Revised Method and Outcomes for Estimating Soil Phosphorus Losses from Agricultural Land in the Chesapeake Bay Watershed Model

Current restoration efforts for the Chesapeake Bay watershed mandate a timeline for reducing the load of nutrients and sediment to receiving waters. The Chesapeake Bay Watershed Model (WSM) has been used for two decades to simulate hydrology and nutrient and sediment transport; however, spatial limitations of the WSM preclude edge-of-field (EOF) scale representation of P losses. Rather, the WSM relies on literature-derived county-scale target rates of P loss. An independent, field-scale modeling tool (APLE) was used as an alternative to the current WSM approach. Identical assumptions of county-level acreage, soil properties, nutrient management practices, and transport factors from the WSM were used as inputs to APLE. Incorporation of APLE P loss estimates resulted in greater estimated total P loss and a revised spatial pattern of P loss compared to the WSM’s original targets. Subsequently, APLE’s revised estimates for P loss were substituted into the WSM and resulted in improved WSM calibration performance at 74% of tributary monitoring stations. The incorporation of APLE into the WSM will improve its ability to assess P loss and the impact of field management on Chesapeake Bay water quality.