainfall retention on the land surface, ranges from 0 to 0.2.
Default is set to zero.
Pollutant Load Estimation Tool
Title
Example Watershed
State
Georgia
Watershed
030601020101 (Headwaters Talk s
Share Model I Copy Model I Delete Model I Download Input Data Server Data
Add watershed
; c
Delete watersheds
Gullies and Streambanks
County
RABUN
Weather Station
COWEETA EXP STATION
Rainfall Correction Factor Raindays Correction Factor Rainf
0.9332 0.5616
Urban BMP Tool
Manure Application
BMP Calculator
Inputs BMPs Total Loads Additional Reference Tables
Mandatory Inputs
Figure 8. PLET Model Navigation Dashboard.
The Add Watershed button brings up a pop-up that will display a new Watershed row, populated with the
watershed name and acreages from the initial watershed that was added (Figure 9). These can be overwritten
here with new watershed data, or once the watershed is added to the model. Click Submit to add a new
watershed or click Cancel to leave the pop-up without adding a new watershed.
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Georgia
030601020101 (Headwaters Talk
Q. Lookup
COWEETA EXP STATION
Add Watershed
il Loads
Xrea (ac) and Preci
select a Hydrobgic So
Urban
Add row
0
g
Watershed
Cropland
Pastureland
Forest
Urban
Feedlots
User Defined
0
=
03060102010...
1.33
89.62
19431.05
194.59
.0737
0
1 rows selected
ual Rainfall Rain Day:
11 140.72
Figure 9. View of the Add Watershed pop-up, used to input additional watersheds and their land use acreages.
If more than one watershed needs to be added, click on the menu icon and select "+ Add Row" to add additional
rows in the pop-up table (Figure 10).
lit
Add Watershed
v = Watershed
Clf
So
Cropland
= 03060102010... 1.33
Add row
Pastureland Forest Urban
89.62 19431.05 194.59
Feed lots
.0737
User Defined
0
IHjUU Single Row View
Total 1
Add Row
Submit
[q Duplicate Row
Cancel
Tuf Delete Row
0 Refresh Row
Revert Changes
Figure 10. Add row button in the Add Watershed pop-up used to create additional rows for more watersheds.
Delete Watershed will bring up a pop-up with all of the watersheds in the model listed. Click the trash can icon
next to the one to be deleted, and another pop-up wili ask the user to confirm deletion of the watershed from
the model (Figure 11). Click OK to delete the watershed and click Cancel to return to the list of watersheds. If
Cancel is selected, to exit the Delete pop-up, hit the Esc key on the keyboard.
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Delete PLET Watersheds
Q"
Go
Actions v
lur
md
4}
Watershed
Urban Cropland Pastureland Forest
Defined FeedlotS
0 1.4901
f
030601060607 - Beaverdi
Savannah River
>43.83
Are you sure?
Cancel foi^
1-1
Close
Figure 11. Delete watershed pop-up window, shown with delete confirmation message.
The previously described Navigation Dashboard buttons are used to setup the modeled watersheds. The
remaining Navigation Dashboard buttons, which are green, are used to provide inputs to the model, These input
buttons, briefly described below, are discussed in more detail in the appropriate section of this User's Guide.
Gullies and Streambanks opens a pop-up window with a form for adding gullies and streambanks to the model.
Urban BMP Tool opens a pop-up to the Urban BMP and LID input form. BMPs applied to urban land uses are
added using this form.
Manure Application opens up the manure application calculator, which allows the user to calculate a land use-
wide average manure application period for both pasture and cropland.
BMP Calculator opens the BMP Calculator tool that provides a workspace to configure multiple BMPs on a given
land use to determine that combined efficiency value.
Input Data Server
For users interested in gathering initial input data for a watershed of interest, the Input Data Server is available.
The Input Data Server provides initial model input data that can be used as a starting point in populating the
PLET model. It provides HUC12-level land use acreages, agricultural animals, septic system information, and HSG
information. These data can be downloaded for use in other applications, or the data can be applied
automatically to a PLET model as it is developed. Note: for user's downloading data for HUC12 watersheds that
cross county lines, the data provided is at the HUC12 scale and is not subdivided by county, so requesting the
data for more than one county produces duplicate data. The Input Data Server can be accessed from outside of
the model at https://ofmpub.epa.gov/apex/grts/f?p=109:333.
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The Input Data Server allows the user to select the state and county of interest and then it generates a list of
HUC8 watersheds. Once a HUC8 is selected, the list of HUC12 watersheds within the selected HUC8 watershed
appears. From here, one or more HUC12 watersheds can be selected (Figure 12). To select more than one
watershed, hold down the Shift button while clicking on the desired HUC12 watersheds. After selecting
watersheds, click on Generate and the Input Data Server will provide data tables that can be downloaded
individually by data type, or all together in one Excel spreadsheet (Figure 13).
PLET Input Data Server
Select Geographic Area
HUC12
* PLEASE CLICK HERE TO CONFIRM HUC12 SELECTION PRIOR TO USING THE INPUT DATA SERVER
State
Delaware
District of Columbia
Florida
Georgia
Guam
HUC8
-Select HUC8 (optional)- ^
03070201 (Satilla)
03070202 (Little Satilla)
03070203 (Cumberland-St. Si
03110201 (Upper Suwannee) "*
030702020202 (Lower Sweetwater Creek)
030702020301 (Fishing Creek)
030702020302 (Upper Colemans Creek)
030702020303 (Lower Colemans Creek)
030702020304 (Lower Big Satilla Creek)
030702020401 (Keene Bay Branch-Little Satilla Creek) ¦
030702020402 (Dry Branch-Little Satilla Creek)
030702020403 (Boggy Creek) ]
Ctrt+A to select all
Figure 12. Input Data Server watershed selection fields.
Watershed Name
Upper Colemans Creek
Lower Big Satilla Creek
Fishing Creek
Lower Colemans Creek
HUC12
030702020302
030702020304
030702020301
030702020303
1623.7
90826
1452.01
1174.02
21998.38
16555.04
17903.64
9491.14
Pastureland
1356.6
1717.77
244746
1440
Cropland
3763.14
9595.88
822949
9077.92
Other Landuse
2.8867
2.607
4.2489
105.86
91.63
Agricultural Animals Count
Watershed Name
HUC12
Beef Cattle
Dairy Cattle
Swine Sheep
Horse
Turkey
Chicken
Duck
Upper Colemans Creek
030702020302
337
161
3 0
15
0
196417
0
Lower Big Satilla Creek
030702020304
230
90
3 5
21
0
115366
0
Fishing Creek
030702020301
233
86
3 8
27
0
102971
0
Lower Colemans Creek
030702020303
300
131
2 1
18
0
171495
0
Septic Systems
Watershed Name
Upper Colemans Creek
Lower Big Satilla Creek
Fishing Creek
Lower Colemans Creek
HUC12
030702020302
030702020304
030702020301
030702020303
Population per Septic Syster
% Septic Failure Rate
0.57
0.57
0.57
0.57
Hydrological Soil Group
Figure 13. Example partial outputs from the Input Data Server.
14
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Inputs Module
The Input module has 10 tables (shown on the following pages in Figure 14) with a combination of optional and
required data inputs. Although some tables do not require data to run the model, it is up to the user to decide
whether these optional data tables should be excluded, depending on the purpose of the model run and the
characteristics of the watershed. The tables include:
1. Watershed Land Use Area and Precipitation
2. Agricultural Animals
3. Septic and Illegal Wastewater Discharge
4. Percent Nutrient Content in Soil
5. Universal Soil Loss Equation
6. Reference Runoff Curve Number
a. Detailed Urban Reference Runoff Curve Number
7. Nutrient Concentration in Runoff
a. Nutrient Concentration in Shallow Groundwater
8. Urban Land Use Distribution
9. Input Irrigation Area and Irrigation Amount (optional)
10. Wildlife Density in Cropland (optional)
For most HUC12 watersheds, if they were selected from the drop-down list during model setup, Tables 1-8 will
be automatically populated with initial model input data specific to the watershed and/or county derived from
the Input Data Server and pre-set default values. It is the user's responsibility to verify and refine the initial data,
and EPA highly encourages users to replace initial values with more locally specific and relevant data whenever
it is available.
To manually enter data:
1. Enter land use areas in acres, the dominant hydrologic soil group (HSG1), and the percent
imperviousness of feedlots in Table 1;
2. Enter total number of agricultural animals by type and number of months per year that manure is
applied to croplands in Table 2, refer to the Manure Calculator section below for more information on
calculating the average number of months of manure application for the watershed;
3. Enter values for septic system parameters, population counts that discharge wastewater directly, and
reduction percentages on direct wastewater discharge in Table 3; and
4. Optionally modify the percent nutrient concentrations and USLE parameters associated with the
selected county in Table 4 and Table 5, respectively.
The user may stop here and proceed to the BMP module, unless there is more detailed information on the
watersheds that can be updated in the remaining input tables. The remaining input tables are already populated
with default values or are set to zero (0) and should be updated with local data, if available.
1 HSG A: Low runoff potential and high infiltration rates even when thoroughly wetted. Chiefly deep, well to excessively drained sands or
gravels. High rate of water transmission (<75 cm/hr).
HSG B: Moderate infiltration rates when thoroughly wetted. Chiefly moderately deep to deep, moderately well to well-drained soils with
moderately fine to moderately coarse textures. Moderate rate of water transmission (0.4 to 0.75 cm/hr).
HSG C: Low infiltration rates when thoroughly wetted. Chiefly soils with a layer that impedes downward movement of water, or soils with
moderately fine to fine texture. Low rate of water transmission (0.15 to 0.40 cm/hr).
HSG D: High runoff potential. Very low infiltration rates when thoroughly wetted. Chiefly clay soils with a high swelling potential, soils
with a permanent high water table, soils with a clay pan or clay layer at or near the surface, or shallow soils over nearly impervious
material. Very low rate of water transmission (0 to 0.15 cm/hr).
15
-------
The reference curve number default values in Table 6 and 6a are derived using the land use and soil group inputs
from Table 1. If watershed-specific pollutant concentrations in runoff or groundwater are known, these can be
updated in Tables 7 and 7a. In Table 8, the model will also automatically subdivide the urban land use acreage
into preset proportions of urban land uses, such as residential, industrial and institutional. However, these
distributions can be adjusted to better reflect the actual urban land use distribution in the project area. The land
use proportions in Table 8 must add up to 100. If they are edited and do not add up to 100, a warning will
appear below the table ("ERROR - Total % must equal 100")
If the user wishes to represent irrigation or changes to irrigation, this information should be included in Table 9.
The total cropland acres are automatically populated from Table 1. If there are significant wildlife populations on
cropland that should be represented, these can be included in Table 10.
16
-------
Input Module
Pollutant Load Estimation Tool
Trtle
Example Watershed
State
Georgia
Watershed
030601050607 (Beaverdam Ditcf ~
County
RICHMOND
Weather Station
AUGUSTA BUSH FIELD AP
Share Model I Copy Model I Delete Model I Download Input Data Server Data
Add watershed
31
Delete watersheds
Gullies and Streambanks
Rainfall Correction Factor
0.9163
Rairiflays Correction Factor Rainfall Initial Abstraction
0.5051
Urban BMP Tool
Manure Application
BMP Calculator
BMPs
Inputs
Mandatory Inputs
Additional Reference Tables
Download Inputs
~ 1. Watershed Land Use Area (ac) and Precipitation (in)
Double-click on the "HSG" field to select a Hydrologic Soil Group category [NOTE: hover over column header for more information].
Cropland Pastureland
030601060607 - Beaver...
User
Defined
1.49 67083.2201
Feedlots
Percent
Paved
Annual
Rainfall
Average
Rain/Event
~ 2. Agricultural Animals (Animal Count)
Beef Young
Cattle Beef
Dairy
Cattle
Young
Dairy
Stock
Swine Feeder
(Hog) Pig
030601060607 - Bea...
# Of Months # Of Months
Duck Manure Applied Manure Applied
to Cropland to Pastureland
71.00 54789.00
w 3. Septic and Illegal Wastewater Discharge
Number Of
Septic Systems
030601060607 - Beaverdam Ditch-Savan...
w 4. Percent Nutrient in Soil
030601060607 - Beaverdam Ditch-Savannah River
5. Universal Soil Loss Equation
Population Per
Septic System
Soil N conc.%
Septic Failure
Rate, %
Soil P conc.%
Waste Water
Direct Discharge,
# Of People
Direct Discharge
Reduction, %
Soil BOD conc.%
Cropland Pastureland
Watershed RKLSCPRKLSCPR
03060... 250.00... 0.16918 0.42499 0.28008 0.91096 250.00... 0.16918 0.42499 0.04000 1.00000 250.00...
0.16918 0.42499 0.00300 1.00000 250.00... 0.16918 0.42499 0.28008 1.00000
17
-------
~ Optional Inputs
6. Reference Runoff Curve Number
6a. Detailed Urban Reference Runoff Curve Number
Urban
Cropland
Pastureland
Forest
User Defined
83.00
67.00
49.00
39.00
50.00
89.00
78.00
69.00
60.00
70.00
92.00
85.00
79.00
73.00
80.00
7. Nutrient Concentration in Runoff (mg/L)
93.00
89.00
84.00
79.00
85.00
Landuse T =-
N
P
BOD
1. L-Cropland
1.90
0.30
4.00
1a. w/ manure
8.10
2.00
12.30
2. M-Cropland
2.90
0.40
6.10
2a. w/ manure
12.20
3.00
18.50
3. H-Cropland
4.40
0.50
9.20
3a. w/ manure
18.30
4.00
24.60
4. L-Pastureland
4.00
0.30
13.00
4a. w/ manure
4.00
0.30
13.00
~ 7. More Information
SHG
A
B
C
D
Commercial
89.00
92.00
94.00
95.00
Industrial
Institutional
81.00
81.00
88.00
88.00
91.00
91.00
93.00
93.00
Transportation
98.00
98.00
98.00
98.00
Multi-Family
Single-Family
77.00
57.00
85.00
72.00
90.00
81.00
92.00
86.00
Urban Cultivated
Vacant- Developed
67.00
77.00
78.00
85.00
85.00
90.00
89.00
92.00
Open Space
49.00
69.00
79.00
84.00
7a. Nutrient Concentration in Shallow Groundwater (mg/l)
Landuse
N
P
BOD
Urban
1.50
0.063
0.00
Cropland 1.44
0.063
0.00
Pastureland
1.44
0.11
0.063
0.009
0.00
0.00
Feedlots
User Defined
6.00
0.00
0.07
0.00
0.00
0.00
~ 8. Urban Land Use Distribution
Urban
Area (ac.)
030601060607 - Be...
9. Input Irrigation Area (ac) and Irrigation Amount (in)
Total
Cropland (ac)
030601060607 - Beaverdam Ditch-...
10. Wildlife density in cropland (Cropland / sq. mile)
Cropland
Acres Irrigated
Multi
Family
%
Single
Family
%
Water Depth (in)
per Irrigation
Before BMP
Urban
Cultivated
%
Vacant
(developed)
%
Water Depth (in)
per Irrigation
After BMP
Open
Space
%
Irrigation Frequency
(#/Year)
Figure 14. View of the Input Module Tables 1-10.
18
-------
User-Defined Land Use
The tool can accommodate one user defined land use, which is added in Table 1. When creating a user-defined
land use, the user should be sure to update the USLE values (Table 5), runoff curve number (Table 6) and
nutrient concentration in runoff (Table 7) to represent the land use. As a default, the user-defined land use
nutrient concentrations in runoff are set to zero, so no pollutant loading is generated from this land use until the
concentrations are added by the user.
Replacing initial and default values with local data
EPA strongly encourages the use of local data, when available, to replace default values as well as the initial
inputs that are provided in the Input Data Server. In cases where no other information is available, these values
are useful, but can be improved upon with more specific information at the local level. It is up to the user to
decide whether the default values and initial input data are adequate to represent the watershed being
modeled, based on the other available data and the purposes for running the model.
Updating USLE values
The model calculates annual erosion using the Universal Soil Loss Equation (USLE). The default USLE parameter
values (R, the rainfall erosivity index; K, the soil erodibility factor; LS, the topographic factor; C, the cropping
factor; and P, the conservation practice factor) for different types of rural land uses are included within PLET
based on the State/County selection. The USLE parameters provided in the model are based on the 1992
National Resources Inventory (NRI) database. The user can obtain more recent soil erosion parameter values
from their local Natural Resources Conservation Service office or more accurately calculate them using NRCS's
most recent RUSLE2 tool. The RUSLE2 is a model containing both empirical and process-based science that
estimates long-term, average-annual rates of rill and inter-rill (sheet) soil erosion caused by rainfall and runoff
(https://www.ars.usda.gov/southeast-area/oxford-ms/national-sedimentation-laboratorv/watershed-physical-
processes-research/research/rusle2/revised-universal-soil-loss-equation-2-overview-of-rusle2/). The RUSLE2
program requires more site specific information which in turn results in parameter and erosion estimates which
are more site-specific.
Using PLET for other pollutants
If the user has adequate information on pollutant soil concentrations, concentrations in runoff and BMP
reduction efficiencies, PLET may be used to calculate the loads of pollutants other than nitrogen, phosphorus,
and BOD by replacing the pollutant data for nitrogen, phosphorus or BOD with data for another pollutant.
However, the user should ensure that mg/l is the appropriate unit of expression for the pollutant in question
and should consider the appropriateness of a runoff model when opting to use it for other pollutants. If the
pollutant is not expressed in mg/l, the calculations will be incorrect. PLET cannot account for impacts on
pollutants related to pH, or growth and decay functions. It is the user's responsibility to ensure the model is
being used appropriately for the pollutant of interest.
Manure Application
The Manure Calculator supports data entry in Table 2. It calculates the average number of months for manure
application per year with varying application frequencies across the watershed. It can be applied separately for
cropland and pastureland. All acres of the land use need to be included, even those that do not receive manure,
to accurately calculate the average number of months of manure application across the land use. Acres that do
not receive manure, should be entered with the number of months set to zero.
After clicking the Manure Application button, a pop-up Manure Calculator form will appear (Figure 15). Enter the
total land use acres in the watershed, then enter each set of acres with the number of months during which
manure is applied. Double click to activate the field for data entry. Click Add Row to create additional rows for
19
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data entry. Once all acres are accounted click Calculate. Below the table, the average number of months will
appear, along with a land use acreage validation. If the total land use acres do not match the total acres in the
table, a warning will state "Check to ensure total treatment area matches the total land use area." Edit the
acreages and click Calculate again. The Total Landuse Area Check should now read "OK." Select the watershed
and land use where the months of manure will be included from the respective drop-down lists and click Apply
to Selected Watershed. A notice will confirm that the data were applied successfully. Click Esc on the keyboard
to return to the Input module.
Manure Application
Total Land Use Acres 4286
Close
Add row
Area
Number of Months
500 8
BOO
4
3486
0
1 rows selected
1 -3
Total Land Use Acres 4286 # of Months 1
Total Landuse Area check: OK
Apply To Watershed
Watershed 030601060607 - Beaverdam Drtch-Savannah River •
Landuse Cropland •
to selected watershed ||
Figure 15. View of the Manure Calculator.
Gullies and Streambanks
The Gullies and Streambanks button takes the user to the Gullies and Streambanks Form (Figure 16), which is
used for defining the dimensions for the user-specified gully formations and impaired streambanks. This form
contains two tables: the first table will show the gully formations, and the second table will show the impaired
streambanks. To add new a gully or streambank to the form, select gully or streambank and click Submitto
generate a blank form. For each new gully or streambank the required information can be defined in following
steps:
20
-------
Gully:
1. Select the watershed to apply the gully
2. Name the gully
3. Specify the gully dimensions (length, top width, bottom width, and depth).
4. Specify the time (number of years) that the gully has taken to form the current size.
5. Specify the gully stabilization (BMP) efficiency (0-1) and the gully soil textural class.
Streambank:
1. Select the watershed to apply the streambank
2. Name the streambank
3. Specify the streambank dimensions (length, height).
4. Specify the lateral recession rate (ft/yr) of the eroding streambank.
5. Specify the streambank stabilization (BMP) efficiency (0-1) and the streambank soil textural class.
The form calculates both the initial loading from gullies and streambanks and also the reductions from
stabilization and restoration BMPs. Gully loads are based on annual erosion rates, which are determined by the
volume of the gully and the soil weight times the soil nutrient concentration. Stream calculations are similar,
except they also include a lateral recession rate. The streambank form requires separate entries for each side of
a streambank, while the gully form represents the entire gully in one entry.
Once the soil textural class is selected, the soil dry weight (density) and nutrient correction factor will
automatically populate. For streambanks a lateral recession factor is also required. The user can select from
categories ranging from slight to severe, corresponding to estimated lateral recession rates, which will populate
automatically. For more information about the soil texture classes and associated density and nutrient
correction factors and the lateral recession rates, click on the More Info drop-down on the right hand side of the
Gullies & Streambanks Form. If local data or project-specific data are available on the soil density, nutrient
correction factor or lateral recession rate, these can be edited by the user to better reflect local conditions.
The annual load and load reduction will automatically be calculated based on the inputs provided. If the model
represents a gully or streambank under current unimproved conditions (i.e. without a restoration BMP) the BMP
efficiency field should be set to zero (0). The load reduction will calculate as 0. If the model represents a
restoration BMP, the BMP efficiency value should be set to a non-zero value, where 1 equals 100 percent
removal of the gully or streambank load.
Click Submit for the gully or streambank entry to appear in the Report rows on the Gullies & Streambanks Form.
A pop-up in the upper right corner will confirm that the gully or streambank has been submitted. To delete a
previously entered gully or streambank, click on the trash can icon on the left side of the row to be deleted, and
a pop-up window will ask for confirmation before deleting the row (Figure 17). To add another gully or
streambank, select the gully or streambank radio button and enter another gully or streambank. When all
entries are complete, click the Close button to exit the Gullies & Streambanks Form and return to the main
model view.
21
-------
Gullies and Streambanks Form
~ Gully and Streambank PollutJ
\/ Gully successfully submitted
Use this form to add a new gully or streambank entry
Select One: O Gu"y Streamban
G u I ly/Strea mbank Name
This sheet contains two input tables: the first table is for inputting the gully dimensions, and the second is for
inputting the eroding streambank dimensions.
GULLY:
Step 1: Specify the gully dimensions and assign each gully to a watershed.
Step 2: Specify the time (number of years) that the gully has taken to form the current size.
Step 3: Specify the gully stabilization (BMP) efficiency (0-1) and the gully soil textural class.
STREAMBANK:
Step 1: Specify the streambank dimensions and assign each bank to a watershed.
Step 2: Specify the lateral recession rate (ft/yr) of the eroding streambank.
Step 3: Specify the streambank stabilization (BMP) efficiency (0-1) and the streambank soil textural class.
BMP Efficiency (0-1)
Soil Textural Class
Soil Dry Weight (ton/ft5)
Annual Load (ton)
Nutrient Correction Factor
Load Reduction (ton)
Gullies Report
Watershed
030601060607 - Be...
Gully
Width
Bottom
Width
BMP
Efficiency
Soil
Class
Soil
Weight
Nutrient
Correction
Factor
Annual
Reduction
0 Loams, sandy ~
Streambanks Report
Watershed Streamban
Name Name
Length Height
Lateral ..
Recession
Rate
Range
BMP ..
Efficiency
Soil
Class
Nutrient .
Correction
Annual
Load
Load
Reduction
Figure 16. View of the Gullies& Streambanks Form.
^kulli
W
Hies Report
(D 030601060607 - Be...
Gully
Name
Gully 1
Width
Bottom
Width
BMP
Efficiency
Soil
Class
Soil
Weight
Nutrient
Correction
Factor
Annual Load
Load Reduction
0 Loams, sandy...
Figure 17. View of the trash can icon to delete a row.
22
-------
BMPs Module
The BMPs module provides a single table where the BMPs can be entered for all watersheds and all non-urban
land uses. The land uses included in the BMPs module are cropland, feedlots, pasture, forest, and user defined.
At its most basic, you select the BMP and the percent of the land use on which it's applied. The effective
efficiency will be calculated automatically.
The BMP worksheet is set up to accept one BMP per land use per watershed. If only BMP per land use is being
applied in each watershed, this is the only table needed for adding non-urban BMPs. Alternatively, combined
BMP efficiencies can be created to account for multiple BMPs on a single land use using the BMP Calculator,
which is discussed later in this guide.
To add a BMP to the BMP table, click Add BMP on the right side of the table, a pop-up window will open, where
the watershed and land use should be specified for the BMP that is being added (Figure 18). After clicking Add
BMP in the pop-up window, a new line is created in the BMP table. From here, double click on the BMPs cell to
activate a drop-down list of BMPs that can be applied. Once a BMP is selected, the pollutant efficiencies will
automatically populate in the table. For BMPs with no default efficiency value data, "ND" will be displayed in
corresponding efficiency field in the table.
The next step is to add the percentage of the land use on which the BMP is applied to the "% Area BMP Applied"
field. Adding a single BMP to a land use means that only one type of BMP is present, but there may be several
locations throughout the watershed with this BMP. The user should add up the total acreage of the BMP type in
the watershed and determine what percentage of the land use is covered by this BMP.
For example, if Contour Farming is the only cropland BMP in a watershed, add up all the acres of contour
farming throughout all of the cropland and divide this by the total acres of cropland in the watershed to
determine the percent area of the BMP.
The list of BMPs included in the BMPs drop-down menu will include those specific to the selected land use as
well as any custom BMPs the user has previously created.
Pollutant Load Estimation Tool
Title
Example Watershed
State
Georgia
Watershed
030601060607 (Beaverdam Ditcf
Share Model I Copy Model I Delete Model I Download Input Data Server Data
Add watershed
31
Delete watersheds
D
Gullies and Stream banks
County
RICHMOND
Rainfall Correction
0.9163
Weather Station
AUGUSTA BUSH FIELD AP
Urban BMP Tool
Rairaays Correction Facti
0.5051
Rainfall Initial Abstracts
Manure Application
BMP Calculator
Inputs BMPs Total Loads Additional Reference Tables
Land Use Filter By Landuse 8
BMPs and Efficiencies
Watershed
Double-click on the "BMPs' field to select a Best Management Practice that will be applied 1"030601060507 " Beaverdam Ditch-Savannah River
Watershed BMPs ¦ Landuse
Cropland '
Add BMP Cancel
Create a User Defined BMP
% Area BMP Applied
Figure 18. BMP Module layout and popup window to specify the watershed and land use where a BMP will be applied.
23
-------
To delete a BMP from a scenario, click on Delete BMP on the right side of the BMP table. A popup window will
list the BMPs in the scenario. Click the trash can icon next to the BMP that will be deleted. A second popup will
ask "Are you sure?" Click OK to delete the BMP or Cancel to keep it.
User-Defined BMPs (Custom BMPs)
PLET allows the user to create custom BMPs if the user has BMPs that are not represented in the model, or
BMPs with efficiencies that are different from the default values included in the model. The advantage to
creating user-defined BMPs is that it allows the user to create BMPs with more specific local data than is
provided in the default values. Custom BMPs can also represent a suite of practices with a combined BMP
efficiency. User-defined BMPs are saved and can be accessed for future use in other modeling scenarios.
To create a User defined BMP, from the BMPs module, click on Create a User Defined BMP. This opens a dialog
box where the BMP name and pollutant reduction efficiency values can be entered (Figure 19). Efficiencies are
values between 0 and 1.00, where 1.00 equals 100% pollutant removal (e.g. 0.4 is 40% removal). After clicking
Save and Close, the BMP will be saved. To add the new BMP to the scenario, click Add BMP and select the
watershed and land use where the practice will be applied. Then select the practice from the drop-down list in
the BMPs column. Custom BMPs are distinguished from default BMPs with the naming convention "User
Defined/Custom BMP (Custom BMP Name)" where the Custom BMP Name is the name entered in the Custom
BMP creation dialog box.
Pollutant Load Estimation Tool
Title
Example Watershed
State
Georgia
Watershed
030601060607 (Beaverdam Ditct
Share Model I Copy Model I Delete Model I Download Input Data Server Data
Add watershed
Di
Delete watersheds
m
Gullies and Streambanks
Inputs
BMPs and Efficiencies
Double-click on the "BMPs' field to select a Best Management
Watershed
030601060607 - Beaverdam Ditc...
County
RICHMOND
Rainfall Correction Factor
0.9163
Weather Station
AUGUSTA BUSH FIELD AP
Ra nflays Correction Factor
0.5051
Rainfall Initial Abstraction
Urban BMP Tool
1
Custom BMP
Name your custom BMP
N Value
1
P Value
BOD Value
<
Sediment Value
Add a custom BMP to your list of BMPs. The final nai
tie will be listed as 'User Defined/Custom BMP (#YOUR CUSTOM
NAME#). To change the values of a custom BMP, ere.
ate a new custom BMP using the same name as the one you want
to change.
Save Close
BMP Calculator
Landuse
Cropland
Figure 19. View of the Custom BMP popup where the BMP name and pollutant removal efficiencies are specified.
Editing Default Efficiency Values
While the user can manually adjust the efficiency values of the default BMPs, EPA strongly recommends creating
a custom BMP with the desired efficiency values instead. Any manual adjustment to the default values will not
be carried over to other models and may result in incorrect values being applied to areas of less than 100%.
24
-------
Urban BMP Tool
The Urban BMP Tool supports the input of BMPs on urban land uses. All urban BMPs are entered here, rather
than on the BMPs module. The Urban BMP Tool uses event mean concentrations (EMCs) in runoff for each of
the 9 urban land use types.
There is a default distribution provided for urban land uses in Table 8 in the Input module. This table should be
modified to match the desired urban land use distribution in the watershed before working in the Urban BMP
Tool. Default values for urban pollutant concentrations are provided in the Urban BMP Tool. These values can be
modified/refined if local data are available.
To use the Urban BMP Tool, select the watershed from the drop-down and then select the urban land use where
BMPs will be applied (Figure 20). Select the BMP from the Available LID/BMP drop-down list and enter the total
acres treated by that type of BMP. The total available acreage for the land use selected will be shown for
reference, the BMP drainage area cannot exceed the amount of total available acres. Click on Apply LID/BMP
and a pop-up will indicate that the BMP has been applied. As BMPs are entered, they will appear in the Selected
Urban BMPs table, showing the type of practice on each urban land use. Further down, the Effective BMP
Application Area table will indicate the number of acres where BMPs have been applied on each land use. There
is also a Percentage of BMP Effective Area table that will show the proportion of each land use with BMPs.
There are generally three types of urban BMP/LID practices: standard efficiency-based reduction BMP practices,
BMP and LID practices with runoff capture depths, and LID practices with volume reductions. LID practices with
volume reductions are denoted with an asterisk in the Available LID/BMP drop-down list.
Practices that include a runoff capture depth and volume reductions will have additional required inputs. When
a runoff capture depth practice is selected, another input window (Runoff) will appear with fields for the percent
impervious and the runoff capture depth. The default values are set at 100% impervious and 0.5 inch capture
depth. These values should be modified to reflect the local conditions. Based on the BMP drainage area, percent
impervious and the runoff capture depth, the Tool will calculate the percent of captured runoff volume from the
total area of the selected land use and the BMP storage capacity in gallons. An estimated BMP sizing (required
BMP area) is also calculated. The BMP storage volume will be displayed in the Captured Flow Volume table and
the required BMP area will be shown in the BMP Surface Area or Number of Units table.
Similarly, if a runoff volume reduction practice is selected, the Runoff input window will appear, as will a second
popup window called Runoff Volume Reduction. The Runoff input window will now include a third calculated
value - the required number of cisterns/rain barrels to treat the BMP drainage area at the runoff capture depth
specified. The Runoff Volume Reduction popup will provide the runoff volume reduced by the selected practice
based on the use-specified inputs. This information will be presented in the Captured Flow Volume and BMP
Surface Area or Number of Units tables.
Like the BMP module, only one BMP type can be entered for each land use. For example, if there are 12
bioretention systems on the commercial land use, add up the total drainage area for all 12 systems and apply
that as the BMP drainage area.
25
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Urban BMP Tool
Urban BMP/LID
Watershed
030601060607 - Beaverdam Ditch-Savannah River
> Commercial
) Multi-Family
1 Transportation
Available LID/BMP
LIDVCistern
) Industrial O Institutional
! Open-Space Q_) Single-Family
) Urban-Cultivated O Vacant-Developed
BMP Drainage Area (ac):
0.25
Total Available Area (ac):
Percent Impervious (%):
Runoff Capture Depth (in):
0.5
- Percent captured volume =
.01%
- BMP Storage Capacity =
3394.28 gallons
- Required Cistern/Rain barrel
BMP Units = 34 Units
Runoff Volume Reduction
Input the runoff volume (ac-
ft/yr) reduced by the practice
(Baseline Runoff =3015.95 ac-
ft/yr):
.239
Reset All Urban BMPs
Figure 20. View of the Urban BMP Tool input screen.
Combined Urban BMPs and Custom Urban BMPs
The Combined BMPs-calculated selection from the Available LID/BMP drop-down list provides options for
entering either a custom BMP efficiency for a single BMP or a combined BMP efficiency for multiple BMP types.
Upon selecting the Combined BMPs-Calculated, an Efficiencies popup window will appear where the pollutant
removal efficiencies can be entered directly, and where the BMP Calculator can be accessed (Figure 21). If the
custom BMP efficiency is known, the pollutant removal efficiencies should be entered and then click Apply
LID/BMP.
If there are multiple types of BMP on a given urban land use, the user can click on the BMP Calculator button in
the popup. This will open the BMP Calculator, which can be used to calculate a combined BMP efficiency for all
of the BMPs on a given land use. Refer to the BMP Calculator section of this guide for details on its use. Once the
BMP configuration is submitted from the BMP Calculator, it will return the user to the Urban BMP Tool, where
the BMP will automatically be submitted to the scenario. There is no need to click Apply LID/BMP. The combined
BMP efficiency will also be displayed in the Efficiencies popup. Note that with combined BMPs, volume
reductions calculations are not provided.
Click Exitto leave the Urban BMP Tool and return to the main Tool modules.
Urban BMP Tool
Urban BMP/LID
Efficiencies
Watershed
030601060607 - Beaverdam Ditch-Savannah River •
Landuse O Commercial
O Multi-Family
(J) Transportation
Available LID/BMP
Combined BMPs-Calculated
1 Industrial
1 Open-Space
1 Urban-Cultivated
Institutional
Single-Family
Vacant-Developed
BMP Drainage Area (ac):
Total Available Area (ac):
Enter the calculated BMP
efficiencies:
N Removal Efficiency (0-1):
P Removal Efficiency (0-1):
BOD Removal Efficiency (0-1):
TSS Removal Efficiency (0-1):
Reset All Urban BMPs
Use the BMP Calculator
Figure 21. View of the additional input fields for custom and combined BMPs in the Urban BMP Tool.
26
-------
BMP Calculator
The BMP Calculator calculates the combined BMP efficiency of combinations of multiple BMPs that can then be
applied in the model. The calculator can be used to represent BMPs both in series and parallel. For an
explanation of BMPs in series and parallel, refer to Appendix A. To access the BMP calculator, either click on the
BMP Calculator button, or from the BMP drop-down list, select Combined BMPs-Calculated.
Navigating the BMP Calculator
As shown in Figure 22, when the BMP Calculator is opened, the main screen is a gridded workspace. There are
two modules in the BMP Calculator, which can be found along the top of the window: BMP Editor, where BMP
configurations can be created and modified, and Configuration List, where the user can review previously
created BMP configurations and view the BMPs and efficiency value in each configuration.
BMP Calculator
BMP Editor Configuration List
BMP Calculator
Cancel Save Calculator Configuration
Watershed Landuse
030601060607 - Beaverd; « Cropland *
If nodes are appearing past the edge of the grid, please zoom in or out to make them visible
Node Info
Configuration Name
Node name
BMP Type
Area (acres)
NEff.
Save Node Changes
Node Connections
H
Load a Configuration
Config select
Figure 22. The BMP Calculator layout.
BMP Editor
When in the BMP Editor, the gridded workspace, each BMP is represented graphically by a node. Connecting a
series of nodes creates a configuration, which represents multiple BMPs with a single set of combined BMP
efficiency values. To build a BMP configuration, click on Create Node, and a New Node box will appear in the
workspace. Click on the node box to activate data entry for the node. On the righthand side, there is a panel
called Node Info, which provides input fields for the various required information for each BMP in the
configuration. In the Node Info pane, the user enters the Configuration Name, the name of the node, the BMP in
the node, the acres on which it will be applied, and then the efficiency values will automatically populate when a
BMP is selected from the BMP Type drop-down list. Click on Save Node Changes, to save and move on to the
next node. If Save Node Changes is not clicked, the data entered will be lost. Continue building the nodes by
clicking on Create Node again and repeating the process as many times as is necessary to represent all the BMPs.
To create the configuration of the BMPs to represent how they are organized on the ground, click on the black
27
-------
half circle on the right side of a node and drag it to the white half circle on the left side of the node that it will
drain to. A line will connect the two nodes. Multiple BMPs can connect to the same node, if needed. The final
node in the configuration will provide the resulting total area and combined BMP efficiencies. A more detailed
explanation of BMP configurations is provided in Appendix A. Once the configuration is complete, click Save
Calculator Configuration. This configuration is now available to use in any scenario and will appear in the BMP
drop-down list in the BMPs module. Figure 23 shows the finished product of a parallel BMP configuration with
three BMPs and a blank final node that connects all three BMPs together and provides the combined efficiency
values. The final node in this instance represents the receiving water.
NOTE: When creating a BMP configuration, if Save Calculator Configuration is selected, the BMP workspace wili
be cleared prior to Submitting the BMP to the selected land use. If this occurs reload the BMP from the
Configuration List and click Submit. If Save Calculator Configuration is not clicked, the combined BMP efficiency
will be entered into the scenario, but the configuration will not be saved or reviewable at a later time.
BMP Calculator
BMP Editor Configuration List
BMP Calculator Create Node Cancel Save Calculator Configuration Node Info
Contour Farming
BMP Type - Cropland - Buffe
BMP Type - Cropland - Conto.
Area - 250
N Elf. - .279
P Eff - .398
BOD Eff. - 0
Sediment Eff. - 341
BMP Type - Cropland - Cover.
Area - 300
N Eft-196
P Eff . - .07
BOD Eff. - 0
Sediment Eff. -1
r N Eff - 478
L P Eff - .465
BOD Eff. - 0
Sediment Eff - .586
BMP Type - 0...
Area - 0
N Eff. - 0
P Eff. - 0
BOD Eff. - 0
Sediment Eff. -0
Area: 575.000
N Eff. : 0.244
P Eff : 0.230
BOD Eff : 0.000
SED Eff : 0.226
Forest Buffer
Configuration Name
Parallel BMP 1
Node name
New Node
BMP Type
Area (acres)
Save Node Changes
Node Connections
| Select a Connection
Load a Configuration
Watershed Landuse
030601060607 - Beaverdi * Cropland *
If nodes are appearing past the edge of the grid, please zoom in or out to make them visible
Figure 23. Example Combined BMP efficiency value configuration using the BMP Calculator BMP Editor.
Reviewing and Editing Previously Created Custom or Combined BMPs
To bring up a previously created configuration in the BMP Calculator, at the bottom of the Node Info is a "Load a
Configuration" section with a drop-down list of all previously created BMP configurations. Select the BMP
configuration of interest and it will load in the workspace.
To change elements of a previously created configuration, once it is loaded in the workspace, make the desired
edits and then click Save Calculator Configuration. To create a new BMP configuration, based on a previous BMP
configuration, load the initial configuration, and then edit the configuration name. Once the name is changed,
changes to the configuration will be reflected in the new configuration, while still maintaining the original
configuration of the initial BMP configuration.
28
-------
Configuration List
The Configuration List Provides a summary of all the combined BMP configurations created by (or accessible to)
the user. The list shows the combined efficiency value for each configuration. Click View Configuration to see an
image of the BMP configuration below the list. This is intended as a reference for the user to assist in reviewing
and selecting the appropriate BMP configuration from those previously created.
29
-------
Total Loads Module
The Total Loads module shows the final results of the modeled calculations in terms of watershed pollutant
loads and load reduction from BMPs, shown in Figure 24. There are several tables summarizing the results of the
scenario:
• Table 1 - A summary of the total pollutant loads for each watershed in the scenario, both with and
without pollutant reduction practices. Also provides the percent reduction in pollutants from load
reduction practices.
• Table 2 - The total loads by land use for all watersheds cumulatively. Gullies, streambanks, septics and
groundwater are each broken out separately from land uses.
• Table 3 - The pollutant loads by land use by watershed with pollutant reduction practices
• Table 4 - The loads from groundwater by land use with BMPs included
• Table 5 - The pollutant loads from just urban areas.
As shown in Figure 24, there is a Download button on the top right of the Total Loads module. This will generate
an Excel file with the results from the 5 tables, each in its own tab. In addition to the results tables, there are
two check boxes in the upper left of the Total Loads module: Groundwater Load Calculation and Treat All
Subwatersheds as Part of a Single Watershedalso visible in Figure 24.
Groundwater loads are optional - check the box to include groundwater loads. These values are based on the
infiltration volume using the selected HSG for each watershed in the scenario. Based on the infiltration rate, the
annual infiltration volume is calculated for the different land uses. The annual infiltration volume is assumed to
be equivalent to the annual groundwater output in the local hydrological cycle.
The groundwater load is not impacted by any of the BMPs entered into the scenario. Changes to groundwater
loads can be manually adjusted by the user by editing the HSG applied and/or editing the nutrient
concentrations in shallow groundwater to be reflective of the desired practices.
The Treat All Subwatersheds as Part of a Single Watershed check box changes the sediment delivery ratio. This
box is only relevant if there is more than one watershed in the model. Checking the box allows the sediment
delivery ratio to be calculated using the total watershed area of all watersheds included the model. Important:
This feature does not represent routing through the watersheds in a particular order. Unchecking the box allows
the sediment delivery ratio to be calculated independently for each watershed in the model.
30
-------
Pollutant Load Estimation Tool
Eranpte Wsteriwd
'Watershed
030601OCC607 fflcj.'cidam Ditc
Copy Model I Delete Model I Dcnaribad Input Data Server Data
Delete watersheds |
Gullies and Strcamhanks
Inputs BMPs Tot^l Loads Additional Reference Table;
Loads Calculated
I I Groundwater foac cafculaton Q Treat ail sutmatenheds as part of a single watershed
County
RICHMOND
Westfieir Station
AUGUSTA BUSH R&D AP
RiWfcl Corruseri =acfor ftandays Gatocitfi Fiec»
0S163 0.SC51
Urban BMP Tool
Mjtfiurc Application
1. Total load by subwatershed(s)
M Lad PL"a 800
(toc/yaar) (lbc/yw)
Sediment
Load
tHo
BW)
(towyaar)
H
RKluoton
Obcjyaan
Raduetan
mwrMO
BOO
RadueMon
(Itwlyaan
Ssdbnont
R«Ouo9cm
(tnWVBHl
N Load
(Wtth
Ubbywl
PLoad
»»
CfcUyaart
BOO
(WNh
Cfcc,Vo*r)
Load
(WW)
on
itontynar)
«N
Reduction
*P
RBdinthm
% BOD
*
&*
BOO Low) (Hwyr)
Badtant Load (tfyr)
Urban
17256640
2S&42.33
670154. W
3362.42
Crcpisnd
25GG7.C8
5473J9
61574.17
133124
27350SG
22® B0
235.73
—
1191677
58BS23
2S478JZ2
19607
Food OB
835301
1713 BO
11425.35
0X0
UiarMlnad
0.00
ooo
OjOB
0.00
Saptc
61825
242.1G
2524 53
0.00
3.67
1.41
734
2.70
Srwntunk
0.D0
goo
OOG
n.oo
QnviMAr
UB
000
OJOO
0.00
TOTAL
2460B1.13
42246.27
363140.1*
6728-32
3. Nutrient and sediment load by land i
man
WMarcrtad N | P | BOO | Sodlmont
uses wi1
Lj
th BMP (lbs/year)
P | BOD | tndlmafii
Lj
Pacha
rtXwa
BOD |
Lj
For
LJ
net
Lj
F«
u
dot
^BO^I
N
0306010GQ607 ¦
BaaMroamOtdv
SauraahRvar
17256640
26612.39
670154.14
732484230
25057 0B
5473.73
61574.17
26636T7J12
273ffi.HR
2285.89
"ST."1.
471571.82
1T016.77
GBSS.23
2947H22
35214321
3569 01
1713.80
1142535
0.00
ooo
4. Load from groundwater by land uses with BMP (lbs/year)
Urban Cropland
Watarehod H | P BOO Sadmant M P BOD | Sad) merit
Lj
Pasha
mm
Lj
FOTBtl
P | BOO |
Sediment
L
Fm
U
dot
^BO^I
H
03060106C607 -
Si-«orfiafi FOvaf
0.00
CLG0
(mo
•»
300
0.00 0.00
oco
OOO
OOO
aco
„
oca
-
MB
<9
OOO
aoo
OOO
5. Pollutant loads from urban (lbs/year)
WatonlujC P l 800 l T,i
Lj
Load fla
LJ
~
iJ
Lj
P | BOO
O3O6Q1Q6C607 -
BaavanMm CMdv
172566.43
3=642.96
670(5414
7924842.30
aoo
aoo
a.x>
ooo
172566.49
2E64299
57015414
7924842.30
< ~
Figure 24. Total Loads Module. Note that full visibility of tables 3 and 4 requires scrolling to see all columns.
31
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Additional Reference Tables Module
The Additional Reference Tables module provides default values for animal weights used to calculate animal
equivalent units, soil infiltration rates, feedlot nutrient ratios, septic overcharge pollutant concentrations
reaching streams, and wastewater nutrients and volume (Figure 25). These values are used in conjunction with
the data entered on the Inputs module to generate the loading rates for the land uses and activities. The default
values can be adjusted by the user if more accurate local data are available.
32
-------
Pollutant Load Estimation Tool
Titfe
Example Vfotenhed
Watershed
030601060607 l/day/penonfrarvge of 45 to 100)
Wastewater per capita:
Parameter
Wastewater
Remarks
Total Nitrogen
40
mg/t {range of 20 to 85)
Total Phosphorus
8
rng/L (range of 4 to 15)
Organic: I BOD)
220
mg/L (range of 110 to 400)
Typical septic overcharge flow rate
75
gal/day/person(range of 75 Id 125)
Figure 25. View of the Additional Reference Tables module.
33
-------
APPENDIX A - BMPs in Series and Parallel
Series and parallel describe different configurations of BMP placement on the landscape. As shown in Figure 26,
parallel BMPs are BMPs that treat areas separately and individual loads are drained to the same waterbody. In a
series BMP configuration, more than one BMP treats the same area of the watershed. In other words, the same
runoff flows through two or more different BMPs before reaching the waterbody. BMPs in series can also be
thought of as a treatment train. Using the BMP calculator tool modelers can develop a singular BMP for
common treatment trains applied in their watershed.
There can also be combinations of parallel and series BMPs where one portion of the watershed is treated by
multiple BMPs and another portion is treated by one or more other different BMPs.
Parallel
Series
Cover Crop
Conservation tillage
Conservation tillage
Conservation Tillage
Cover Crop
Combination
Grass Buffer
Settling Basi
Figure 26. Illustration of BMPs in parallel and series configurations, and a combination of both.
Parallel BMPs Example
The first example is a watershed with parallel BMPs only (Figure 27). In this example, there are 2,000 acres of
cropland in the watershed. It has 3 BMPs: forest riparian buffer (25 acres), contour farming (250 acres) and
cover crops (300 acres), leaving 1,425 acres of cropland untreated by BMPs. Each of these BMPs treats a
separate area of the watershed. To build this configuration in the BMP calculator, create separate nodes for
each of the BMPs and connect them to a node representing the remaining acres in the watershed, as shown in
Figure 28. The results box below the final node in the configuration shows that there are 2,000 acres. In this
configuration 100 percent of the cropland is represented. The resulting efficiency values can be applied to
cropland and denote that the Percent Applied in the BMP module row for crop land is 100%. Alternatively, a
scenario can be built that only represents the areas of the watershed treated by BMPs, as shown in Figure 29. In
this case, the node at the end of the configuration is left blank and the area reflected is only 575 acres, the sum
of the BMP acreages. Notice that since all acreage in the calculator is treated by a BMP, the efficiencies are
higher. However, if this configuration is to be added to the same scenario, the Percent Area Applied would be
28.75% (575/2000 acres). Both of these configurations will result in identical load reductions in this scenario.
However, the first configuration representing all cropland may be most useful when building a single watershed
scenario, where the second option could be used to build a BMP combination that is saved and frequently
applied across many watersheds and different scenarios.
34
-------
Parallel BMPs only
2,000 acres of
cropland
Conventional tilla
(no BMP)
Cover
crops
300 acres
Conventional tillage
(no BMP)
Figure 27. Example of parallel BMPs on the landscape.
BMP Calculator
BMP Editor Configuration Li
BMP Calculator
030601060607 - Beaverdam Ditch-Sa • Cropland •
If nodes are appearing past the edge of the grid, please z<
m in or out to make them visible
Cancel Save Calculator Configuratjo
Configuration Name
Parallel BMPs
Entire Watershed
BMP Type
BMP Type - Cropland - Bufie
Area - 25
^ N Elf - 478
BMP Type - Cropland - Conto
Area - 250
ft N Eft - 279 L
s P Elf - 398
BOD Eff - 0
. Sediment Eff - 341
BMP Type - Cropland - Cover-
Area - 300
^ N Eff. - 204
^ P Eff - 15
BOD Elf - 0
Sedimeni Eft - 2
NEff -0
PEff -0
BOD Eff - 0
Sediment Eff. - 0
Area: 2000.000
N Eff. 0.071
P Eff.: 0.078
BOD Eff.; 0.000
SED Eff. - 0.080
Node Connections
Select a Connection
Load a Configuration
Conftg select
Parallel BMPs
Figure 28. Example configuration of parallel BMPs in the BMP Calculator representing all cropland.
35
-------
BMP Calculator
BMP Editor Configuration Li
BMP Calculator
ncel Save Calcu!
Configuration Name
Parallel BMPs
New Node
BMP Type
Node Connections
I Select a Connection
Load a Configuration
Config select
Parallel SMPs
Figure 29. Example configuration of parallel BMPs in the BMP Calculator representing only cropland with BMPs.
Series BMPs Example
This example demonstrates a watershed with series BMPs (Figure 30). There are 625 acres of pasture in this
watershed. All of the pasture flows through a forested riparian buffer, but 525 acres also have livestock
exclusion fencing. The practices are arranged in such a way that runoff from the fenced pasture will also travel
through the buffer - two practices in series. For this configuration, it is necessary to ensure the proper number
of acres are represented in the BMP Calculator. The acres treated by fencing are routed to the riparian buffer,
which treats an additional 100 acres of pasture. The configuration is shown in Figure 31. The 525 fenced acres
are routed to the riparian buffer, where those 525 acres plus an additional 100 acres only treated by the buffer
are represented. The results box below the buffer node confirms that the total acreage in the scenario is 625
acres. A common mistake in this type of configuration would be to enter 525 acres treated by the fencing and
then 625 acres treated by buffer. However, the results box provides an indication that this is incorrect because
the total acreage would be shown as 1,150 acres rather than 625 acres.
Contour Fanning
Watershed Unduse
030601060607 - Beaverdam Ditch-Sa » Cropland »
If nodes are appearing past the edge of the grid, please zoom in or c
BMP Type - Cropland - Conks
Area - 250
N Elf - 279
P Elf - - 398
BMP Type - Cropland - Cover.
Area 575.000
NEff 0.249
P EfT 0 272
BOD Eff 0.000
SED Elf. : 0.278
36
-------
Series BMPs Only
625 acres of
pasture
525 acres
Livestock exclusion
fencing
pl
m
Forested Riparian
buffer
625 acres
Figure 30. Example of series BMPs on the landscape.
BMP Calculator
BMP Editor Configuration Li
BMP Calculator
030601060607 - Beaverdam Ditch-Sa »
If nodes are appearing past the edge of the grid, pli
Node Info
Configuration Name 13)
Node Connections
I Select a Connection
Load a Configuration
Config select
Figure 31. Example of series BMP configuration in the BMP Calculator.
37
-------
Parallel and Series BMPs Combined
Next is an example of parallel and series BMPs in the same configuration. In this scenario there are 975 acres of
pasture in the watershed. There are several BMPs across the pastureland: 100 acres of forage planting high in
the watershed, 25 acres draining to forested riparian buffer on one side of the stream, and 750 acres of livestock
exclusion fenced pasture draining through a forest buffer before reaching the stream. This leaves 100 acres of
pasture untreated by BMPs (Figure 32).
In the BMP Calculator, the forage planting and 25 acres of forested riparian buffer are two parallel BMPs, so
they can be linked directly to a downstream node representing the entire watershed. The livestock exclusion
fencing and riparian buffer represent a BMP series, so these should be linked together, with 0 acres in the buffer
node to denote that the buffer is treating the same pasture acres as the fencing. This series BMP can also be
connected to the node representing the entire watershed. This final downstream node represents all of the
upstream acreages and BMPs plus the 100 untreated acres, so the 100 untreated acres are entered but the BMP
type is left blank. The results box shows the combined efficiencies for this configuration on 975 acres. Figure 33
illustrates the completed configuration for this scenario.
Parallel and Series BMPs
Figure 32. Example of combined series and parallel BMPs.
38
-------
Node Connections
| Select a Connection
Load a Configuration
Figure 33. BMP Calculator configuration for series and parallel BMPs on the same watershed.
39
-------
APPENDIX B - References Used in PLET
The following references describe where the default values and data assumptions are derived.
HUC12
USEPA/OW/WBD_WMERC
(Map Server)
EPA ArcGIS Server
3s ://wate rsqeo ,e pa ,q ov/ArcG I S/rest/services
Weather Data
Land Use
Agricultural Animal Counts
Animal weights
Feed lot Nutrient
Concentrations associated
with animal weights
Nutrient Adjustment Factor by
Animal Density
Number of septic systems
Population per septic system
USEPA, 2019
Calculated based on daily
and hourly timeseries from
WDM files provided within
BASINS.
Multi-Resolution Land
Characteristics Consortium.
NLCD and USDA Cropland
Data Layer (CDL) 2011,
HICCAP_land_cover_2005_
2011_Enviroatlasversion
AK_NLCD_2011 landcover
USDA, National Agricultural
Statistics Service. 2014 &
USDA 2012.
ASAE 1998
DEQ 1999
Evans et al. 2001
1992 and 1998 National
Small Flows Clearinghouse
(Defunct, now part of the
National Environmental
Services Center)
(iMtewjejawMdl/)
1992 and 1998 National
Small Flows Clearinghouse
(Defunct, now part of the
National Environmental
Services Center)
Based on long-term (30 years) quality controlled
precipitation data from USEPA's BASINS
system
2011 landcover
2014 animal count data
The agricultural animal data source is at the
County level and is summarized at the HUC12
level based on the pastureland area weighted
ratio
Standard animal weights
Nutrient Ratios
Based on the density of agricultural animals in
the study area (PLET calculates animal density
automatically)
The septic system data source is at the County
level and is summarized at the HUC12 level
based on the low-density residential area
weighted ratio.
The septic system data source is at the County
level and is summarized at the HUC12 level
based on the low-density residential area
weighted ratio
40
-------
Septic failure rate
Soil cover Curve Number for
feed lot based on percent
paved
Infiltration Fraction by
Hydrologic Soil Group for
groundwater infiltration
volume calculation
Sediment delivery ratio
Urban Runoff Curve Numbers
Urban Land pollutant
concentrations in runoff
Septic overcharge
concentrations reaching
stream
Wastewater per capita
concentrations
USLE parameters: USDA-
NRCS, National Resources
Inventory (no date)
Hydrologic soil group (HSG)
Soil N, P, BOD concentrations
Runoff Curve Numbers
1992 and 1998 National
Small Flows Clearinghouse
(Defunct, now part of the
National Environmental
Services Center)
(iMfelJejawMdM/
Horsley & Whitten, 1996,
USEPA, 1980
Metcalf & Eddy, 2003
NRI, 1992
USDA NRCS 2002 SSURGO
database
Haith et al. 1992
USDA NRCS, 1986
DEQ, 1999
Caraco, 2001.
USDA-NRCS 1983, Vanoni,
1975
USDA NRCS, 1986
Caraco, 2001.
The septic system data source is at the County
level and is summarized at the HUC12 level
based on the low-density residential area
weighted ratio.
2nd Edition. Table - Typical composition of
untreated domestic wastewater
National Resource Inventory study 1992 by county
Area-weighted within each HUC12
National average from GWLF manual.
TR55 manual
Table 2-2b
Cropland - Row Crops - Good condition
Table 2-2b
Pasture - Pasture - Fair condition
Forest - Woods - Fair condition
Page 39, Figure 8 in DEQ, 1999
Taken from Page 7-16, Table 7.7 of Caraco, 2021
(by HSG; same for all land use types).
Urban land use infiltration fraction value was
assumed to be 20 percent less for each HSG.
Drainage area power function relationships
TR55 manual Table 2-2a
Default suggested TN, TP, and TSS
concentrations taken from Tables 6.2, 6.3, and 6.4
for Commercial, Industrial, Transportation, and
Residential.
41
-------
Nutrient Concentration in
Runoff
Haith et al. 1992, Haith &
Shoemaker. 1987
Nutrient Concentration in
Shallow Groundwater
Haith et al. 1992, Haith &
Shoemaker. 1987
Urban Land Use Distribution
Gully and Streambank Soil dry
density and correction factor
DEQ, 1999
Dissolved Nutrients in Agricultural Runoff (Table 2
in Haith & Shoemaker, 1987)
Cropland-Low
o N & P taken from Small Grains
o N with manure estimated from ratios
estimated from reported values of
Cropland-Medium with and without
manure (see Cropland-Medium)
Cropland-Medium
o N with and without manure taken from
Corn,
o P scaled based on ratio of Cropland Lo
and High since reported Corn P was less
than small grain P, ratio of ~1.5 was used
to scale Cropland L to Cropland M for P)
Cropland-High
o N and P estimated based on ratio of 1.5
from Cropland- Medium
o Cropland-High N estimated based ratio of
N with and without manure for Medium
condition
• Manured P concentrations taken based on
best professional judgment using the range
provided for Corn, Small Grains, and Hay
from manured lands (reported range was 1.9
to 8.7 mg/L). P concentration of 2, 3, and 4
mg/L were assigned for low, medium and
High with manure condition.
• BOD was taken as ~2 times N for without
manure and ~1.5 times N for with manure
applied.
• Pasture values are currently same for all low,
medium and high with and without manure.
These should be updated is literature is
available. Haith & Shoemaker 1987 report N
and P as 3 mg/L and 0.25 mg/L which is
slightly lower than that in PLET which has 4
mg/L and 0.3 mg/L. BOD was taken as 13
mg/L for which we do not have reference.
Most likely based on best professional
judgement.
Taken as mean value of reported (inorganic forms
of N and P) Central, Eastern, and Western US in
Table 3 for mean dissolved concentration in
streamflow
Forested~ 90% forested; Agriculture~ >=75%;
Pasture taken same as that of Agriculture; Urban
N from Caraco, 2001, P taken same as Pasture.
Percent Urban land use distribution. These area
Placeholder values provided that should be
adjusted by the user based on the distribution
found in their watershed
Figure 1. Lateral Recession Rate
Exhibit 1 Dry Density Soil Weights & Exhibit 2. Soil
Textural Class
42
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References:
HUC12 Watershed Boundary
USEPA/OW/WBD_WMERC (Map Server)
https://watersgeo.epa.gov/ArcGIS/rest/services/OW/WBD WMERC/MapServer
Weather Data:
US EPA (2019). BASINS 4.5 (Better Assessment Science Integrating point & Non-point Sources) Modeling
Framework. National Exposure Research Laboratory, RTP, North Carolina.
https://www.epa.gov/sites/production/files/2019-03/documents/basins4.5coremanual.2019.03.pdf
Land Use:
Multi-Resolution Land Characteristics Consortium (MRLC) and Crop Data Layer (CDL), 2011 CONUS Land
Cover.
MRLC. 2011 AK Land Cover
National Oceanic and Atmospheric Administration, Office for Coastal Management. "Name of Data Set."
Coastal Change Analysis Program (C-CAP) High-Resolution Land Cover. Charleston, SC: NOAA Office for
Coastal Management.
Agricultural Animal Counts
USDA National Agricultural Statistics Service, 2012 Census of Agriculture. Complete data available at
https://agcensus.library.cornell.edu/census year/2012-census/.
USDA, National Agricultural Statistics Service. 2014. 2012 Agricultural Census Data by County
AEU equivalents (animal weights):
ASAE (American Society of Agricultural Engineers). 1998. ASAE standards: Standards, engineering
practice, and data. 45th ed. American Society of Agricultural Engineers. St. Joseph, Michigan.
Animal nutrient concentrations by AEU:
Evans, B.M., S. A. Sheeder, K. J. Corradini, and W. S. Brown. 2001. AVGWLF version 3.2, users guide.
Software. Environmental Resources Research Institute, Pennsylvania State University, University Park,
Pennsylvania.
DEQ, 1999. Pollutants Controlled Calculation and Documentation for Section 319 Watersheds Training
Manual, June 1999 Revision; Michigan Department of Environmental Quality - Surface Water Quality
Division - Nonpoint Source Unit. EQP 5841.
Number of septic systems:
1992 and 1998 National Small Flows Clearinghouse (Defunct, now part of the National Environmental
Services Center) (https://www.nesc.wvu.edu/)
Population per septic system:
1992 and 1998 National Small Flows Clearinghouse (Defunct, now part of the National Environmental
Services Center) (https://www.nesc.wvu.edu/)
43
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Septic failure rate:
1992 and 1998 National Small Flows Clearinghouse (Defunct, now part of the National Environmental
Services Center) (https://www.nesc.wvu.edu/)
Septic overcharge concentrations reaching stream:
Horsley & Whitten, Inc., 1996. Identification and Evaluation of Nutrient Bacterial Loadings to Maquiot
Bay, Brunswick and Freeport, Maine. Casco Bay Estuary Project.
EPA Design Manual, Onsite Wastewater Treatment and Disposal Systems, 1980
Wastewater per capita concentrations:
Metcalf and Eddy, Inc. 2003. Wastewater Engineering: Treatment and Reuse. 4th Edition. McGraw-Hill,
New York. ISBN 007112250-8
USLE parameters: USDA-NRCS,
National Resources Inventory, 1992. 1992 NRI database.
Hydrologic soil group:
USDA NRCS SSURGO 2002 database, https ://websoilsurvey.sc.egov. usda.gov/App/HomePage. htm
Soil N, P, BOD concentrations:
Haith, D.A., Mandel, R., and Wu, R.S. 1992. GWLF: Generalized watershed loading functions (User's
Manual, version 2.0). Cornell University.
Runoff Curve Numbers:
United State Department of Agriculture, Natural Resources Conservation Service (USDA NRCS, 1986).
Urban Hydrology for Small Watersheds, Technical Release 55 (TR-55).
Infiltration Rate by Hydrologic Soil Group
Caraco, D. 2001. The Watershed Treatment Model. Version 3.0. Center for Watershed Protection.
Ellicott City, Maryland.
Sediment delivery ratio: USDA-NRCS 1983
USDA-NRCS (U.S. Department of Agriculture, Natural Resources Conservation Service). 1983Sediment
sources, yields, and delivery ratios. In National Engineering Handbook, Chapter 6, Section 3,
Sedimentation
Vanoni, V.A., (ed). (1975) Sedimentation Engineering. ASCE Manuals and Reports on Engineering
Practice No.54. American Society of Civil Engineers, New York.
Urban Runoff Curve Numbers:
United State Department of Agriculture, Natural Resources Conservation Service (USDA NRCS, 1986).
Urban Hydrology for Small Watersheds, Technical Release 55 (TR-55).
Urban Land pollutant concentrations in runoff:
Caraco, D. 2001. The Watershed Treatment Model. Version 3.0. Center for Watershed Protection.
Ellicott City, Maryland.
44
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Nutrient Concentration in Runoff:
Beaulac, M.N. and K.H. Reckhow. 1982. An examination of landuse- nutrient export relationships. Water
Resources Bulletin 18:1013-1024.
Haith, D.A. and L.L. Shoemaker. 1987. Generalized watershed loading functions for stream flow
nutrients. Water Resources Bulletin 23:471-478.
Haith, D.A., R. Mandal, and R.S. Wu. 1992. GWLF: General watershed loading functions, user's manual,
version 2.0. Cornell University, Ithaca, NY, USA.
Horner, R. R. 1992. Water quality criteria/pollutant loading estimation/treatment effectiveness
estimation. In R.W. Beck and Associates. Covington Master Drainage Plan. King County Surface Water
Manage. Div., Seattle, WA.
Maidment, D. R. 1993. Handbook of hydrology. McGraw-Hill, New York.
Evans, B.M., S. A. Sheeder, K. J. Corradini, and W. S. Brown. 2001. AVGWLF version 3.2, users guide.
Environmental Resources Research Institute, Pennsylvania State University, University Park,
Pennsylvania.
Nutrient Concentration in Shallow Groundwater
Haith, D.A., Mandel, R., and Wu, R.S. 1992. GWLF: Generalized watershed loading functions (User's
Manual, version 2.0). Cornell University.
Haith, D.A. and L.L. Shoemaker. 1987. Generalized watershed loading functions for stream flow
nutrients. Water Resources Bulletin 23:471-478.
Gully and Streambank Soil dry density and correction factor
DEQ, 1999. Pollutants Controlled Calculation and Documentation for Section 319 Watersheds Training
Manual, June 1999 Revision; Michigan Department of Environmental Quality - Surface Water Quality
Division - Nonpoint Source Unit. EQP 5841.
45
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APPENDIX C - PLET Underlying Formulas Documentation
46
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1. Introduction
The purposed of this document is to provide all relevant equations and methods used in the PLET model.
PLET employs simple algorithms to calculate nutrient and sediment loads from different land uses and
the load reductions that would result from the implementation of various best management practices
(BMPs), including Low Impact Development practices (LIDs) for urban areas. It computes surface
runoff; nutrient loads, including nitrogen, phosphorus, and 5-day biological oxygen demand (BOD); and
sediment delivery based on various land uses and management practices. The land uses considered are
urban land, cropland, pastureland, feedlot, forest, and a user-defined type. The pollutant sources include
major nonpoint sources such as cropland, pastureland, farm animals, feedlots, urban runoff, and failing
septic systems. The types of animals considered in the calculation are beef cattle, dairy cattle, swine,
horses, sheep, chickens, turkeys, and ducks. For each watershed, the annual nutrient loading is calculated
based on the runoff volume and the pollutant concentrations in the runoff water as influenced by factors
such as the land use distribution and management practices. The annual sediment load (from sheet and rill
erosion only) is calculated based on the Universal Soil Loss Equation (USLE) and the sediment delivery
ratio. The sediment and pollutant load reductions that result from the implementation of BMPs are
computed using the known BMP efficiencies.
Total Load from various Sources Equation 1
= Urban + Cropland + Pastureland + Forest + Feedlots
+ User — Defined + Septic + Gully + Streambank
+ Groundwater
47
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2. Surface Runoff
The Runoff Curve Number (CN) method is used to estimate runoff from urban land, cropland,
pastureland, forest and a user-defined land use. The runoff equation used is:
(P - la)2 (P-a¦ S)2
——- or Q = — —-
Equation 2
Where:
Q = Surface Runoff (in/day)
P = Rainfall (in) per event.
PLET calculates the average rainfall per even as follows:
P = (Annual Rainfall x Rainfall correction factor) (Rain days x Rain day correction factor)
P = (AR ¦ Rcor)/(Rdays ¦ RDcor)
Equation 3
Where:
AR = Average Annual rainfall
Rdays = Average rain days in a year
Rcor = Rainfall correction factor refers to the percentage of rainfall events that exceed 5 mm/event
RD cor Rain day correction factor refers to the percentage of rain day events that generate runoff
la = Initial abstraction which determines the initial rainfall retention on the land surface. Ia is
given by aS (where a ranges from 0 to 0.2). Note that PLET uses zero initial abstraction factor as
a default value. This is because rainfall and rainy days correction factors are already considering
that runoff occurs when it rains more than 5mm in a day, a criterion used to calculate the
correction factors. For example, for a value of a = 0 Equation 2 reduces to P2/(P+S)
S = Potential maximum retention after runoff begins (in). S is related to the soil and cover
conditions of the drainage area through the CN. CN has a range of 0 to 100, and S is related to
CN by:
Equation 4
48
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The estimated average daily runoff volume is multiplied by the corrected number of average rain days in
a year to calculate the annual runoff volume.
Annual Runoff Volume (ac — ft) = x (Rdays ¦ RDcor) Equation 5
Where:
Q = surface runoff (in)
A = area of land use (acres)
Rdays = Average rain days in a year
RD cor Rain day correction factor refers to the percentage of rain day events that generate runoff
PLET also includes the Irrigation runoff contribution to Croplands when irrigation is applied. The
irrigation runoff depth (Qirr) is calculated using Equation 1. The water depth per irrigation (inches) is used
instead of the rainfall (P). The annual runoff volume for cropland is calculated as the sum of the surface
runoff volume and irrigation volume.
Annual Runoff Volume Cropland (ac — ft) Equation 6
= Runoff Volume of Cropland + Irrigated Runooff volume
= 12 X A X (RdayS ' RDcor) + X Airr X IF
Where:
Qirr = Irrigation runoff (in)
Airr Cropland acres irrigated
IF = Irrigation frequency (#/year)
Urban load is calculated based on the loading from nine separate land use categories - Commercial,
Industrial, Institutional, Transportation, Multi-Family, Single-Family, Urban-Cultivated, Vacant
(developed), and Open Space. The surface runoff depth for each Urban category is calculated using
Equation 5.
49
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3. Nutrient Load from Runoff
Urban
The following equations refers to the total load calculation for Urban land. The loading is calculated using
the calculated annual runoff volume and assumed nutrient EMC concentration in runoff, for each of the
nine urban land use categories:
WN = [V ¦ CN] x 4047 ¦ 0.3048/1000 Equation 7
WP = [V ¦ CP] x 4047 • 0.3048/1000 Equation 8
WB0D = [V ¦ CB0D] x 4047 ¦ 0.3048/1000 Equation 9
Wrss = W ' Ctss] x 4047 ¦ 0.3048/1000 Equation 10
Where:
Wn, Wp, Wbod, Wtss = annual N, P, BOD, and TSS (sediment) loads from Urban land (Kg)
V = the total calculated annual runoff volume from the various urban land use categories (ac-ft)
Cn, Cp, Cbod, Ctss = urban pollutant (N, P, BOD, and TSS) concentration in runoff in mg/L
Cropland and Pasture
The following equations refer to the total load calculation for Cropland and Pastureland. The loading is
calculated using the calculated annual runoff and assumed nutrient concentration in runoff.
WN = V x
WP = Vx
Wbod = ^ x
Where:
.n.„ 0.3048 Equation 11
X 4047 — 4
454
0.3048 Equation 12
X 4047
12/ \12 B0D"
454
0.3048 Equation 13
X 4047
454
Wn, Wp, Wbod = annual N, P, and BOD loads from Cropland and Pastureland (lbs)
V = the calculated annual runoff volume (ac-ft)
Nm = number of months manure is applied
Cn, Cp, Cbod = N, P, or BOD nutrient concentration in agricultural area or pasture area in mg/L
Cn nm, Cp mn, Cbod mn = N, P, or BOD nutrient concentration in manured agricultural area or
manured pasture area in mg/L
50
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Note that nutrient concentrations are calculated based on input of number of agricultural animals in the
watershed. Animal Equivalent Counts (AEU; 1 AEU = 1000 lb/ac) are first calculated based on typical
animal mass (lb) and counts of animals. The calculated AEU for each watershed is then used to estimate
the nutrients in cropland and pasture runoff, based on literature values (for manured and non-manured
areas).
Forested and User Defined
The Forested and User-Defined land use annual runoff loading is calculated using the calculated annual
runoff and assumed nutrient concentration in runoff for forested land use.
WN= [V ¦ CN] X 4047 ¦ 0.3048/454
Equation 14
Wp= [V ¦ Cp] X 4047 ¦ 0.3048/454
Equation 15
WB0D = [V ¦ CB0D] X 4047 ¦ 0.3048/454
Equation 16
Where:
Wn, Wp, Wbod = annual N, P, and BOD loads from Forested or User-Defined land use (lbs)
V = the calculated annual runoff volume (ac-ft)
Cn, Cp, Cbod = N, P, or BOD nutrient concentration in Forested or User-Defined land use area
51
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4. Sediment Loading Calculations
Sediment loads (from Cropland, Pastureland, Forest, and User-defined land uses) are calculated based on
the Universal Soil Loss Equation (USLE) equation and sediment delivery ratio (DR). The USLE equation
to calculate the mean annual soil loss (E) is calculated as follows:
E = R ¦ K ¦ LS ¦ C ¦ P ¦ DA Equation 17
Where:
E = the computed annual soil loss (sheet and rill erosion) in tons/year
R = the rainfall factor
K = the soil erodibility factor
LS = the topographic factor which combines the slope length and gradient
C = the cropping management factor
P = the erosion control practice factor
Wsed = E ¦ DR Equation 18
Where:
DR = sediment delivery ratio,
Wsed = Sediment Load in tons/year
Sediment delivery ratio is calculated using:
DR = 0.42 ¦ i4"0125 if A < 200 acres Equation 19
DR = 0.417662 ¦ A"0'134958 - 0.127097 if A > 200 acres Equation 20
Where:
A = watershed area (mi2).
PLET calculates only sheet and rill erosion using USLE. Gully erosion and stream bank erosion are
calculated separately.
52
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5. Nutrient and Sediment Runoff Loads with BMP application
Pasture, Forested, and User-Defined
The nutrient, BOD, and sediment load calculations after BMP application for Pasture, Forested and User-
Defined land use are given below:
WNi = WN — WN ¦ e + SEDn X 2000
vN vvN
N
WP1 = Wp — Wp ¦ e + SEDp X 2000
Wbodi ~ WBod WBod ' e + SEDfton X 2000
SED = E -DR (1 - e) ¦ 2000
BOD
Equation 21
Equation 22
Equation 23
Equation 24
Where:
Wn, Wp, Wbod = annual N, P, or BOD loads (lbs)
SED = sediment load (lbs/year)
SEDn, SEDp, SEDbod, are the nutrient loading (tons/year) from the sediment
SEDn, SEDp, and SEDbod are calculated as follows:
SEDn = E ¦ DR (1 — e) ¦ %soil N cone ¦ 2/100
SEDp = E ¦ DR (1 — e) ¦ %soil P cone ¦ 2/100
SEDbod = E ¦ DR (1 — e) ¦ %soil BOD cone ¦ 2/100
Where:
Equation 25
Equation 26
Equation 27
e = BMP efficiency
Cropland
The nutrient, BOD, and sediment load calculations after BMP application for Cropland are as follows:
WNt = (WN — (Virr ¦ CN) x 4047 ¦ 0.3048/454) ¦ e + (Virr ¦ CN) x 4047 Equation 28
¦ 0.3048/454 + SEDN X 2000
Wpi = (WN — (Virr ¦ CP) x 4047 ¦ 0.3048/454) ¦ e + (Virr ¦ CP) x 4047 Equation 29
¦ 0.3048/454 + SEDP X 2000
WB0Dl = (WB0D — (Virr ¦ CB0D) x 4047 ¦ 0.3048/454) ¦ e + (Virr ¦ CB0D) x 4047 Equation 30
¦ 0.3048/454 + SEDbod X 2000
Where:
Cn, Cp, Cbod = N, P, or BOD nutrient concentration in agricultural area
53
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Urban
The nutrient, BOD, and sediment load calculations after BMP application for Urban land areas are given
below. A separate loading value is calculated for each of the Urban land use categories depending on
whether a BMP is applied on it or not. The resulting loading from each of the nine Urban land use
categories is then summed up to calculate the total Urban load.
WN1 = WN — WN ¦ e ¦ %Aeff Equation 31
Wpi = WP — WP ¦ e ¦ %Aeff Equation 32
Wbodi = Wbod — WBOd ' e ' %-^e// Equation 33
WTSS1 — 1^7^55 ^ Equation 34
Where:
Wni, Wpi, Wbodi, Wtssi = annual N, P, BOD, and TSS (sediment) loads from Urban land (Kg)
after application of BMP
Wn, Wp, Wbod, Wtss = annual N, P, BOD, and TSS (sediment) loads from Urban land (Kg)
e = BMP efficiency application to the Urban land use category
%Aeff = percentage of BMP effective area for each of the Urban land use categories.
9 Equation 35
Total N Load (lb/year) = 2000 x N WN1
i = 1
9 Equation 36
Total P Load (lb/year) = 2000 x Wpi
i = 1
JL, Equation 37
Total BOD Load (lb/year) = 2000 x WB0D1
Equation 33
i = 1
9
Total TSS Load (lb/year) = 2000 x ^
TSS 1
i = 1
Where:
i refers to each of individual Urban land use category
PLET also calculates flow volume reductions for selected Urban LID and infiltration BMP practices for
each of the Urban land use categories.
The approach involves calculation of BMP storage capacity and the runoff volume per event. The
computed BMP storage capacity is then compared with the runoff volume per event to determine the
captured volume per event for the BMP (based on the minimum of both the computed volumes.
54
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BMP storage capacity (ac — ft) = DA ¦ PI ¦ RD/12
Equation 39
Runoff volume per event (ac — ft) = DA ¦ PI ¦ P/12
Equation 40
Captured volume per event = minimum(BMP storage capacity, Runoff volume)
Equation 41
Captured volume per year (gal)
= ( Per_captured_Vol_per_Event/100
¦ DA_Runoff_volume) x 325850.58
Where:
Equation 42
DA = BMP drainage area (acre)
PI = Percent imperviousness within the drainage area, assuming 100% by default (%
RD = Impervious area runoff depth to be captured (in)
P = Rainfall (in) per event.
Percent Captured Volume per Event
= (BMP storage capacity/Runoff volume per event)xl00
Equation 43
Note that when the capture volume per event is equal to the runoff volume in Equation 41 then the
percent captured volume is 100 percent.
DA_Runoff_volume = LU_Runoff_volume * BMPDAAr / LUArea
Equation 44
In addition, PLET also provides an estimate of the required BMP surface area or the required number of
BMP units depending the type of BMP chosen.
Required BMP surface area (acres) =
BMP storage capacity(ac-ft)
Typical design BMP storage depth(ft)
Equation 45
Required BMP units =
BMP storage capacity(ac-ft) ¦ 325850.58 (gal/acft)
Typical design unit volume (e. g., rain barrel) (gal)
Equation 46
55
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6. Groundwater
Groundwater infiltration is estimated as a fraction of the precipitation. PLET uses reference soil
infiltration fractions for precipitation (P) for the various land uses based on hydrologic soil group (HSG)
to calculate the amount of infiltration to groundwater.
Infiltration (in) = Infiltration Fraction x P (in) Equation 47
The infiltrated volume or groundwater volume is then calculated as follows:
Infiltration (in) Equation 43
Infiltration Volume (ac-ft) = — x A (ac) x (Rdays ¦ RDcor)
Note that for urban areas, in order to calculate the amount infiltrated, the pervious areas are first
calculated. The pervious area is calculated based on the difference between the total urban area and
impervious area. The impervious area is calculated based on assumed percent imperviousness for the
various urban land use categories.
Pervious Urban Area(ac) Equation 49
= Total Urban Area — (Commercial ¦ 0.85 + Industrial ¦ 0.7
+ Institutional ¦ 0.5 + Transportation ¦ 0.95 + Multi — Family
¦ 0.75 + Single — Family * 0.3 + Urban — Cultivated ¦ 0.01
+ Vacant — developed ¦ 0.7 + Open Space ¦ 0.01)
Groundwater volumes from Feedlot areas are calculated using the calculated infiltration from Urban areas
and the Feedlot pervious area. The Feedlot pervious area is calculated based on the contribution from
Feedlot areas and a fraction based on Feedlot percent paved area as shown below:
Feedlot Pervious Area(ac) Equation 50
= Feedlot Area pervious x fraction based on feedlot % paved
Feedlot Percent Paved and associated fraction used in PLET:
Percent paved
Pervious Fraction
0-24%
0.875
25-49%
0.625
50-74%
0.375
75-100%
0.125
56
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7. Feedlot Calculations
Pollutant loads from feedlots in PLET are based on animal types, weight, and average rainfall. Runoff
volume from the feedlots are calculated based on contributing area in acres, feedlot percent paved, and
average event rainfall in inches.
V (ac- in) = Q (in) ¦ A(ac) Equation 51
Where:
A = contributing feedlot area (acres)
Q = surface runoff (inches) and is calculated as
The surface runoff (Q) calculations are based on the SCS runoff curve number method. Note that the CN
used in the runoff calculations is estimated based on the selected range of percent imperviousness in the
feedlot.
Nutrient contributions in cropland from animals are used to derive load estimates for feedlots. The
equivalent animal units (EAU) for N, P, and BOD are first calculated using the equation below for each
watershed.
EAU = No.x Factor Equation 52
Where:
No. = number of animals
Factor = Ratio of nutrients produced by animals relative to 1000 lb. of slaughter steer
The Animal Unit Density (AUD) and % manure pack using the following equation:
AUD = EAU / A Equation 53
If AUD < 100, percent manure pack = AUD;
If AUD > 100, percent of manure pack = 100%
Finally, the pollutant concentration of in the feedlot runoff is calculated using the following equation:
Cfeediot = Fraction of manure pack x Constant Equation 54
Where:
Cfeediot = runoff concentration from Feedlot (mg/L)
Fraction of manure pack = manure pack/100
Constant is pollutant specific and based on 100% manure pack. N constant = 1500 mg/L, P constant =
300 mg/L, and BOD constant = 2000 mg/L
The calculated runoff volume and concentration from Feedlots (Equation 51 and Equation 54) are then
used to calculate the Feedlot loading.
Wfeedi0t(lb/year) = V (ac- in) ¦ (Relays ¦ RDcor) ¦ Cfeedlot(mg/L) ¦ 0.227 Equation 55
57
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3. Septic Load
The septic load is calculated as the sum of the failing septic load and the direct wastewater loading.
/ lb \ Equation 56
Septic Load
Vyear/
/lb\ /lbv
= Failing Septic Load I —) + Direct Wastewater Load I —)
x 24 x 365
The Failing Septic Load and Direct Wastewater Load calculations are shown below.
Failing Septic Load
The failing septic load is calculated using the failing septic flow and an average concentration reaching
the stream from septic overcharge. The failing septic flow is calculated using the number of septic
systems (tanks), the failure rates (percentage), and the ratio of persons per septic system.
/lb\
Failing Septic Load I —)
Failing Septic Flow ^
/m§\
x Avg. concentration reaching stream from septic overcharge (—— 1
^ ]_j '
/453592
Equation 57
Where:
Failing Septic Flow (L/hr)
= Population of Failing Septic (persons)
x Typical Septic Overcharge Flow Rate (gal/day/person)
X (3.785412/24)
Equation 58
The population of failing septic is calculated as follows:
Population of Failing Septic (persons) Equation 59
= No. of Septic Systems x Population per Septic System
x Septic Failure Rate%
Typical Septic Overcharge Flow Rate in PLET is 70 gal/day/person (range of 45 to 100).
The average concentration reaching the stream from septic overcharge are determined based on ranges
observed in literature for Total Nitrogen, Total Phosphorous, and Organics (BOD) (as specified in the
PLET model).
58
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Direct Wastewater Load
Direct Wastewater Load
Equation 60
= [Direct Wastewater Flow J x Avg. co nee titration (-j—) ]
/453592
Direct wastewater flow is calculated based on per capita flow 75 gal/day/person (range of 75 to 125) and
the specified direct discharge population as:
Direct Wastewater Flow
= percapita flow (gal/day/person)
x direct discharge population(persons) x (3.785412/24)
Average concentrations reaching the stream from wastewater load as specified in PLET for Total
Nitrogen, Total Phosphorous, and Organics (BOD).
Equation 61
59
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9. Gully Erosion Load
The annual load due to Gully Erosion (GE) for each watershed is calculated as the sum of the all the
impaired Gully loading as follows:
Ji-, Equation 62
GE Sediment Load (lb/year) = 2000 x } [(TW + BW) ¦ D ¦ L ¦ Wt/T/2]
i = 1
GE Nitrogen Load (lb/year) Equation 63
n
= 2000 x % Soil N Cone x ^[(TW + BW) ¦ D ¦ L ¦ Wt ¦ NCF/T/2]
i=1
GE Phosphorous Load (lb/year) Equation 64
n
= 2000 x % Soil P Cone x ^[(TW + BW) ¦ D ¦ L ¦ Wt ¦ NCF/T/2]
i=1
GE BOD Load (lb/year) Equation 65
n
= 2000 x % Soil BOD Cone x ^[(TW + BW) ¦ D ¦ L ¦ Wt ¦ NCF/T/2]
i = 1
Where:
TW = top width (ft)
BW = bottom width (ft)
D = depth (ft)
L = length (ft)
Wt. = Soil Dry Weight (ton/ft3) - based on soil textural class
NCF = Nutrient correction factor - based on soil textural class
T = time (number of years) that the gully has taken to form the current size
PLET uses default 0.08, 0.031, and 0.160 % soil nitrogen, phosphorous and BOD values which can be
updated.
The gully erosion load reduction is calculated using a specified BMP efficiency due to gully stabilization
(0 to 1) as follows:
GullyErosionLoad Reduction = GullyErosionLoad x BMP Efficiency Equation 66
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10. Impaired Streambank Load
The annual load due to Streambank (SB) Erosion for each watershed is calculated as the sum of the all the
impaired stream bank loading as follows:
Ji-, Equation 67
SB Sediment Load (lb/year) = 2000 x yL ¦ H ¦ LRR ¦ Wt
i=1
Ji-, Equation 68
SB Nitrogen Load (lb/year) = 2000 x % Soil N Cone x } L ¦ H ¦ LRR ¦ Wt ¦ NCF
i=1
Equation 69
SB Phosphorous Load (lb/year) = 2000 x % Soil P Cone x yL ¦ H ¦ LRR ¦ Wt ¦ NCF
i = 1
Ji-, Equation 70
SB BOD Load (lb/year) = 2000 x % Soil BOD Cone x } L ¦ H ¦ LRR ¦ Wt ¦ NCF
i = 1
Where:
L = length (ft)
H = height (ft)
LRR = lateral recession rate (ft/yr) - based on categorization of LRR i.e. slight, moderate, severe
or very severe
Wt. = Soil Dry Weight (ton/ft3) - based on soil textural class
NCF = Nutrient correction factor - based on soil textural class
PLET uses default 0.08, 0.031, and 0.160 % soil nitrogen, phosphorous and BOD values.
The stream bank load reduction is calculated using a specified BMP efficiency due to stream bank
stabilization (0 to 1) as follows:
StreamBankLoad Reduction = StreamBankErosionLoad x BMP Efficiency Equation 71
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