Identification of Field-Level Critical Management Areas for Implementing Best Management Practices for the Central Claypan Area.

Targeting critical management areas (CMAs) within fields is essential due to an ever increasing need to maximize cultivation area while simultaneously implementing conservation management practices to minimize impacts on downstream water quality. The objective of this study was to identify CMAs in a 36 ha field within Major Land Resource Area 113, Central Claypan, and evaluate their effects on simulated water quality using the Agricultural Policy Environmental eXtender (APEX) model. The field was characterized by an abrupt argillic horizon with clay content greater than 50% occurring within the upper 30 to 50 cm, referred to as the claypan. Depth to the argillic horizon is one of the most sensitive parameters affecting runoff, nutrient and herbicide transport. The selected field was under a corn (Zea mays)-soybean (Glycine max) crop rotation since 1991 with a V-notch weir installed at the field outlet for measurement of runoff and sediment, nutrient and herbicide transport. The field was divided into 35 subareas based on slope, depth to claypan, and soil mapping unit (1:5,000 scale). The APEX model was calibrated and validated from 1993 to 2002 for runoff and sediment, atrazine, nitrogen and phosphorus concentrations. After calibration, statistical analyses were conducted to assess differences in simulated amounts of runoff as well as sediment, atrazine, nitrogen and phosphorus loads produced from each subarea. Subareas that generated excessive amounts of runoff and selected pollutants per unit area were considered as CMAs. Simulated effects of selected placement of conservation management practices based on CMAs were evaluated using the APEX model. Use of these techniques to identify and assess CMAs can provide a management system for fields to significantly reduce runoff and pollutants.