Study On Agricultural Non-Point Source Pollution And Its Ecological Effects In Qiandao Lake Watershed

As the agricultural NFS is the most important factor for water environmental deterioration and lake eutrophication in China, more and more scientists and technologists focus on the control countermeasurc and pollution mechanism of agricultural non-point source (NFS). Based on the investigation of natural resource and agricultural distribution, the loss rule of nitrogen and phosphorus in the typical land of Qiandao Lake watershed and distribution rule of the phytoplankton and physico-chemical characteristic in the lake were studied, and the input loading of nitrogen and phosphorus was also calculated by the AnnAGNPS model in this thesis.According to the routine monitoring data of the water environment in Qiandao Lake from 1989 to 2001, the concentrations of toxic substance and heavy metal were under the monitoring limit, but the total nitrogen (TN) and total phosphorus (TP) were much higher than the criterion for lake and reservoir. In spatial distribution, the water in Xin'an River and southwest of lake was polluted seriously than that in other parts of lake.The results of monitoring, from June to August in 2001, showed that the average concentration of nitrogen and phosphorus in runoff varied strongly, the loss concentrations in the cultivated lands such as sweet potato land and orchard land were higher than those in uncultivated lands such as grassland and woodland. The correlation between daily rainfall and N and P concentrations under different sloping land use pattern reached 95%, which showed that the rainfall played an important role in nutrient loss from soil.Annual average input loading was simulated by AnnAGNPS model. The results showed that there were 2380.879 tons sediment-attached nitrogen, 10432.959 tons soluble nitrogen, 40.922 tons sediment-attached phosphorus and 295.679 tons soluble phosphorus inputted into the Qiandao Lake per year. Monthly inputted loading changed very strongly, the loading was interrelated positively with rainfall and 70% of total pollutant was exported from the watershed during the April and September.According to the 2 years monitoring data of the Qiandao Lake, TN and TP concentrations monthly varied strongly, TN and TP concentrations in spring were higher than other seasons, the variation tendency was: spring > summer > autumn> winter, and the spatial distribution of concentrations was: inlet part> inside part> outlet part. The relationship analysis showed thatthe TN concentrations and TP concentrations in Xin'an River came from the agricultural non-point source.The spatial and temporal concentration of chlorophyll a in the lake varied very strongly, the concentrations in the sites located in Xin'an River and effected by human activity were raised obviously from April to August, and were much higher than those in other sites. Monthly average concentration for the lake has the same tendency as the rainfall.According to the modified Carlson's trophic state index, the lake has recently been varied between oligotrophic and mesotrophic.Cyanophyta, Bacillariophyta and Crytophyta were the superiority population in Qiandao Lake, but Cyanophyta was the superiority population in algae bloomed month such as March, June and July. Compared with 1992, the superiority population has changed. The quantity of algae in Qiandao Lake varied strongly in different season, summer> autumn> spring> winter. The quantity in the sites effected by human activity were much higher than that in other sites.