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The purpose of the Conservation Effects Assessment Project (CEAP) is to provide people involved with environmental policy issues an accounting of the environmental benefits obtained from United States Department of Agriculture (USDA) conservation program expenditures. There are two main components in CEAP: (1) The national assessment and (2) watershed assessment. This paper will focus on the calibration aspects of modeling component of the national assessment part of the project.

The primary focus of the project is on cropland where most of the conservation practices were implemented. Hence field level modeling for CEAP will be conducted by APEX (Agricultural Policy/Environmental eXtender). Outputs from the APEX model runs will be input to the watershed scale model Soil and Water Assessment Tool (SWAT) in HUMUS (Hydrologic Unit Modeling for the United States) setup to route flow, sediment, and pollutants to the outlet of each watershed in different agricultural regions. Environmental benefits at the national scale will be estimated based on the differences in output between the two scenarios before conservation practices and after conservation practices. In this modeling approach a calibration is necessary to compensate for the uncertainties in input data. The scale of the study, limits manual calibration. Hence an automated calibration procedure is developed to manage the large-scale calibration efforts. The auto-calibration procedure automatically adjusts the input parameters in HUMUS-SWAT so that the annual average stream flow and sediment yield values from HUMUS-SWAT system match to that of SPARROW (SPAtially Referenced Regression On Watershed attributes) model estimates for each 8-digit watershed.