Modeling the Average Annual Nutrient Losses of Two Watersheds in Indiana Using Gleams-Napra

From 1973 to 1984 the Black Creek Project assessed the contributions of agricultural production to theagrochemical loading levels to drainage water. GLEAMS-NAPRA calibration and validation were conducted using measuredwater quantity and quality data from 1975 to 1977 from two watersheds within the Black Creek watershed, the Driesbach andSmith-Fry. The model was calibrated and validated for monthly runoff, nitrate loading, sediment loss, sediment phosphorus,and total phosphorus. Modeling the land use as it existed at the time of the original Black Creek Project usingGLEAMS-NAPRA resulted in model predictions that were similar to observed monthly results. In the Driesbach watershed,the Nash-Sutcliffe model efficiencies for monthly runoff, sediment, nitrate, sediment phosphorus, and total phosphorus were0.89, 0.78, 0.69, 0.57, and 0.70, respectively. Additionally, the R2 values for monthly runoff, sediment, nitrate, sedimentphosphorus, and total phosphorus were 0.90, 0.86, 0.81, 0.79, and 0.75, respectively, for the same watershed. Since themid-1970s, the land use within the watershed has changed, with an increase in urban and farmstead areas and changes incropping systems. When applying the model with 2003 land use data, there was a predicted average annual decrease in nitrateloss for the Smith-Fry and Driesbach watersheds by 6 kg/ha and 4 kg/ha, respectively. Little impact was predicted for sedimentphosphorus and total phosphorus loss, with differences in average annual loss of 0.05 and 0.08 kg/ha for Smith-Fry and 0.08and 0.26 kg/ha for Driesbach, respectively. The results of this study indicate that GLEAMS-NAPRA has the ability to predictmonthly runoff, nitrate, sediment, sediment phosphorus, and total phosphorus losses on a small watershed scale. The resultsalso indicate the possibility of using GLEAMS-NAPRA to estimate losses in other watersheds with similar soil, land use, anddrainage characteristics.