Modeling the Effect of Landuse Change on Phosphorus Loading to Lake Allatoona.
David Radcliffe and Zhulu Lin. Dept. of Crop & Soil Sci., University of Georgia-3111 Miller Hall, Athens, GA 30602
Lake Allatoona watershed in North Georgia covers an area of 2,800 square km and includes the Etowah River and eleven smaller rivers. Lake Allatoona is classified as in transition to eutrophic and the state has placed a cap on annual phosphorus (P) load to the lake. Our objective was to estimate the effect on landuse change on P loading to Lake Allatoona using the Soil Water Assessment Tool (SWAT). We calibrated SWAT using data from a USGS gauge station on the Etowah River with daily flow and approximately monthly samples of sediment and P concentrations from 1992-1996. We also used monthly samples of flow, sediment concentration, and P concentration collected on each of the major tributaries in a study conducted from 1992-1996. Land use data was from 1992 and 2001. In 1992, the dominant land use was forest ranging from 72.8 to 93.8% of the area among watersheds. Next was pasture land use (4.0-13.3%), and then low density residential (urban) land use (0.2-12.1%). Pasture land use consisted, for the most part, of combined poultry and beef cattle operations. The annual load of total P predicted by SWAT for 1992 was 157,816 kg, which was about 73% of the cap. Point sources accounted for 10%, urban land use 10%, forest land use 30%, and pasture land use 47% of the total P loading to the lake. From 1992 to 2001, urban land use increased significantly in the Lake Allatoona basin. In the Etowah River basin, urban landuse went from 0.6 to 8.8% and in the Little River / Noon Day Creek watershed urban landuse went from 11.6 to 40.2%. The predicted total P load to the lake in 2001 was 219,052 kg, an increase of 38.8% over 1992 and 101% of the cap.