EVALUATION OF NITROGEN VERSION OF LEACHM FOR PREDICTING NITRATE LEACHING

The abilities of the Richards and convection-dispersion equations approach (LEACHNR) and the capacity model approach (LEACH-NA) of the nitrogen version (LEACHN) of the LEACHM model to simulate nitrate leaching were evaluated using field data from a 5-year nitrate leaching experiment conducted in central Pennsylvania on Hagerstown silt loam soil (fine, mixed, mesic, Typic Hapludalf). Nitrate leaching losses from N-fertilized and manured corn below the 1.2-m depth were measured with zero-tension pan lysimeters. Three N-fertilized and manured treatments for 1988-1989, 1989-1990, and 1990-1991 and two N-fertilized treatments for 1991-1992 and 1992-1993 were used from the leaching experiment to evaluate both approaches of LEACHN. The individual monthly simulations of nitrate leaching were compared with the mean of pan efficiency corrected-measured data for these 5 years. Both approaches of the model were calibrated to the site conditions using the data of 1989-1990 and were then evaluated using 1988-1989, 1990-1991, 1991-1992 and 1992-1993 nitrate leaching data. Simulated results for the calibration year for both models were reasonably accurate (31 of 36 months simulated within the experimental 95% confidence limits). The statistical analysis used in this study indicated that both LEACHNA and LEACHNR adequately (91 to 120 months within the 95% confidence limits) predicted nitrate leaching below the 1.2-m depth for treatments in the refinement years. Much of the simulation error in some treatments in the refinement years seemed to be related to the sub-routine controlling soil nitrogen transformation processes and their rate constants in the model. The large deviations in NO3--N leached in some winter months may be related, in part, to problems with simulated water flow associated with the frozen soil conditions and snow accumulation. The addition of a dual-pore water flow option (LEACHNA) to the nitrogen version of LEACHM did not improve prediction of nitrate leaching beyond the rooting zone of corn under Pennsylvania conditions.