Effects of agricultural conservation practices on N loads in the Mississippi-atchafalaya river basin.

A modeling framework consisting of a farm-scale model, Agricultural Policy Environmental Extender (APEX); a watershed-scale model, Soil and Water Assessment Tool (SWAT); and databases was used in the Conservation Effects Assessment Project to quantify the environmental benefits of conservation practices on cropland. APEX is used to simulate conservation practices on cultivated cropland and Conservation Reserve Program land to assess the edge-of-field water-quality benefits. Flow and pollutant loadings from APEX are input to SWAT. SWAT simulates the remaining noncultivated land and routes flow and loads generated from noncultivated land, point sources, and cropland to the basin outlet. SWAT is used for assessing the effects of practices on local and in-stream water-quality benefits. Each river basin is calibrated and validated for streamflow and loads at multiple gauging stations. The objectives of the current study are to estimate the effects of currently existing and additional conservation practices on total N (TN) loads in the Mississippi-Atchafalaya River Basin (MARB) and draw insights on TN load reductions necessary for reducing the hypoxic zone in the Gulf of Mexico. The effects of conservation practice scenarios on local and in-stream (riverine) water quality are evaluated. Model results indicate that conservation practices currently on cropland have reduced the TN losses to local waters between 20 and 59% in the six river basins within MARB and the TN load discharged to the Gulf by 17%. Further water-quality improvement can be obtained in the MARB with additional conservation treatment.