EVALUATION OF SWAT IN SIMULATING NITRATE NITROGEN AND ATRAZINE FATES IN A WATERSHED WITH TILES AND POTHOLES

Abstract

We evaluated a version of the Soil Water Assessment Tool (SWAT-M) that was modified to more accurately simulatetile drainage and water flow in a landscape dominated by closed surface depressions or potholes at a watershed scale usingten years of measured nitrate-nitrogen (NO3-N) and atrazine data in stream discharge in the Walnut Creek watershed (WCW).The model was calibrated during the period of 1992 to 1995 and validated during the period of 1996 to 2001. Stream sitesin the middle and outlet of the WCW were selected to assess overall performance of the model, while one drainage districtdrain was used for investigating chemical loads in subsurface flows. With the introduction of an independent tile drain lagtime parameter, the performance of SWAT-M for daily flow simulation was improved. In comparison to our previous results,the Nash-Sutcliffe E values for the calibrated daily flow at the mid-watershed and outlet simulated by the enhanced SWATmodel rose from 0.55 to 0.69 and from 0.51 to 0.63, respectively. Of special note, the E value for calibrated flow rose from-0.23 to 0.40 for the drainage district drain, which was dominated by tile and subsurface flow. Both the predicted corn yieldsand N uptake by corn were very similar to the measured data. The predicted yield and N uptake by soybean were relativelylower than the measured values. The monthly NO3-N loads in stream discharges at the center and outlet of the Walnut Creekwatershed were accurately predicted with good Nash-Sutcliffe E values of 0.91/0.80 and 0.85/0.67 in calibration/validation,respectively. Nevertheless, the models simulation of the daily NO3-N loads was not as good as the monthly simulation. Thegood agreement between the simulated and measured monthly NO3-N loads from the drainage district site leads us to concludethat SWAT can reasonably simulate tile flow from pothole-dominated landscapes, although the model needs to be improvedin the simulation of daily subsurface NO3-N fluxes. The enhanced SWAT-M model simulated the NO3-N loads in a watershedwith intensive tile drainage systems much more accurately than the original SWAT2000 version. A second pesticidedegradation half-life in soil was added for SWAT-M, which greatly improved the model performance for predicting atrazinelosses from the watershed. Overall, SWAT-M is capable of simulating atrazine loads in the stream discharge of the WCW andis a much-improved tool over SWAT2000 for predicting both daily and monthly atrazine losses in nearly level, tile-drainedwatersheds.