|Evaluation of Sediment Transport Models and Comparative Application of Two Watershed Models (81 pp, 1.6 MB) (EPA/600/R-03/139) September 2003
Suspended solids and sediments, regarded as the two leading pollutants of our nation's bodies of water, are carriers of various pesticides, radioactive materials, and nutrients. Section 303(d) of the 1972 Clean Water Act requires states, territories, and authorized tribes to identify and list areas of impaired water every two years and to develop Total Maximum Daily Loads (TMDLs) for pollutants in the water.
Mathematical models in hydrology, hydrodynamics, and recently, water quality are widely accepted, effective, and powerful tools for TMDL development and for evaluating performances of best management practices (BMPs). The selection of the right model requires a comprehensive knowledge of the capabilities and features of available models. This report provides an overview and evaluation of sediment models and compares two distributed, watershed-scale models by applying them to an experimental watershed. A probabilistic, risk-based mathematical optimization framework is presented and proposed as a strategy, especially when multiple stressors are involved, for obtaining cost-effective, optimal load reductions using BMPs.
The report has two parts. The first part evaluates and summarizes the key features of the most widely cited watershed-scale, hydrodynamic, and water quality models with the emphasis on TMDLs and BMPs. Reviewed models were selected based on minimum criteria. Water quality models, specifically those that can simulate nutrients in the environment, are also considered because transport and fate of sediments and nutrients are intimately related phenomena.
Among the reviewed loading models, SWAT and AGNPS offer the most BMP alternatives at agricultural watersheds.
For urban areas, SWMM is identified as the most suitable loading model; for mixed land uses (i.e., rural and urban) HSPF is the most suitable. These models need to be used with hydrodynamic and water quality models for a complete TMDL analysis and BMP development.
BASINS and MIKE-SHE are comprehensive watershed water quality modeling systems, with varying degrees of complexity.
WMS offers a tractable watershed-modeling platform; if fully developed, it can be used for sediment TMDLs allocation.
Available and potential model linkages between loading, hydrodynamic, and water quality models are also discussed. It is recommended that future model development include more linkages between loading and hydrodynamic and water quality models. It is observed that most physically based models are incapable of generating a complete BMP assessment. It is recommended that future models have the capability of simulating more BMPs.
The second part of the report evaluates, by application to an experimental watershed, two promising distributed watershed-scale sediment models: KINEROS-2 and GSSHA. Sensitivity of KINEROS-2 to model parameters was evaluated within a probabilistic framework using Monte Carlo simulations to identify key model parameters for calibration. It was shown that the order of parameter sensitivities changes with the quantity of interest (e.g., peak flow and total sediment yield). The calibration/verification procedure performed over KINEROS-2 showed that the Manning's roughness and soil erosion parameters demonstrated systematic seasonal variations.
Both models were calibrated and verified, and the results clearly highlight the challenges modelers face when applying complex, distributed watershed models. The results emphasize the importance of using the numerical application of different watershed models as a means of evaluating the models. Future efforts to evaluate hydrologic and water quality models should include actual comparative applications to real case studies.
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