Determining phosphorus loading rates based on land use in an urban watershed

Abstract

Non-point phosphorus (P) loading to surface water can degrade water quality and impair habitat. As urban areas continue to grow in the United States, critical P source loading areas need to be identified to assess their impact on water quality. A 332 ha urban watershed in Ithaca, NY was selected and monitored for two years, with above average precipitation. Runoff collected from 98 precipitation events and three land uses was analyzed for dissolved P (DP), particulate P (PP), and total P (TP), with mass losses calculated. Monitored land uses included fertilized lawns (FL), urban barren (UB) areas and wooded (FR) areas. Stream gauges were installed at the stream entrance to the urban area and the watershed outlet to monitor the impact of the urban area on stream water quality. A multivariate analysis of the data revealed that the FL land use had higher DP losses than the other land uses on the shallow, low storage, runoff prone urban soils. If runoff volumes were low, the FL land use had similar or considerably lower DP losses. Particulate P mass losses were highest from the FR and UB land uses due to little or no ground cover to prevent erosive P losses. Total P losses from the land uses were highest on the shallow, low storage soils and surprisingly similar among land uses. As the stream flowed through the urban area, there was a two fold increase in P loads when compared to the undeveloped upper area of the watershed. This indicated that the urban area was contributing P to the stream. Best management practices in this and other similar watersheds should focus on reducing runoff and P loss from source areas to realize the largest reduction in P loading to surface water. Ultimately, it is imperative to assess land use performance under varying conditions in order to reduce P loads in surface water. © 2008 American Chemical Society.