Long-term stream chemistry trends in the southern Georgia Little River Experimental Watershed

Abstract

Long-term stream water quality data may provide opportunities to study the effectiveness of conservation practices. The first three decades of data for the Little River in southwestern Georgia were analyzed for trends as part of the Conservation Effects Assessment Project. Concentrations and loads for chloride, ammonium-N, nitrate plus nitrite-N, total Kjeldahl N, total P, and dissolved molybdate reactive phosphorus were determined from 1974 through 2003 for eight nested subwatersheds in the Little River Experimental Watershed. There was a statistically significant downward trend for annual mean total phosphorus concentration in five subwatersheds and an upward trend for chloride in three subwatersheds. The decrease in total phosphorus concentration occurred primarily in winter. Trends in phosphorus and chloride concentrations did not appear to be related to land use. There were no statistical differences in annual streamflow or nutrient loads expressed on a per area basis among the nested subwatersheds. Annual and seasonal flow-weighted mean concentrations were different among the subwatersheds for nitrate-N and chloride. The larger subwatersheds had significantly higher nitrate-N in winter and spring. The nutrient loads and concentrations from these subwatersheds were an order of magnitude less compared to other agricultural watersheds. Conservation practices were implemented on 11% of the watershed area from 1980 to 2003; however, the affects of the practices on watershed water quality was not clear. Earlier short-term studies attributed the low levels of nutrient transport to the presence of extensive riparian forests and the general prevalence of forest in these mixed land use watersheds.