Proper calibration and validation of computer models can inexpensively and quickly assess the impacts of different agricultural management practices on water quality. This study used Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) version 2.1 to determine the effects of two nitrogen (N) application rates (168 kg N/ha (168PM) and 336 kg –N/ha (336PM)) from poultry manure and one N application rate (168 kg N/ha (168UAN)) from urea ammonium nitrate (UAN) fertilizer on NO3–N loss with subsurface drainage. The simulated NO3–N losses by the GLEAMS model were compared with two–year (1999 and 2000) monthly measured NO3–N losses from six 2.1–m 2 field lysimeters under continuous corn production.

Results indicated that the two–year average simulated subsurface drainage consistently followed the average measured subsurface drainage from 168UAN, 168PM, and 336PM treatments, with mean errors of 9%, 3%, and –2%, respectively. The model was capable of predicting overall NO3–N concentration in subsurface drainage from lysimeters under 168UAN, 168PM, and 336PM treatments reasonably well, giving mean errors of 19%, –29%, and 9%, respectively. The overall two–year results also show that there were no significant differences (p= 0.05) between average measured and simulated NO3–N losses with subsurface drainage from poultry manure treatments. The study concluded that the GLEAMS model can be used as a viable management and decision–making tool to assess the impacts of long–term poultry manure application on water quality and agricultural production for Iowa soils.