SIMULATING TILLAGE EFFECTS ON NONPOINT SOURCE POLLUTION FROM AGRICULTURAL LANDS USING GLEAMS

A GLEAMS (ver. 2.10) model was calibrated and validated using three years (1990–1992) of field–measured data to simulate tillage effects on nitrate–nitrogen (NO3–N) and herbicide losses with subsurface drain “tile” water beneath a continuous corn production system. The model was calibrated for chisel plow systems using 1991 field data and was validated against field data of two years (1990 and 1992) for chisel plow and three years (1990–1992) for moldboard plow (MB), ridge till (RT), and no–till (NT) systems. The model simulations were made with a single run using data on measured tile flows; nitrate–nitrogen (NO3–N), atrazine, and alachlor losses with tile flows; N–uptake; and other biomass parameters for the period 1 January 1990 through 31 December 1992 for CP, MB, RT, and NT systems. All four tillage treatments, each replicated three times, were applied in a randomized complete block design at Iowa State University’s Northeastern Research Center, Nashua, Iowa, to collect field data. A specific calibration procedure for the nutrient component of the GLEAMS model was developed adjusting the N–uptake coefficient and using steady–state N–pool values obtained after multiple years of the model runs. The results of this study indicated that model simulations on subsurface drain water quality and plant parameters were adequate for chisel plow systems and the percent difference, on the average, was <10% between predicted and measured data with R 2 = 0.99. The change in tillage management option for MB, RT, and NT systems slightly affected the model predictions on subsurface drain water quality, probably due to the lack of a macropore option in the model and perhaps also due to no fall plowing in 1991 because of wet weather. However, calibration of the nutrient submodel using steady–state N–pool values was successful and may be tested to simulate the long–term effects of different cropping systems on soil and water quality.