Assessing nonpoint phosphorus control in the laplatte river watershed

Abstract

Phosphorus loading from agricultural activities such as manure and fertilizer applications often contributes to eutrophication of surface waters. The primary goal of the LaPlatte River Watershed Project in northwestern Vermont was to reduce phosphorus loading from farmland through implementation of best management practices (BMPs). Eleven years of monitoring did not show a dramatic decrease in phosphorus concentration or load from the watershed. However, analysis controlling for hydrologic variability suggested significant decreases in phosphorus load from some subwatersheds following completion of the land treatment program. Post-BMP phosphorus load reductions of 26–44% (0.01–0.14 kg/ha/yr) were estimated using a paired regression technique that accounts for discharge differences between years. Phosphorus export was reduced under most circumstances, except under the highest runoff conditions, suggesting that the capacity of the land treatment system to control phosphorus may have been exceeded occasionally. Observed phosphorus reductions in treated watersheds appeared to be related to the degree of treatment after a minimum threshold level of land treatment had been achieved. © 1993 Taylor and Francis Group, LLC.