USE OF THE WATER, ENERGY, AND BIOGEOCHEMICAL MODEL (WEBMOD) AND THE SOIL AND WATER ASSESSMENT TOOL (SWAT) TO IDENTIFY HYDROLOGIC FLOW PATHS AT FIVE AGRICULTURAL SITES INCLUDED IN THE U.S. GEOLOGICAL SURVEY NATIONAL WATER-QUALITY ASSESSMENT

As part of the U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program, the transport and fate of nutrients and agricultural chemicals are being studied at five sites with significantly different climate, surficial geology, crops, and agricultural practices: Mustang Creek in California; Granger Drain in Washington; Maple Creek in Nebraska; Sugar Creek in Indiana; and Morgan Creek in Maryland. WEBMOD and SWAT are being tested to see how well they simulate variations in discharge and water quality measured at the sites. This presentation addresses three questions. 1) What are the primary streamflow-generation processes in generally flat agricultural landscapes? 2) How do the fluxes of water and conservative solutes predicted by SWAT, which uses a curve-number method, compare with those predicted by WEBMOD, which is a more process based model? 3) Is the reliability of model-estimated discharge and water quality improved by calibrating the models to measured concentrations of chemical tracers, such as chloride and base cations?

WEBMOD simulates hydrologic fluxes and solute concentrations using process modules coupled within the USGS Modular Modeling System (MMS). The hydrologic fluxes are modeled using routines developed for the USGS Precipitation Runoff Modeling System, the National Weather Service Hydro-17 snow model, and a TOPography-based hydrological MODEL (TOPMODEL). Equilibrium and kinetic reactions are simulated using the USGS aqueous geochemical model PHREEQC. Modifications to several modules were made to simulate irrigation practices.

Initial calibrations were completed by fitting the simulated hydrographs with those measured at the watershed outlets. Model performance then was refined by comparing the predicted export of conservative chemical tracers with those measured at the watershed outlets.