Alternative Simulation of Soil Phosphorus for Agricultural Land Uses in the Chesapeake Bay Watershed Model.

Current restoration efforts for the Chesapeake Bay (Bay) include a comprehensive Total Maximum Daily Load (TMDL) designed by the USEPA to address continued eutrophication concerns caused by excess nutrients and sediment entering the Bay. The magnitude of the undertaking requires a multi-jurisdictional partnership for achieving the TMDL goals.  Central to such efforts is the Bay Watershed Model (WSM) that simulates nutrient and sediment concentrations in soil and water, and ultimate transport and load delivery to the Bay across varying management scenarios.  Within the WSM, phosphorus (P) nutrient cycling for cropland is simulated in three pools: soluble phosphates (PO₄), insoluble inorganic P, and organic P. Transformations among these P pools occurs primarily by adjusting model parameters to dictate the rate of exchange, and subsequently associate each pool with a hydrologic transport mechanism.  The methodology results in loads of nutrients and sediment lost from the landscape and delivered to a stream that can be calibrated to observed hydrology and water quality data over the simulation period to produce an output of normalized annual P loads. Increasingly, studies on the sources and transport of P at the watershed scale continue to affirm greater complexity of soil P and the important connections between nonpoint source P and flow across the landscape. The primary objective of this research is a thorough review of the soil P simulation within the WSM to identify opportunities for improving its scientific representation of soil P dynamics.  Alternative soil P parameters and adjustments to the WSM soil P nutrient sequences will be proposed and validated by an independent data set. Subsequently, outputs of predicted annual loads generated by the current WSM and the modified WSM containing alternative soil P representation will be analyzed and compared relative to deviation from established TMDL limits.