Integrated modeling system for evaluating water quality benefits of agricultural watershed management practices: Case study in the Midwest

Nonpoint source pollution (NPS) resulting from agricultural production is one of the major causes of impaired natural resources, adversely affecting the provision of ecosystem services. Best management practices (BMPs) are commonly implemented in watersheds to mitigate agricultural NPS. Appropriate selection of BMP type and its placement in agricultural watersheds are crucial for successful mitigation efforts. In this study, an integrated modeling system was developed to identify scenarios of best management practices such as nutrient management, constructed wetlands and filter strips that provide downstream water quality improvements. The modeling system is a coupling of Soil and Water Assessment Tool (SWAT) for simulating watershed responses and impacts of BMPs, with cost evaluation model solved using Archived Micro-Genetic Algorithm (AMGA2) for generating optimal reduction strategies of NPS pollution at a watershed scale. Its application was demonstrated using tributary watersheds of Lake Decatur - a water supply reservoir located in Illinois, which is listed as impaired for nitrate-nitrogen (N) and total phosphorus (P) in the 2004 Section 303(d) List of Clean Water Act. Study results indicate that nutrient management is the best alternative practice to provide water quality benefits with annual nitrate-N loss reduction of 14.9 percent and cost savings of $6.42 per kg of nitrate-N reduction per hectare (ha). In contrast, constructed wetlands and filter strips were found to incur implementation costs of $10.89/kg N/ha and $1.74/kg N/ha, respectively, including associated land revenue losses. The effectiveness of the filter strips was very limited because of extensive tile drainage in the study watersheds. The integrated modeling system generates valuable information for developing sound watershed management plan and its framework can be adapted to investigate other environmental challenges detrimental to sustainability of water resources.