Comparing Watershed Scale P Losses from Manure Spreading in Temperate Climates across Mechanistic Soil P Models

Watershed scale modeling is commonly used to evaluate nutrient fluxes to surface waterbodies and the appropriateness of regulatory intervention, such as the implementation of a total maximum daily load (TMDL). The Soil and Water Assessment Tool (SWAT) is a widely used watershed-scale model employed to simulate streamflow and nutrient fluxes, including phosphorus (P), from nonpoint sources, which has become ubiquitous throughout nutrient management legislation in the US. A number of studies have shown that manure spread on fields has high potential for nutrient loss in surface runoff directly following application. The authors evaluated the influence of the seasonality of hydrologic fluxes on P losses from dairy manure spread fields during this especially labile period across three mechanistic soil P loss models: SWAT v2012, SWAT v2016, and JoFlo. Calibration was performed using a DDS algorithm for SWAT v2012 and JoFlo and results of the SWAT v2012 calibration were applied to SWAT v2016. Calibration results were non-ideal; however, we propose that our methodology appropriately isolates the processes of concern and furthers the debate on what remains a critical open question. SWAT v2016 and JoFlo show a strong influence of seasonality with respect to P losses from manure-spread field, in agreement with recent field-scale studies. SWAT v2012, commonly utilized for establishing nutrient management policy, demonstrated a limited influence of seasonal hydrology on P losses, suggesting issues in the underlying mechanisms driving labile P mobilization. Our study reinforces the idea that development of hydrologic model structure should strive to parallel the intent of environmental policy. (c) 2019 American Society of Civil Engineers.