[Risk assessment of nitrogen and phosphorus export in upper reach of Daliao river watershed]

With the development of point pollution control, non-point source pollution has become an important environmental and water quality management problem. Land cover composition across a watershed is a dominant factor in controlling the amount of nitrogen and phosphorus exported from a watershed. Generally, urban and agricultural land covers are considered as principal sources of excess loads of nitrogen (N) and phosphorous (P) in receiving waters. A well developed literature of nutrient export coefficients by land-cover class was used to model the risk of equaling or exceeding specified levels of nutrient export in drainage basin of Dahuofang Reservoir. The model was applied to about 513 comparatively small watersheds mapped for the drainage basin of Dahuofang Reservoir for environmental analysis and planning. The results suggest that the probabilities of risk are 19.31% and 8.95% for N and P nutrient respectively. As the spatial distribution concerned, risk estimates generally differed with different slope degrees and districts. Risk estimates generally increased from the places where lies near the rivers to the places where are far from the river, but numerous areas of high variability were evident.