Validating soil phosphorus routines in the SWAT model

Authors: Vadas, Peter; White, Michael

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2010
Publication Date: 11/12/2010
Citation: Vadas, P.A., White, M.J. 2010. Validating soil phosphorus routines in the SWAT model. Transactions of the ASABE. 53(5):1469-1476.

Interpretive Summary: Phosphorus loss from agricultural soils to surface waters is an important environmental issue. Computer models like the Soil & Water Assessment Tool (SWAT) that are commonly used to predict phosphorus loss are not always updated to reflect current scientific knowledge. Using data from 40 published studies, we found that the model routines in SWAT under-predict soil total phosphorus, which could result in under-prediction of phosphorus loss in runoff with eroded sediment. Conversely, we developed alternative routines that accurately predict soil total phosphorus. Our results also showed that both the SWAT routines and our alternative routines accurately predict long-term changes in soil-labile phosphorus. However, routines in both the 2000 and 2005 versions of SWAT may under-predict labile phosphorus for several weeks after phosphorus is added to soils. This could result in under-prediction of dissolved inorganic phosphorus loss in runoff soon after a phosphorus application to soils. Our alternative routines would not result in similar under-predictions. Overall, our new routines should help SWAT and similar models to more accurately predict phosphorus loss from agricultural soils. This, in turn, will help scientists and policy makers develop management practices to reduce the environmental impacts of agricultural phosphorus.

Technical Abstract: Phosphorus transfer from agricultural soils to surface waters is an important environmental issue. Commonly used models like SWAT have not always been updated to reflect improved understanding of soil P transformations and transfer to runoff. Our objective was to validate the ability of the P routines in both SWAT2000 and SWAT2005 to initialize the size of the soil P pools (and thus, soil total P) and simulate long-term changes in the simulated Solution P pool. We used data from 40 published studies. Results showed that currently published equations to estimate the soil PSP parameter and SWAT’s method of summing four soil P pools under-predict soil total P. An under-prediction of soil total could subsequently result in under-prediction of P loss in runoff with eroded sediment. Conversely, our proposed alternative for estimating soil total P, which includes a new equation to estimate the soil PSP parameter and including the Solution P pool when summing soil P pools, resulted in accurate predictions of soil total P for 484 topsoil samples from 35 published studies. Our results also showed that both the SWAT routines and routines proposed by Vadas et al. (2006) for simulating changes in soil P accurately predict long-term changes in soil Solution P. However, routines in both SWAT 2000 and SWAT 2005 may under-predict Solution P for several weeks after P is added to soils. This could result in under-prediction of dissolved inorganic P loss in runoff soon after a P application to soils. The routines proposed by Vadas et al. (2006) would not result in similar under-predictions.