ARS | Publication request: Watershed-level comparison of predictability and sensitivity of two phosphorus models

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 9, 2012
Publication Date: September 6, 2012
Citation: Sen, S., Srivastava, P., Vadas, P.A., Kalin, L. 2012. Watershed-level comparison of predictability and sensitivity of two phosphorus models. Journal of Environmental Quality. 41:1642-1652.

Interpretive Summary: Buildup of phosphorus in agricultural soils and transport of phosphorus to surface waters from application of poultry manure is an environmental concern. Computer models are often used to quantify the impacts of poultry manure application, but estimates and related recommendations for decreasing impacts differ depending on the model used. In this study, we evaluated two phosphorus loss models, the Soil and Water Assessment Tool (SWAT) and a newly developed model called SurPhos, in a small agricultural watershed in Alabama where poultry manure is applied to hayfields. Both models simulated greater phosphorus loss as both the initial amount of phosphorus in the soil and the manure application rate increased. However, SWAT phosphorus loss varied more as a function of initial soil phosphorus, and SWAT simulated faster accumulation of phosphorus in the top 1-cm soil layer from over-application of manure. Overall, SWAT predicted less phosphorus loss in early years of manure application, but greater phosphorus loss in later years. This is due to how SWAT simulates assimilation of manure into soil and soil phosphorus accumulation, which is quite different from SurPhos. Results showed SurPhos simulates phosphorus loss and soil phosphorus accumulation processes from applied manure more appropriately, suggesting that the SWAT phosphorus model be replaced by the SurPhos model to more accurately determine watershed-level effectiveness of phosphorus management.

Technical Abstract: Buildup of phosphorus (P) in agricultural soils and transport of P to nearby surface waters due to excessive, long-term application of poultry litter is a concern in poultry-producing states. Watershed models are often used to quantify soil and water quality impacts of poultry litter applications. Depending on how P transport is simulated in the models, the anticipated impact could be quite different. The objectives of this study were to: a) determine the sensitivity of two P models (Soil and Water Assessment Tool (SWAT) P model and a newly developed, state-of-the-art manure P model called SurPhos) from a poultry litter-applied pasture watershed; and b) quantify potentially achievable watershed-level reductions in P using these P models. A small agricultural watershed in Alabama was used for this study. SWAT was calibrated with 2004 and 2005 flow data with Nash-Sutcliffe coefficient of 0.70 for both surface runoff and total streamflow. Dissolved P loss simulated by SWAT and SurPhos from hayfields were compared for different poultry litter application rates. Both models showed sensitivity to litter application rates. SWAT simulated an average increase of 0.21 and 0.48 kg/ha/yr of total dissolved P (TDP) export for every 1121 kg/ha increase in application rate at initial soil labile P (SLP) levels of 0.5 mg/kg and above 5 mg/kg, respectively. SurPhos simulated an average increase of 0.42 and 0.52 kg/ha/yr at different initial SLP levels. Simulation scenarios suggested SWAT over-estimates TDP exports from higher SLP fields. SurPhos more appropriately identifies manure P as the main source of dissolved P in runoff. Also, SWAT simulates faster buildup of soil solution P in the top 1 cm as compared to SurPhos. Since SurPhos appears to simulate P transport processes from applied manure more accurately, the study suggests that the SWAT P model be replaced by the SurPhos model to more accurately determine watershed-level effectiveness of P management measures.