Abstract:

Nutrients (mainly as nitrogen and phosphorus) transported from watershed is one of the most important nutrients sources for lake eutrophication. Assessment and control of the watershed exogenous nutrients has been the key to resolve the problem of lake eutrophication. The Taihu Lake Basin,located at the lowerreach of the Yangtze River,with a watershed area over 36,000 km2 was chosen as the study area for its serious eutrophication. Precious studies of this area were either on small scale of subwatershed simulation,or using monitored data to estimate the nutrients loading statistically. Numerical simulation over all the upstream has seldom been done before. In this study,the watershed distributed hydrological model SWAT 2005 (Soil Water Assessment Tool) was applied to simulate the watershed nutrients transportation during 1987-2007,to assess the nutrients transportation trend,spatial and temporal distribution. Database composed of topography (1∶250 000),climate (daily precipitation,temperature,humidity and wind speed),hydrology (flow speed,flow flux),soil (1∶1000 000 with physical and chemical properties),landuse/cover (1∶100 000) and variety of nutrient sources were established to simulate the nutrients loading. Both natural process and human activities within the basin were considered,such as reclamation of lake,the construction of reservoirs. And the simulation was divided into two experiments. The hydrological simulation was the most basic one,and then the nutrients simulation,with background,nonpoint source pollution (agricultural),and point source pollution were added and simulated in a specific sequence. Data from hydrological gauge station and the monitored data were used for the validation and calibration of simulation. Correlation coefficient (R) and NashSutcliffe efficiency coefficients (Ens) were introduced to evaluate the simulation results. In calibration,the correlation coefficients of average monthly value were all over 85%,the efficiency coefficients were between 0.24 and 0.90,and the relative error was 3.2%. In validation period,the correlation coefficients of average monthly value were all over 80%,and the relative error of annual value was 8.1%. And both the hydrological and nutrients simulation could match well with the data in references. The simulation results showed that the annually averaged runoff of the Taihu Lake Basin was 104.15×108m3. The nutrients production was 42 730 t/a for TN,3 075 t/a for TP ,with a trend of decreasing after 2000. The simulated concentrations of TN and TP in Zhexi area were 2.38 mg/L and 0.18 mg/L,respectively; 5.68 mg/L and 0.43 mg/L in Huxi area; and 6.14 mg/L,0.40 mg/L in Wuyuxicheng area. The simulation results futher showed a significant spatial variation. And among all the upstream input rivers,Chen DonggangDa Pukou,Xiao MeikouChang Dougang,Wu JingangZhi Hugang were of the most importance,with the contribution ratio of 27%,24% and 20% respectively. The area mentioned above should be regarded as the key areas for external nutrients source control. The study results could provide basic data for watershed nutrients total amount control policy