SIMULATION OF SEDIMENT AND NITRATE LOSS ON A VERTISOL WITH CONSERVATION TILLAGE PRACTICES

Shrinking and swelling clay soils are dominant in the Blackland Prairi e of Central Texas and comprise a vast majority of agricultural produc tion land in the area. An agricultural field scale simulation model (E PIC) was applied on six small watersheds located in Riesel, Texas. A n on-calibrated model performance evaluation of the runoff sediment yiel d, nutrient transport, and crop growth components was completed. Manag ement practices included no-till and conventional till systems. Annual and monthly predicted parameter values were compared with measured da ta for a 5-year period. Annual comparisons indicate close agreement be tween means and standard deviations for runoff erosion, and nitrate-ni trogen. Significant correlation existed between monthly measured and s imulated runoff and erosion. Significant correlation for nitrate-nitro gen was present in a majority of the cases studied. Prediction efficie ncy was significant for all elements except nitrate-nitrogen on two wa tersheds. The results of this study indicate EPIC's ability to simulat e natural processes without calibration on shrinking and swelling clay soils with varying management practices.