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Validating Soil Phosphorus Routines in the SWAT Model

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Transactions of the ASABE. 53(5): 1469-1476. (doi: 10.13031/2013.34897) @2010
Authors:   P. A. Vadas, M. J. White
Keywords:   Modeling, Phosphorus, Soil, SWAT

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 current understanding of soil P transformations and transfer to runoff. The objective of this study was to validate the ability of routines in both SWAT2000 and SWAT2005 to initialize the quantity of P in different soil P pools (and thus soil total P) and simulate changes in the simulated solution P pool. Based on data from 40 published studies, results show that currently published equations to estimate the soil PSP parameter and SWAT's method of summing four soil P pools underpredict soil total P. An underprediction of soil total could result in underprediction of P loss in runoff with eroded sediment. Conversely, the proposed alternative for estimating soil total P, which includes a new equation to estimate the soil PSP parameter and includes 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. Results also show that both the SWAT routines and the routines proposed by a previous study for simulating changes in soil P were able to accurately predict long-term changes in soil solution P. However, routines in both SWAT2000 and SWAT2005 may underpredict solution P for several weeks after P is added to soils. This could result in underprediction of dissolved inorganic P loss in runoff soon after a P application to soils. The routines proposed by the previous study would not result in similar underpredictions.

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