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United States Department of Agriculture

Agricultural Research Service

Title: Identification of Critical Source Areas for Phosphorus Export from Agricultural Catchments

Authors
item Gburek, William
item Sharpley, Andrew
item Pionke, Harry

Submitted to: Advances in Hillslope Processes
Publication Type: Book / Chapter
Publication Acceptance Date: October 7, 1997
Publication Date: N/A

Interpretive Summary: In the northeastern U.S., agricultural activities are resulting in more phosphorus (P) from fertilizer and manure being applied to land than is needed by the crops. While the application of these excessive amounts of P continues year after year, there is a greater chance for runoff removing P from farmland to streams. When streams carry this P to freshwater reservoirs, lakes, and estuaries, it can accelerate algae and weed growth in the water, a process called eutrophication. The aquatic weeds can reduce the levels of oxygen dissolved in water to less than those required by fish. Research on farmland within Pennsylvania, Oklahoma, and Texas shows that runoff from soils with high P levels occurs from relatively small and identifiable areas of the landscape. The findings enable us to develop better ways to locate these areas, and construct computer models to predict the locations and losses of P in runoff. We are then able to use these models to more cheaply and effectively change management practices s that farm income and environmental quality are both maintained.

Technical Abstract: Inputs of phosphorus (P) to agricultural systems can increase P export from catchments. Export of P is controlled to a large extent by the interaction of its source factors (soil P and amount applied) with its transport processes (runoff and erosion) over the catchment. To develop cost-effective remedial strategies, we must be able to target critical source-areas of P within catchments, areas that are vulnerable to P loss i runoff and sediment. To this end, a Phosphorus Index (PI) was developed to quantify site vulnerability to P export in runoff by ranking the relative importance of the P source and transport factors. Site vulnerability to P loss determined by the PI was compared to measured P export in runoff for 30 unit-source agricultural areas (<5 ha) in Oklahoma and Texas (USA), and in streamflow from a 26-ha hill-land catchment in Pennsylvania (USA). Vulnerability to P loss from the unit-source areas was found to be closely related to actual losses. Actual losses in these cases were strongly related to erosion, emphasizing the importance of incorporating accurate estimates of erosion, particularly, and runoff in any ranking or predictive scheme. The PI was also applied to individual fields within the larger mixed land use catchment to identify sites that are potentially vulnerable to P loss via runoff and erosion. The rankings were examined in context of measured and simulated P losses to identify runoff-producing zones within the catchment and their potential to move sediment and P to the stream channels. The comparisons illustrate the importance of targeting controls to the hydrologically active P source-areas within a larger catchment area to provide the greatest opportunity for minimizing P export.

Last Modified: 7/22/2014
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