An Approach for Estimating Water Quality Benefits of Conservation Practices at the National Level

Abstract

The United States Department of Agriculture has initiated the Conservation Effects Assessment Project (CEAP) to quantify the environmental benefits of conservation practices at the national scale. This paper provides an overview of the analytical approach being used in the CEAP national assessment to estimate off-site water quality benefits. For the assessment, a sampling and modeling approach is used. The farm-scale model Agricultural Policy/Environmental EXtender (APEX) is used to simulate conservation practices for cultivated cropland. Farmer surveys conducted on a subset of National Resource Inventory sample points provide information on current farming activities and conservation practices for APEX. Output from APEX will be input into the watershed scale model, Soil and Water Assessment Tool (SWAT) in the HUMUS (Hydrologic Unit Modeling for the United States) system for routing the pollutants to the 8-digit watershed outlet. SWAT will be calibrated and validated using the United States Geological Survey’s SPAtially Referenced Regressions On Watershed attributes (SPARROW) model output, streamflow and pollutant data. The HUMUS system simulates in-stream effects for (a) a baseline scenario with conservation practices and (b) an alternative scenario without conservation practices. The off-site water quality benefits of conservation practices currently in use will be determined by comparing outputs for the alternative scenario to the baseline outputs at each 8-digit watershed. Benefits will be reported as reductions in in-stream concentrations and loadings of sediment, nutrients and pesticides, and reductions in the number of days that concentrations exceed human health and ecological thresholds.