Testing a connectivity factor for the Georgia P Index
Bryant, John Howard
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Nonpoint source pollution in the Etowah River Watershed located in North Georgia is leading to eutrophic conditions in Lake Allatoona and a major source of P has been the poultry industry, specifically the 2 billion pounds of litter land applied within the watershed (Cazier and Salmore, 1998). The Georgia Phosphorus Index (PI) provides a tool to predict bioavailable P loss from agriculturally based land use to surface waters. Gburek et al. (2000b) suggested a connectivity factor to improve the PI based on distance of a field from a stream. Romeis (2008) monitored first-order streams on three forested and nine agricultural sites for flow and P concentration. The objective of this study was to compare measured P loads from the agricultural sites to estimates based on the Georgia PI, with and without modifications to include a connectivity factor. Survey data on management practices and soil measurements provided input for the PI and site characteristics. Curve numbers (CN) were calculated from the runoff data of Romeis (2008) and compared to those estimated following standard methods (NRCS, 2001b). Our study showed that the current Georgia PI consistently underestimated runoff from the agricultural and forested sites. Three methods were applied to create connectivity factors including method that corrected the runoff estimate. Each method provided acceptable correlations to TP loads within the watersheds, but the current PI proved effective as well.