Abstract:

Diffuse water pollution has become one of the most important factors for environmental pollution. Research on emission, transportation, transformation, and management of nonpoint source water pollution is helpful for effectively preventing lake pollution. The Leanhe Watershed is a typical agriculturalbased watershed of Poyang Lake watershed. The nonpoint source pollution in this watershed impacts water quality issue which influences ecology and society. Based on observed data from 17 monitoring stations within the Leanhe Watershed from 2009 to 2011, the temporal and spatial variation patterns were analyzed. The WATLAC Model and MESAW Model are combined to simulate the nutrient emission and transformation in the Leanhe watershed, and to calculate the nutrient load and retention rate in river course. The main conclusions of this paper are as follows. (1)The distribution and seasonal changes of nitrogen and phosphorus are basically similar. They increased from April by the beginning of rainfall and agricultural season, reached a high level in June, and then dropped dramatically to the lowest value in September for annually maximum water volume in the Leanhe Watershed. After September, the values recovered for the dry season and reached the highest point for the annually minimum water value int December. The nutrient concentrations were generally higher in downstream than in upstream. This result indicated that agricultural fertilization was the main source of diffuse N in soils, the rainfall runoff was the main driving factor for Nlosses, and forest land had an obviously constraint effect on nonpoint pollution. (2)Nonpoint source based nitrogen and phosphorus emission and transformation in the Leanhe watershed was simulated by the MESAW Model. The result shows that: the nitrogen emission from the Leanhe Watershed was 10 4380 t/a, the contribution to the watershed mouth was 5 855.7 t, and the retention rate of nitrogen in the Leanhe Watershed was 439%. In terms of phosphorus, the emission from the Leanhe Watershed was 1 0037 t/a, the contribution to the watershed mouth was 3449 t, and the retention rate of phosphorus in the Leanhe Watershed was 656%. Those data indicated the nitrogen had higher emission and contribution value than phosphorus because of its higher solubility and fluxility. The main physical form of phosphorus was particle, which could be detained by sedimentation. Phosphorus had therefore a higher retention rate than nitrogen. (3)The retention rates of nitrogen and phosphorus in the wet season were higher than in the dry season, because less precipitation in the dry season leaded to a lower amount of runoff flowing down the river course slowly. However, the storm in June of 2011 caused huge dissolved nitrogen going into river course in short time. The N retention rate in this season was 15%, lower than that in 2010. The storm also moved some particle phosphorus that has been absorbed in the soil for a long time to the river, so P retention rate in this season was 10%, higher than that in 2010.