Answers-2000: Non-point-source nutrient planning model

ANSWERS-2000, a non-point-source planning model was modified to simulate lo ng-term nitrogen (N) and phosphorus (P) transport from rural watersheds. Th e model simulates infiltration, evapotranspiration, percolation, and runoff and losses of nitrate, adsorbed and dissolved ammonium, adsorbed total Kje ldahl N, and adsorbed and dissolved P losses. Eight soil nutrient pools are modeled: stable organic N, active organic N, nitrate, ammonium, and stable mineral P, active mineral P, organic P, and exchangeable P. The model was validated on two small watersheds without calibration and on a large waters hed with calibration of only the sediment detachment parameters. Predicted cumulative runoff, sediment, nitrate, dissolved ammonium, adsorbed total Kj eldahl N, and orthophosphorus P losses were within a factor of 2 of observe d values (-40 to +44% of observed values). Predictions of individual runoff event losses were not as accurate (-98 to +250%). The model seriously unde rpredicted adsorbed ammonium losses by up to 97%, and additional work is re commended on this submodel. In a practical application, the use of the mode l in evaluating the cost-effectiveness of alternative management scenarios was demonstrated.