Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/3/2002
Publication Date: 11/7/2002
Citation: SHARPLEY, A.N., KLEINMAN, P.J., MCDOWELL, R.W., GITAU, M., BRYANT, R.B. MODELING PHOSPHORUS TRANSPORT IN AGRICULTURAL WATERSHEDS; PROCESSES AND POSSIBILITIES. JOURNAL OF SOIL AND WATER CONSERVATION. 2002. V. 57. P. 425-439. Interpretive Summary: Phosphorus, an essential nutrient for crop and animal production, can accelerate freshwater eutrophication, which has recently been identified as the most ubiquitous water quality impairment in the U.S., with agriculture a major contributor of phosphorus. Given general environmental concerns and regulatory pressure to reduce phosphorus loadings to surface waters, much research is now focused on a better understanding of factors controlling phosphorus loss from agricultural watersheds. Because of the time and expense involved in field studies, computer models often represent a more efficient means of evaluating management alternatives. Scientists involved in this modeling endeavor, however, are often limited by a lack of information to reliably predict phosphorus losses at a large watershed scale over a period of several years. We show that not only is there a lot of new information available but existing data could be better utilized to refine current water quality models. However, one the main problems that still remains is the use of inappropriate models. It is essential that the most appropriate model be carefully selected to meet a user's needs, in terms of level of predictive accuracy needed, input data available, and scale of simulation being considered.
Technical Abstract: Modeling phosphorus (P) loss from agricultural watersheds is key to quantifying the long-term water quality benefits of alternative best management practices. Scientists engaged in this endeavor struggle to represent processes controlling P transport at scales and time frames that are meaningful to farmers, resource managers, and policy makers. To help overcome these challenges, we review salient issues facing scientists that model P transport, providing a conceptual framework from which process- based P transport models may be evaluated. Recent advances in quantifying the release of soil P to overland and subsurface flow show that extraction coefficients relating soil and flow P are variable but can be represented as a function of land cover or erosion. Existing information on best management effects on P export can be linked to watershed models to better represent changes in P transport. The main needs of P transport models are einclusion of flexible coefficients relating soil and overland flow P, manure management and P loss, stream channel effects on edge-of-field P losses prior to water body input, and linkage of watershed and water-body response models. Even so, it is essential that the most appropriate model be carefully selected to meet a user's needs, in terms of level of predictive accuracy needed, input data available, and scale of simulation being considered.