SEPA
United States
Environmental Protection
Agency
Technical Memorandum #1
Adjusting for Depreciation of
Land Treatment When Planning
Watershed Projects
Introduction
Watershed-based planning helps address water quality
problems in a holistic manner by fully assessing the
potential contributing causes and sources of pollution,
then prioritizing restoration and protection strategies
to address the problems (USEPA 2013). The U.S. Environ-
mental Protection Agency (EPA) requires that watershed
projects funded directly under section 319 of the Clean
Water Act implement a watershed-based plan (WBP)
addressing the nine key elements identified in EPA's Hand-
book for Developing Watershed Plans to Restore and Protect
ThisTechnical Memorandum is one of a series of
publications designed to assist watershed projects,
particularly those addressing nonpoint sources of
pollution. Many of the lessons learned from the
Clean Water Act Section 319 National Nonpoint
Source Monitoring Program are incorporated in these
publications.
October 2015
Donald W. Meals and Steven A. Dressing. 2015. Technical
Memorandum #1: Adjusting for Depreciation of Land
Treatment When Planning Watershed Projects, October 2015.
Developed for U.S. Environmental Protection Agency by Tetra
Tech, Inc., Fairfax, VA, 16 p. Available online at https://www.
epa.gov/pollu ted-runoff-nonpoin t-source-pollution/wa tershed-
approach-technical-resources.
our Waters (USEPA 2008), EPA further recommends that all
other watershed plans intended to address water quality
impairments also include the nine elements. The first	Relds near Seneca Lake'New York-
element calls for the identification of causes and sources
of impairment that must be controlled to achieve needed
ioad reductions. Related elements include a description of the nonpoint source (NPS) management
measures—or best management practices (BMPs)—needed to achieve required pollutant load
reductions, a description of the critical areas in which the BMPs should be implemented, and an
estimate of the load reductions expected from the BMPs.
Once the causes and sources of water resource impairment are assessed, identifying the appropriate
BMPs to address the identified problems, the best locations for additional BMPs, and the pollutant
ioad reductions likely to be achieved with the BMPs depends on accurate information on the perfor-
mance levels of both BMPs already in place and BMPs to be implemented as part of the watershed
project. All too often, watershed managers and Agency staff have assumed that, once certified as
installed or adopted according to specifications, a BMP continues to perform its pollutant reduction
function at the same efficiency (percent pollutant reduction) throughout its design or contract life,
sometimes longer. An important corollary to this assumption is that BMPs in place during project
planning are performing as originally intended. Experience in NPS watershed projects across the
nation, however, shows that, without diligent operation and maintenance, BMPs and their effects
probably will depreciate overtime, resulting in less efficient pollution reduction. Recognition of this
fact is important at the project planning phase, for both existing and planned BMPs.
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Technical Memorandum #1 | Adjusting for Depreciation of Land Treatment When Planning Watershed Projects
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Knowledge of land treatment depreciation is important to ensure project success through the adap-
tive management process (USEPA 2008). BMPs credited during the planning phase of a watershed
project will be expected to achieve specific load reductions or other water quality benefits as part
of the overall plan to protect or restore a water body. Verification that BMPs are still performing their
functions at anticipated levels is essential to keeping a project on track to achieve its overall goals.
Through adaptive management, verification results can be used to inform decisions about needs
for additional BMPs or maintenance or repair of existing BMPs. In a watershed project that includes
short-term (3-5 years) monitoring, subtle changes in BMP performance level might not be detect-
able or critical, but planners must account for catastrophic failures, BMP
removal or discontinuation, and major maintenance shortcomings. Over
the longer term, however, gradual changes in BMP performance level can
be significant in terms of BMP-specific pollutant control or the role of single
BMPs within a BMP system or train. The weakest link in a BMP train can be
the driving force in overall BMP performance.
This technical memorandum addresses the major causes of land treatment depreciation, ways to
assess the extent of depreciation, and options for adjusting for depreciation. While depreciation
occurs throughout the life of a watershed project, the emphasis is on the planning phase and the
short term (i.e., 3-5 years).
Causes of Depreciation
Depreciation of land treatment function occurs as a result of many factors and processes.
Three of the primary causes are natural variability, lack of proper maintenance, and unforeseen
consequences.
Natural Variability
Climate and soil variations across the nation influence how BMPs perform. Tiessen et al. (2010), for
example, reported that management practices designed to improve water quality by reducing
sediment and sediment-bound nutrient export from agricultural fields can be less effective in cold,
dry regions where nutrient export is primarily snowmelt driven and in the dissolved form, compared
to similar practices in warm, humid regions. Performance levels of vegetation-based BMPs in both
agricultural and urban settings can vary significantly through the year due to seasonal dormancy.
In a single locale, year-to-year variation in precipitation affects both agricultural management and
BMP performance levels. Drought, for example, can suppress crop yields, reduce nutrient uptake, and
result in nutrient surpluses left in the soil after harvest where they are vulnerable to runoff or leaching
loss despite careful nutrient management. Increasing incidence of extreme weather and intense
storms can overwhelm otherwise well-designed stormwater management facilities in urban areas.
Lack of Proper Maintenance
Most BMPs—both structural and management—must be operated and maintained properly to
continue to function as designed. Otherwise, treatment effectiveness can depreciate over time. For
example, in a properly functioning detention pond, sediment typically accumulates in the forebay.
Without proper maintenance to remove accumulated sediment, the capacity of the BMP to contain
Application of and methods for
BMP tracking in NPS watershed
projects are described in detail in
Tech Notes 77 (Meals et al. 2014).
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and treat stormwater is diminished. Similarly, a nutrient management plan is only as effective as its
implementation. Failure to adhere to phosphorus (P) application limits, for example, can result in soil P
buildup and increased surface and subsurface losses of P rather than the loss reductions anticipated.
Jackson-Smith et al. (2010) reported that over 20 percent of implemented BMPs in a Utah watershed
project appeared to be no longer maintained or in use when evaluated just 5 years after project
completion. BMPs related to crop production enterprises and irrigation systems had the lowest rate
of continued use and maintenance (-75 percent of implemented BMPs were still in use), followed by
pasture and grazing planting and management BMPs (81 percent of implemented BMPs were still in
use). Management practices (e.g., nutrient management) were found to be particularly susceptible
to failure.
Practices are sometimes simply abandoned as a result of changes in
landowner circumstances or attitudes. In a Kansas watershed project,
farmers abandoned a nutrient management program because of
perceived restrictive reporting requirements (Osmond et al. 2012).
In the urban arena, a study of more than 250 stormwater facilities in
Maryland found that nearly one-third of stormwater BMPs were not
functioning as designed and that most needed maintenance (Lindsey et
al. 1992). Sedimentation was a major problem and had occurred at nearly
half of the facilities; those problems could have been prevented with
timely maintenance.
Hunt and Lord (2006) describe basic maintenance requirements for bioretention practices and the
consequences of failing to perform those tasks. For example, they indicate that mulch should be
removed every 1-2 years to both maintain available water storage volume and increase the surface
infiltration rate of fill soil. In addition, biological films might need to be removed every 2-3 years
because they can cause the bioretention cell to clog.
In plot studies, Diilaha et al. (1986) observed that vegetative filter strip-effectiveness for sediment
removal appeared to decrease with time as sediment accumulated within the filter strips. One set
of the filters was almost totally inundated with sediment during the cropland experiments and
filter effectiveness dropped 30-60 percent between the first and second experiments. Dosskey
et al. (2002) reported that up to 99 percent of sediment was removed from cropland runoff when
uniformly distributed over a buffer area, but as concentrated flow paths developed over time (due
to lack of maintenance), sediment removal dropped to 15-45 percent. In the end, most structural
BMPs have a design life (i.e., the length of time the item is expected to work within its specified
parameters). This period is measured from when the BMP is placed into service until the end of its
full pollutant reduction function.
Unforeseen Consequences
The effects of a BMP can change directly or indirectly due to unexpected interactions with site
conditions or other activities. Incorporating manure into cropland soils to reduce nutrient runoff,
for example, can increase erosion and soil loss due to soil disturbance, especially in comparison
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to reduced tillage. On the other hand, conservation tillage can result in accumulation of fertilizer
nutrients at the soil surface, increasing their availability for loss in runoff (Rhoton et al. 1993). Long-
term reduction in tillage also can promote the formation of soil macropores, enhancing leaching
of soluble nutrients and agrichemicals into ground water (Shipitalo et al. 2000). Stutter et al. (2009)
reported that establishment of vegetated buffers between cropland and a watercourse led to
enhanced rates of soil P cycling within the buffer, increasing soil P solubility and the potential for
leaching to watercourses.
Despite widespread adoption of conservation tillage and observed reductions in particulate P loads,
a marked increase in loads of dissolved bioavailable P in agricultural tributaries to Lake Erie has been
documented since the mid-1990s. This shift has been attributed to changes in application rates,
methods, and timing of P fertilizers on cropland in conservation tillage not subject to annual tillage
(Baker 2010; Joosse and Baker 2011). Further complicating matters, recent research on fields in the
St. Joseph River watershed in northeast Indiana has demonstrated that about half of both soluble P
and total P losses from research fields occurred via tile discharge, indicating a need to address both
surface and subsurface loads to reach the goal of 41 percent reduction in P loading for the Lake Erie
Basin (Smith et al. 2015).
Several important project planning lessons were learned from the White Clay Lake, Wisconsin,
demonstration projects in the 1970s, including the need to accurately assess pollutant inputs and
the performance levels of BMPs (NRC 1999). Regarding unforeseen consequences, the project
learned through monitoring that a manure storage pit built according to prevailing specifications
actually caused ground water contamination that threatened a farmer's well water. This illustrates
the importance of monitoring implemented practices over time to ensure that they function prop-
erly and provide the intended benefits.
Control of urban stormwater runoff (e.g., through detention) has been widely implemented to
reduce peak flows from large storms in order to prevent stream channel erosion. Research has
shown, however, that although large peak flows might be controlled effectively by detention
storage, stormflow conditions are extended over a longer period of time. Duration of erosive and
bankfull flows are increased, constituting channel-forming events. Urbonas and Wulliman (2007)
reported that, when captured runoff from a number of individual detention basins in a stream
system is released over time, the flows accumulate as they travel downstream, actually increasing
peak flows along the receiving waters. This situation can diminish the collective effectiveness of
detention basins as a watershed management strategy.
Assessment of Depreciation
The first—and possibly most important—step in adjusting for depreciation of implemented BMPs is
to determine its extent and magnitude through BMP verification.
BMP Verification
At its core, BMP verification confirms that a BMP is in place and functioning properly as expected
based on contract, permit, or other implementation evidence. A BMP verification process that docu-
ments the presence and function of BMPs over time should be included in all NPS watershed projects.
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At the project planning phase, verification is important both to ensure accurate assessment of
existing BMP performance levels and to determine additional BMP and maintenance needs. Verifica-
tion over time is necessary to determine if BMPs are maintained and operated during the period of
interest.
Documenting the presence of a BMP is generally simpler than determining how well it functions,
but both elements of verification must be considered to determine if land treatment goals are
being met and whether BMP performance is depreciating. Although land treatment goals might
not be highly specific in many watershed projects, it is important to document what treatment is
implemented. Verification is described in detail in Tech Notes 11 (Meals et al. 2014). This technical
memorandum focuses on specific approaches to assessing depreciation within the context of an
overall verification process.
Methods for Assessing BMP Presence and Performance Level
Whether a complete enumeration or a statistical sampling approach is used, methods for tracking
BMPs generally include direct measurements (e.g., soil tests, onsite inspections, remote sensing) and
indirect methods (e.g., landowner self-reporting or third-party surveys). Several of these methods
are discussed in Tech Notes 11 (Meals et al. 2014). Two general factors must be considered when veri-
fying a BMP: the presence of the BMP and its pollutant removal efficiency. Different types of BMPs
require different verification methods, and no single approach is likely to provide all the information
needed in planning a watershed project.
Certification
The first step in the process is to determine whether BMPs have been designed and installed/
adopted according to appropriate standards and specifications. Certification can either be the
final step in a contract between a landowner and a funding agency or be a component of a permit
requirement.
Certification provides assurance that a BMP is fully functional for its setting at a particular time. For
example, a stormwater detention pond or water and sediment control basin must be properly sized
for its contributing area and designed for a specific retention-and-release performance level. A
nutrient management plan must account for all sources of nutrients, consider current soil nutrient
levels, and support a reasonable yield goal. A cover crop must be planted in a particular time
window to provide erosion control and/or nutrient uptake during a critical time of year. Some juris-
dictions might apply different nutrient reduction efficiency credits for cover crops based on planting
date. Some structural BMPs like parallel tile outlet terraces require up to 2 years to fully settle and
achieve full efficiency; in those cases, certification is delayed until full stability is reached. Knowledge
that a BMP has been applied according to a specific standard supports an assumption that the BMP
will perform at a certain level of pollutant reduction efficiency, providing a baseline against which
future depreciation can be compared. Practices voluntarily implemented by landowners without
any technical or financial assistance could require special efforts to determine compliance with
applicable specifications (or functional equivalence). Pollution reduction by practices not meeting
specifications might need to be discounted or not counted at all even when first installed.
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Depreciation assessment indicators
Ideally, assessment of BMP depreciation would be based on actual measurement of each BMP's
performance level (e.g., monitoring of input and output pollutant loads for each practice). Except in
very rare circumstances, this type of monitoring is impractical. Rather, a watershed project generally
must depend on the use of indicators to assess BMP performance level.
The most useful indicators for assessing depreciation are determined primarily by the type of BMP
and pollutants controlled, but indicators might be limited by the general verification approach used.
For example, inflow and outflow measurements of pollutant load can be used to determine the
effectiveness of constructed wetlands, but a verification effort that uses only visual observations
will not provide that data or other information about wetland functionality. A central challenge,
therefore, is to identify meaningful indicators of BMP performance level that can be tracked under
different verification schemes. This technical memorandum provides examples of how to accom-
plish that end.
Nonvegetative structural practices
Performance levels of nonvegetative structural practices—such as animal waste lagoons, digesters,
terraces, irrigation tailwater management, stormwater detention ponds, and pervious pavement—
can be assessed using the following types of indicators:
•	Measured on-site performance data (e.g., infiltration capacity of pervious pavement),
•	Structural integrity (e.g., condition of berms or other containment structures), and
•	Water volume capacity (e.g., existing pond volume vs. design) and mass or volume of
captured material removed (e.g., sediment removed from stormwater pond forebay at
cleanout).
In some cases, useful indicators can be identified directly from practice standards. For example, the
Natural Resources Conservation Service lists operation and maintenance elements for a water and
sediment control basin (WASCoB) (USDA-NRCS 2008) that include:
•	Maintenance of basin ridge height and outlet elevations,
•	Removal of sediment that has accumulated in the basin to maintain capacity and grade,
•	Removal of sediment around inlets to ensure that the inlet remains the lowest spot in the
basin, and
•	Regular mowing and control of trees and brush.
These elements suggest that ridge and outlet elevations, sediment accumulation, inlet integrity, and
vegetation control would be important indicators of WASCoB performance level.
Required maintenance checklists contained in stormwater permits also can suggest useful indi-
cators. For example, the Virginia Stormwater Management Handbook (VA DCR 1999) provides an
extensive checklist for annual operation and maintenance inspection of wet ponds. The list includes
many elements that could serve as BMP performance level indicators:
•	Excessive sediment, debris, or trash accumulated at inlet,
•	Clogging of outlet structures,
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•	Cracking, erosion, or animal burrows in berms, and
•	More than 1 foot of sediment accumulated in permanent pool.
Assessment of these and other indicators would require on-site inspection and/or measurement by
landowners, permit-holders, or oversight agencies.
Vegetative structural practices
Performance levels of vegetative structural
practices—such as constructed wetlands, swales,
rain gardens, riparian buffers, and filter strips—can
be assessed using the following types of indicators;
•	Extent and health of vegetation (e.g.,
measurements of soil cover or plant density),
•	Quality of overland flow filtering (e.g.,
evidence of short-circuiting by concentrated
flow or gullies through buffers or filter strips),
•	On-site capacity testing of rain gardens
using inliltrometers or similar devices, and
•	Visual observations (e.g., presence of water
in swales and rain gardens).
Parking lot rain garden.
As for non-vegetative structural practices, assessment of these indicators would require on-site
inspection and/or measurement by landowners, permit-holders, or oversight agencies.
Nonstructural vegetative practices
Performance levels of nonstructural vegetative practices—such as cover crops, reforestation
of logged tracts, and construction site seeding—can be assessed using the following types of
indicators:
•	Density of cover crop planting (e.g., plant count),
•	Percent of area covered by cover crop, and
•	Extent and vitality of tree seedlings.
These indicators could be assessed by on-site inspection or, in some cases, by remote sensing, either
from satellite imagery or aerial photography.
Management practices
Performance levels of management practices—such as nutrient management, conservation
tillage, pesticide management, and street sweeping—can be assessed using the following types of
indicators:
•	Records of street sweeping frequency and mass of material collected,
•	Area or percent of cropland under conservation tillage,
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•	Extent of crop residue coverage on conservation tillage cropland, and
•	Fertilizer and/or manure application rates and schedules, crop yields, soil test data, plant
tissue test results, and fall residual nitrate tests.
Assessment of these indicators would generally
require reporting by private landowners or munic-
ipalities, reporting that is required under some
regulatory programs. Visual observation of indi-
cators such as residue cover, however, can also be
made by on-site inspection or windshield survey.
Data analysis
Data on indicators can be expressed and analyzed
in several ways, depending on the nature of the
indicators used, indicators reporting continuous
numerical data—such as acres of cover crop or
conservation tillage, manure application rates, miles
of street sweeping, mass of material removed from
catch basins or detention ponds, or acres of logging roads/landings revegetated—can be expressed
either in the raw form (e.g., acres with 30 percent or more residue cover) or as a percentage of the
design or target quantity (e.g., percent of contracted acres achieving 30 percent or more of residue
cover). These metrics can be tracked year to year as a measure of BMP depreciation (or achievement).
During the planning phase of a watershed project, it might be appropriate to collect indicator data
for multiple years prior to project startup to enable calculation of averages or ranges to better esti-
mate BMP performance levels over crop rotation cycles or variable weather conditions.
Indicators reporting categorical data—such as maintenance of detention basin ridge height and
outlet elevations, condition of berms or terraces, or observations of water accumulation and flow—
are more difficult to express quantitatively. It might be necessary to establish an ordinal scale (e.g.,
condition rated on a scale of 1-10) or a binary yes/no condition, then use best professional judgment
to assess influence on BMP performance
In some cases, it might be possible to use modeling or other quantitative analysis to estimate
individual or watershed-ievei BMP performance levels based on verification data. In an analysis
ofstormwater BMP performance levels, Tetra Tech (2010) presented a series of BMP performance
curves based on monitoring and modeling data that relate pollutant removal efficiency to depth of
runoff treated (Figure 1). Where depreciation indicators track changes in depth of runoff treated as
the capacity of a BMP decreases (e.g., from sedimentation), resulting changes in pollutant removal
could be determined from a performance curve. This type of information can be particularly useful
during the planning phase of a watershed project to estimate realistic performance levels for
existing BMPs that have been in place for a substantial portion of their expected lifespans.
The performance levels of structural agricul tural BMPs in varying condition can be estimated by
altering input parameters in the Soil and Water Assessment Tool (SWAT) model (Texas A&M University
2015a); other models such as the Agricultural Policy/Environmental extender (APEX) model (Texas A&M
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Technical Memorandum #1 | Adjusting for Depreciation of Land Treatment When Planning Watershed Projects
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University 2015b) also can be used in this way
(including application to some urban BMPs).
For urban stormwater, engineering models like
HvdroCAD (HydroCAD Software Solutions 2011)
can be used to simulate hydrologic response
to stormwater BMPs with different physical
characteristics (e.g., to compare performance
levels under actual vs. design conditions).
Even simple spreadsheet models such as the
Spreadsheet Tool for Estimating Pollutant Load
(STEPL) (USEPA 2015) can be used to quantify
the effects of BMP depreciation by varying the
effectiveness coefficients in the model.
Data from verification efforts and analysis of the
effects of depreciation on BMP performance
levels must be qualified based on data confi-
dence. "Confidence" refers mainly to a quantitative assessment of the accuracy of a verification result.
For example, the number of acres of cover crops or the continuity of streamside buffers on logging
sites determined from aerial photography could be determined by ground-truthing to be within +10
percent of the true value at the 95 percent confidence level. Confidence also can refer to the level
of trust that BMPs previously implemented continue to function (e.g., the proportion of BMPs still in
place and meeting performance standards). For example, reporting that 75 percent of planned BMPs
have been verified is a measure of confidence that the desired level of treatment has been applied.
While specific methods to evaluate data confidence are beyond the scope of this memo, it is
essential to be able to express some degree of confidence in verification results—both during the
planning phase and over time as the project is implemented. For example, an assessment of relative
uncertainty of BMP performance during the planning phase can be used as direct follow-up to veri-
fication efforts to those practices for which greater quantification of performance level is needed.
In addition, plans to implement new BMPs also can be developed with full consideration of the
reliability of BMPs already in place.
Adjusting for Depreciation
Information on BMP depreciation can be used to improve both project management and project
evaluation.
Project Planning and Management
Establishing baseline conditions
Baseline conditions of pollutant loading include not only pollutant source activity but also the
influence of BMPs already in place at the start of the project. Adjustments based on knowledge of
BMP depreciation can provide a more realistic estimate of baseline pollutant loads than assuming
that existing land treatment has reduced NPS pollutant loads by some standard efficiency value.
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
BMP Performance Curve: Bioretention
Land Use: Low Density Residential
1 J
		1
^ Jf ___—-— ~
—t	
f		
1

A

x

/

A


	
-TSS -
0.6 0.8 1 1.2 1.4 1.6
Depth of Runoff Treated
Figure 1. BMP Performance Curve for Bioretention BMP
(Tetra Tech 2010).
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Technical Memorandum #1 | Adjusting for Depreciation of Land Treatment When Planning Watershed Projects
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Establishing an accurate starting point will make load reduction targets—and, therefore, land
treatment design—more accurate. Selecting appropriate BMPs, identifying critical source areas, and
prioritizing land treatment sites will all benefit from an accurate assessment of baseline conditions.
Knowledge of depreciation of existing BMPs can be factored into models used for project planning
(e.g., by adjusting pollutant removal efficiencies), resulting in improved understanding of overall
baseline NPS loads and their sources.
While not a depreciation issue per se, when a BMP is first installed—especially a vegetative BMP
like a buffer or filter strip—it usually takes a certain amount of time before its pollutant reduction
capacity is fully realized. For example, Dosskey et al. (2007) reported that the nutrient reduction
performance of newly established vegetated filter strips increased over the first 3 years as dense
stands of vegetation grew in and soil infiltration improved; thereafter, performance level was stable
over a decade. When planning a watershed project, vegetative practices should be examined to
determine the proper level of effectiveness to assume based on growth stage. Also, because of
weather or management conditions, some practices (e.g., trees) might take longer to reach their
full effectiveness or might never reach it. The Stroud Preserve, Pennsylvania, section 319 National
Nonpoint Source Monitoring Program (NNPSMP) project (1992-2007) found that slow tree growth in
a newly established riparian forest buffer delayed significant N03-N (nitrate) removal from ground
water until about 10 years after the trees were planted (Newbold et al. 2008).
The performance of practices can change in multiple ways over time. For example, excessive depo-
sition in a detention pond that is not properly maintained could reduce overall percent removal of
sediment because of reduced capacity as illustrated in Figure I.The relative and absolute removal
efficiencies for various particle size fractions (and associated pollutants) also can change due to
reduced hydraulic retention time. Fine particles generally require longer settling times than larger
particles, so removal efficiency of fine particles (e.g., silt, clay) can be disproportionally reduced as
a detention pond or similar BMP fills with sediment and retention time deteriorates. Expert assess-
ment of the condition and likely current performance level of existing BMPs, particularly those for
which a significant amount of pollutant removal is assumed, is essential to establishing an accurate
baseline for project planning.
Adaptive watershed management
Watershed planning and management is an iterative process; project goals might not all be fully
met during the first project cycle and management efforts usually need to be adjusted in light of
ongoing changes. In many cases, several cycles—including mid-course corrections—might be
needed for a project to achieve its goals. Consequently, EPA recommends that watershed projects
pursue a dynamic and adaptive approach so that implementation of a watershed plan can proceed
and be modified as new information becomes available (USEPA 2008). Measures of BMP implemen-
tation commonly used as part of progress assessment should be augmented with indicators of
BMP depreciation. Combining this information with other relevant project data can provide reliable
progress assessments that will indicate gaps and weaknesses that need to be addressed to achieve
project goals.
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BMP design and delivery system
Patterns in BMP depreciation might yield information on systematic failures in BMP design or
management that can be addressed through changes to standards and specifications, contract
terms, or permit requirements. This information could be particularly helpful during the project
planning phase when both the BMPs and their implementation mechanisms are being considered.
For example, a cost-sharing schedule that has traditionally provided all or most funding upon initial
installation of a BMP could be adjusted to distribute a portion of the funds over time if operation
and maintenance are determined to be a significant issue based on pre-project information. Some
BMP components, on the other hand, might need to be dropped or changed to make them more
appealing to or easier to manage by landowners. Within the context of a permit program, for
example, corrective actions reports might indicate specific changes that should be made to BMPs to
ensure their proper performance.
Project Evaluation
Monitoring
Although short-term (3-5 year) NPS watershed projects will not usually have a sufficiently long
data record to evaluate incremental project effects, data on BMP depreciation might still improve
interpretation of collected water quality data. Even in the short term, water quality monitoring data
might reflect cases in which BMPs have suffered catastrophic failures (e.g., an animal waste lagoon
breach), been abandoned, or been maintained poorly. Meals (2001), for example, was able to interpret
unexpected spikes in stream P and suspended sediment concentrations by walking the watershed
and discovering that a landowner had over-applied manure and plowed soil directly into the stream.
Longer-term efforts (e.g., total maximum daily loads1) might engage in sustained monitoring
beyond individual watershed project lifetime(s).The extended monitoring period will generally
allow detection of more subtle water quality impacts for which interpretation could be enhanced
with information on BMP depreciation. While not designed as BMP depreciation studies, the
following two examples illustrate how changes in BMP performance can be related to water quality.
In a New York dairy watershed treated with multiple BMPs, Lewis and Makarewicz (2009) reported
that the suspension of a ban on winter manure application 3 years into the monitoring study led to
dramatic increases in stream nitrogen and phosphorus concentrations. First and foremost, knowl-
edge of that suspension provided a reasonable explanation for the observed increase in nutrient
levels. Secondly, the study was able to use data from the documented depreciation of land treat-
ment to determine that the winter spreading ban had yielded 60-75 percent reductions in average
stream nutrient concentrations.
The Walnut Creek, Iowa, Section 319 NNPSMP project promoted conversion of row crop land to
native prairie to reduce stream N03-N levels and used simple linear regression to show association
of two monitored variables: tracked conversion of row crop land to restored prairie vegetation
and stream N03-N concentrations (Schilling and Spooner 2006). Because some of the restored
prairie was plowed back into cropland during the project period—and because that change was
1 "Total maximum daily loads" as defined in §303(d) of the Clean Water Act.
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documented—the project was able to show
not only that converting crop land to prairie
reduced stream N03-N concentrations but also
that increasing row crop land led to increased
N03-N levels (Figure 2).
Modeling
When watershed management projects are
guided or supported by modeling, knowledge
of BMP depreciation should be part of model
inputs and parameterization.
The magnitude of implementation (e.g., acres
of treatment) and the spatial distribution of
both annual and structural BMPs should be
part of model input and should not be static
parameters. Where BMPs are represented by
pollutant reduction efficiencies, those percentages can be adjusted based on verification of land
treatment performance levels in the watershed. Incorporating BMP depreciation factors into models
might require setting up a tiered approach for BMP efficiencies (e.g., different efficiency values
for BMPs determined to be in fair, good, or excellent condition) rather than the currently common
practice of setting a single efficiency value for a practice assumed to exist. This approach could be
particularly important for management practices such as agricultural nutrient management or street
sweeping, in which degree of treatment is highly variable. For structural practices, a depreciation
schedule could be incorporated into the project, similar to depreciating business assets. In the
planning phase of a watershed project, multiple scenarios could be modeled to reflect the potential
range of performance levels for BMPs already in place.
Recommendations
The importance of having accurate information on BMP depreciation varies across projects and
during the timeline of a single project. During the project planning phase, when plans for the
achievement of pollutant reduction targets rely heavily on existing BMPs, it is essential to obtain
good information on the level of performance of the BMPs to ensure that plan development is prop-
erly informed. If existing BMPs are a trivial part of the overall watershed plan, knowledge of BMP
depreciation might not be critical during planning. As projects move forward, however, the types
of BMPs implemented, their relative costs and contributions to achievement of project pollutant
reduction goals, and the likelihood that BMP depreciation will occur during the period of interest
will largely determine the type and extent of BMP verification required over time. The following
recommendations should be considered within this context:
• For improved characterization of overall baseline NPS loads, better identification of critical
source areas, and more effective prioritization of new land treatment during project
planning, collect accurate and complete information about:
O Land use,
•s 12 -
y = 0.195x+ 1.57
r2 = 0.70
	1	1	1	1	1	1	1	
40 -30 -20 -10 0 10 20 30 40
Change in Row Crop Land Cover
in Watershed Area (%), 1990 to 2005
Figure 2. Relating Changes in Stream Nitrate Concentrations to
Changes in Row Crop Land Cover in Walnut Creek, Iowa
(Schilling and Spooner 2006)).
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Technical Memorandum #1 | Adjusting for Depreciation of Land Treatment When Planning Watershed Projects
October 2015
O Land management, and
O The implementation and operation of existing BMPs. This information should include:
•	Original BMP installation dates,
•	Design specifications of individual BMPs,
•	Data on BMP performance levels if available, and
•	The spatial distribution of BMPs across the watershed.
•	Track the factors that influence BMP depreciation in the watershed, including:
O Variations in weather that influence BMP performance levels,
O Changes in land use, land ownership, and land management,
O Inspection and enforcement activities on permitted practices, and
O Operation, maintenance, and management of implemented practices.
•	Develop and use observable indicators of BMP status/performance that:
O Are tailored to the set of BMPs implemented in the watershed and practical within the
scope of the watershed project's resources,
O Can be quantified or scaled to document the extent and magnitude of treatment
depreciation, and
O Are able to be paired with water quality monitoring data.
•	After the implementation phase of the NPS project, conduct verification activities to
document the continued existence and function of implemented practices to assess the
magnitude of depreciation and provide a basis for corrective action. The verification program
should:
O Identify and locate all BMPs of interest, including cost-shared, non-cost-shared, required,
and voluntary practices;
O Capture information on structural, annual, and management BMPs;
O Obtain data on BMP operation and maintenance activities; and
O Include assessment of data accuracy and confidence.
•	To adjust for depreciation of land treatment, apply verification data to watershed project
management and evaluation by:
O Applying results directly to permit compliance programs,
O Relating documented changes in land treatment performance levels to observed water
quality,
O Incorporating measures of depreciated BMP effectiveness into modeling efforts, and
O Using knowledge of treatment depreciation to correct problems and target additional
practices as necessary to meet project goals in an adaptive watershed management
approach.
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Technical Memorandum #1 | Adjusting for Depreciation of Land Treatment When Planning Watershed Projects
October 2015
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