Hydrologic and Water Quality Modeling of Crep-Watersheds For Identifying Critical Source Areas of Sediment and Nutrient Yields

The Conservation Reserve Enhancement Program (CREP) in Illinois is a partnership between United States Department of Agriculture’s Farm Service Agency, Illinois Department of Natural Resources, and the Soil and Water Conservation Districts, providing agricultural producers with financial incentives to conserve and enhance the natural resources of the land. The Illinois CREP goal is to reduce sediment and nutrient runoffs, improve downstream water quality, and create and enhance critical habitat for fish and wildlife populations on private lands within the Illinois and Kaskaskia River watersheds. Since 1999, Hydrologic and water quality monitoring have been conducted by Illinois State Water Survey (ISWS) to assess the effects of conservation practices in the CREP watersheds. Various methods have been developed to estimate watershed sediment and nutrient contributions by different categories of sources and this includes a simple accounting of the mass balance between total watershed inputs and outputs. For example, a watershed nutrient mass balance can be calculated using total nutrient inputs (e.g., fertilizer application, manure, point sources), total outputs (e.g., nutrient removal by plant uptake, riverine nutrient loads) and their difference as total nutrient losses (e.g., due to denitrification, volatilization, soil adsorption). Such methods implicitly assume similar loss mechanisms and proportional relationships of input sources to outputs. Quantifying the sediment and nutrient source contributions, however, requires accounting for landscape effects, degradation and deposition in the channel, and in-stream nutrient transformations that provide a more complete picture of their impact on downstream water quality. Therefore, in this study, a physically-based, semi-distributed watershed model known as the Soil and Water Assessment Tool (SWAT) was used to develop a hydrologic and water quality model for one of Illinois CREP watersheds. Detailed representation of watershed characteristics and processes in the model allows simulating more accurate relationships between watershed land use, soils, topography and climate, and its resulting watershed responses. The CREP watershed model was calibrated and validated for flow, sediment, nitrate-nitrogen and total phosphorus simulations, and was used to identify critical source areas of sediment and nutrient yields in the watershed. In addition, eligible CREP areas were identified using the 100-year floodplain area in the watershed and this would facilitate targeting outreach programs for CREP enrollment in the watershed. The watershed model can further be used to evaluate optimal placements of best management practices the CREP watershed.