Water Quality Modeling
Soil and Water Assessment Tool (SWAT) watershed model
SWAT is a physically-based watershed and landscape simulation model developed by the USDA-ARS. Major components of the model include hydrology, weather, erosion, soil temperature, crop growth, nutrients, pesticides and agricultural management. SWAT also has the ability to predict changes in sediment, nutrients - such as organic and inorganic nitrogen and organic and soluble phosphorus, pesticides, dissolved oxygen, bacteria and algae loadings from different management conditions in large ungauged basins. SWAT operates on a daily time step and can be used for long-term simulations. The model output is now available in daily, monthly and annual time scales, although efforts are being made to account for sub-daily time steps. SWAT coding and subroutines are modular, allowing for addition of new subroutines when necessary. SWAT has been successfully applied to model water quality issues including sediments, nutrients and pesticides in watersheds. SWAT has been applied to model phosphorus in TMDL analysis of the Bosque River watershed in Texas. In the Hydrologic Modeling of the United States Project (HUMUS), SWAT was used to analyze water management scenarios. SWAT is included in EPA's BASINS modeling framework.
SWAT will be used in this study because it represents landscape processes and the impacts of agricultural management and land uses on water quality. Pollutant loading estimated by SWAT will be input into the QUAL-2E steady state in-stream dynamic model. SWAT uses a simplified version of steady-state instream kinetics routines from the QUAL2E model to transform pollutants along river and stream segments. In this study, the QUAL2E model will be used to simulate in-stream water quality dynamics. Part of this study will include determining if SWAT could be improved for use in watershed modeling and projecting in-stream dynamics along river segments. TAES researchers will lead SWAT modeling efforts.
In-stream water-quality models - QUALTX and QUAL2E
QUALTX is a modified version of QUAL-2E, and is a steady-state surface water quality model. QUALTX was developed for Texas to evaluate permit levels for entities that release point source pollutants. Based on QUALTX modeling results, regulatory agencies can recommend if existing permits should be changed. QUAL2E is a one-dimensional steady state in-stream model supported by EPA. It is applicable to sectional well-mixed streams. The model includes the effects of advection, dispersion, dilution and pollutant reactions, interactions, sources and sinks. The model has built-in options to depict the major reactions of nutrient cycles, algal production, benthic and carbonaceous oxygen demand and atmospheric reaeration.
Water Quality Analysis Simulation Program (WASP)
Output loading from the SWAT and QUAL2E models will drive the Water Quality Analysis Simulation Program (WASP) model (version 6.x) (EPA, 2003) to simulate the reservoir water quality. WASP is a finite-difference model used to interpret or predict possible changes in water quality of ponds, lakes, reservoirs, rivers and coastal waters brought about by pollutants. The model can be applied in one, two or three dimensions, and allows flexibility in defining initial and boundary conditions. The model can be utilized in simple or complex reservoir modeling. A WASP model will be developed for each reservoir owned and operated by the district. Espey Consultants, Inc. will lead the in-stream and reservoir modeling effort.