Second Quarterly Progress Report April 2005
Macoupin Creek Watershed
Limno-Tech, Inc. Page 8
Watershed Characterization System (WCS) Sediment Tool
The Watershed Characterization System (WCS) Sediment Tool was developed by EPA
Region 4. The Watershed Characterization System is an ArcView-based application used
to display and analyze GIS data including land use, soil type, ground slope, road
networks, point source discharges, and watershed characteristics. WCS has an extension
called the Sediment Tool that is specifically designed for sediment TMDLs. For each grid
cell within the watershed, the WCS Sediment Tool calculates potential erosion using the
USLE based on the specific cell characteristics. The model then calculates the potential
sediment delivery to the stream grid network. Sediment delivery can be calculated using
one of the four available sediment delivery equations: a distance-based equation, a
distance slope-based equation, an area-based equation, or a WEPP-based regression
equation.
The applicability of WCS for estimating sediment loads for TMDLs is similar to that of
the USLE in terms of data requirements and model results; i.e., it is relatively simple to
apply but has the potential to be inaccurate. It provides three primary enhancements over
the USLE: 1) Model inputs are automatically incorporated into the model through GIS
coverages; 2) Topographic factors are calculated in the model based on digital elevation
data; and 3) The model calculates the fraction of eroded sediment that is delivered to the
surface water. It is only applicable to sediment TMDLs whose target represents long-term
loading conditions. Because its predictions represent average annual conditions, it is not
suitable for predicting loads associated with specific storm events. Like the USLE, it is
does not lend itself to model calibration unless data are available to define annual loading
rates.
Generalized Watershed Loading Functions Model (GWLF)
The Generalized Watershed Loading Functions Model (GWLF) simulates runoff and
sediment loadings from mixed-use watersheds. It is a continuous simulation model (i.e.,
predicts how concentrations change over time) that uses daily time steps for weather data
and water balance calculations. Sediment results are provided on a monthly basis. GWLF
requires the user to divide the watershed into any number of distinct groups, each of
which is labeled as rural or urban. The model does not spatially distribute the source
areas, but simply aggregates the loads from each area into a watershed total; in other
words, there is no spatial routing. Erosion and sediment yield for rural areas are estimated
using monthly erosion calculations based on the USLE (with monthly rainfall-runoff
coefficients). A sediment delivery ratio based on watershed size and a transport capacity
based on average daily runoff are then applied to the calculated erosion to determine how
much of the sediment eroded from each source area is delivered to the watershed outlet.
Erosion from urban areas is considered negligible.
GWLF provides more detailed temporal results than the USLE, but also requires more
input data. Specifically, daily climate data are required as well as data on processes
related to the hydrologic cycle (e.g., evapotranspiration rates, groundwater recession
constants). By performing a water balance, it has the ability to predict concentrations at a
watershed outlet as opposed to just loads. It lacks the ability to calculate the sediment
delivery ratio that is present in the WCS sediment tool. Because the model performs on a
