Estimating the Effects of Agricultural Conservation Practices on Phosphorus Loads in the Mississippi-Atchafalaya River Basin

Abstract

<abstract><title><italic>Abstract.</italic></title> Agriculture in the Mississippi-Atchafalaya River basin (MARB) is important in terms of both the national economy and the nutrients discharged to the basin and the Gulf of Mexico. Conservation practices are installed on cropland to reduce the nutrient losses. A recent study by the Conservation Effects Assessment Project (CEAP) determined the effects of agricultural conservation practices on water quality in the MARB. 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. APEX was used to simulate the conservation practices on cropland and Conservation Reserve Program (CRP) land and assess the edge-of-field water quality benefits. The predicted flow and loads from APEX were input to SWAT, and SWAT was used to simulate the watershed processes and estimate the local and instream water quality benefits. The model was used for scenario assessment after calibration and validation for streamflow and loads. Recent studies indicate that phosphorus influences the formation of the hypoxic zone in the Gulf of Mexico. The major objectives of this article are to: (1) estimate and discuss the effects of currently existing and additional conservation practices on total phosphorus (TP) loads in the MARB, and (2) assess how TP loads discharged by the conservation scenarios can achieve the recommended annual P target for hypoxia reduction. Results indicated that current conservation practices on cropland have reduced TP losses to local waters by 13% to 52% in six basins within the MARB and reduced the TP load discharged to the Gulf of Mexico by 22%. Additional P load reduction is likely required to reach the annual P target for hypoxia reduction.