Watershed  Characterization  System
(WCS)  Modeling  Extensions
The Watershed Characterization System (WCS) is an Ar-
cview-GIS based system that was originally developed to
facilitate the physical characterization of watersheds, evalu-
ate water quality conditions, and assess potential sources of
impairment. This was achieved by providing users the capa-
bility to automatically summarize information in tabular and
map formats in a Microsoft Word document. WCS now also
provides a platform for specialized modeling extensions sup-
porting TMDL development. These extensions provide new
watershed loading models such as the Sediment Budget Model,
and the  Mercury Loading  Model, and the development of
GIS-based interfaces to prepare data for existing models such
as Stormwater Management Model (SWMM) and Loading
Simulation Program in C+ + (LSPC, a TMDL-focused version
of HSPF). WCS and its extensions are currently available for
EPA Region 4.

Sediment Budget Model
The Sediment Budget Model is a WCS extension for estimat-
ing soil erosion and sediment loads from watersheds. EPA
Region 4 developed the model in cooperation with the U.S.
Department of Agriculture's (USDA) Natural Resources Con-
servation Service (NRCS) and Forest Service. The model uses
the Universal Soil Loss Equation (USLE) to estimate the total
potential erosion and its spatial distribution in the watershed.
The fraction of the potential erosion that reaches the stream
(e.g., sediment delivered) is estimated by using one of the four
sediment delivery equations. These equations express sediment
delivery as a function of the distance of the source grid cell
from the stream, and/or difference in elevation. The distance
is computed by tracing the most likely path of the sediment
from the source grid cell to a stream grid cell (e.g., grid cell to
grid cell routing) based on the steepest descent method.

The model can be used to evaluate the magnitude, extent
and spatial distribution of erosion and sediment load in the
watershed under historical, existing, and managed conditions.
An initial set  of data that may be used to represent existing
conditions is distributed with the model. By using Arc View's
standard and WCS customized tools, users can easily supple-
ment the initial data set with their own data sets that may be
more recent and of higher resolution. In addition, users are
provided with customized utilities allowing them to quickly
formulate and evaluate alternative management scenarios con-
sidering such things as land use changes; road and silvicultural
practices; riparian zone  characteristics; human disturbances
(e.g., construction areas); on-site BMPs such as cropping
practices; and off-site BMPs such as ponds and filter strips.
The model has been successfully used for developing sediment
TMDLs in the states of Georgia, Alabama and Tennessee.

Mercury Loading  Model
The WCS Mercury Loading Model is based on a soil-mercury
mass balance model,  IEM v2.05, developed by EPA's Office
of Health and Environmental Assessment and Environmental
Research Lab, Athens. The soil-mercury mass balance model
calculates surface  soil concentrations in  dissolved, sorbed,
and gas phases.

The model accounts for three routes of contaminant entry
into the soil:
    Deposition of particle-bound contaminant through dry
    fall
    Deposition through wet fall
    Diffusion of gas phase  contaminant into the soil sur-
    face.
                      DATA
      watershed boundary, land uses, streams,
             elevation, soils and roads
                Erosion calculation
                   using USLE
       Distances,
        slopes,
        streams
Roads, BMPs,
management
  structures
                Sediment transport
                   and delivery
                    OUTPUTS
       Erosion map
       Sediment delivery map
       Sediment delivery to assessment points
       Automated reports (maps and tables)
                                                                               United States
                                                                               Environmental Protection
                                                                               Agency

-------
The model also accounts for four dissipation pro-
cesses that remove mercury from the surface soils:
    Volatilization (movement of gas phase out of
    the soil surface)
    Runoff of dissolved phase from the soil surface
    Leaching of dissolved phase through the soil
    horizon
    Erosion of particulate phase from the soil sur-
    face.

The watershed mercury load is calculated as the
total of mercury contributed by sediment, runoff
from both pervious and impervious surfaces, direct
deposition on  water surfaces, and point sources.
The algorithm  for estimating erosion and sediment
transport is based on the WCS Sediment Budget
Model. Runoff is calculated for each land use grid
using the USDA-NRCS curve number method that
requires rainfall as an input and calculates other hydrologic
components such as evapotranspiration and infiltration.

The WCS Mercury Loading Model was used to develop mer-
cury TMDLs for the Middle and Lower Savannah River.

WCS SWMM  Extension
EPA's Storm Water Management Model (SWMM) is a com-
prehensive computer model for the analysis of water quantity
and quality problems associated with urban runoff. SWMM
can simulate all aspects of the urban hydrologic and water qual-
ity cycles, including rainfall, snowmelt, surface and subsurface
runoff,  flow routing through drainage networks, storage, and
treatment. The WCS SWMM extension is designed to simplify
and facilitate the creation of SWMM input files, especially for
PC SWMM (Computational Hydraulics Int.). It consists of two
coupled parts: a GIS interface and a SWMM input processor.
The GIS interface processes spatial databases and prepares two
important data sets, land use distribution and stream network
connectivity, for the SWMM input processor. The SWMM
input processor has a Visual Basic interface that reads GIS
output and user-specified simulation information to generate
the required input files for the SWMM model.

WCS LSPC  Extension
The WCS LSPC extension preprocesses GIS, spatial data,
and time series data for input  to the Loading Simulation
Program in C+ + (LSPC). The LSPC system is a stand-alone
watershed modeling system that includes algorithms from the
Hydrologic Simulation Program - FORTRAN (HSPF) plus
other key features to support large-scale watershed modeling
and TMDL calculation. LSPC is used for simulating watershed
pollutant loadings as well as flow and water quality routing
through stream reaches.
Hg
 -i  ) Diffusion
     Volatilization
               Dry Fall
                               Wet Fall
                    Leaching
     The WCS LSPC extension provides many key data processing
     benefits. After delineating the watershed, the LSPC extension
     (1) packages GIS layers for the LSPC GIS interface, (2) com-
     piles landuse and reach geometry/network information by de-
     lineated subwatershed, (3) locates and compiles USGS gage
     data and STORET water quality data for model calibration,
     (4) locates weather stations, (5) locates and compiles point
     source  information from the  Permit Compliance System
     (PCS),  (6) queries the STATSGO database to identify the
     predominant hydrologic soil type in each delineated subwa-
     tershed, and  (7) assigns initial hydrologic model parameters
     based on the STATSGO query results. WCS LSPC extension
     has been used to develop TMDLs in Alabama, Tennessee, and
     South Carolina.
                          Visit the
         Watershed & Water Quality Modeling
            Technical Support Center Website
     http://www.epa.gov/athens/wwqtsc/index.html
                                                                                     United States
                                                                                     Environmental Protection
                                                                                     Agency

-------