Spatial-temporal Simulation Of Surface Runoff Nps Load And Its Environmental Effect Based On L-THIA Model

With the developing of social and economic lead to the environmental problem is more and more serious nowadays, environmental pollution becomes a big challenge in world. So it is particularly important to choose the appropriate ways and methods to solve environmental crisis. Non-point source pollution is mainly affected by the formation of natural and social factors such as rainfall, land use. The unreasonable land use conversion and management mode will cause the watershed nutrient with runoff loss, causing soil erosion, soil and water loss, watershed pollution. Environmental effect caused by the change of land use in different space and time scales become the hotspot of geography research.This study selected two sub watershed of Songhua River basin(Hulan River watershed and the Ashihe watershed, hereinafter referred to as the Hu-A watershed) as the study area. The land use change and the driving force from 1980 to 2010 in the watershed are analyzed. Non-point source pollution caused by land use change in the four periods were simulated and estimated, and achieved the quantitative evaluation for NPS pollution in this study area, the main pollutants were also identified. On this basis, combined with system dynamics(SD) and changes of land space model(CLUE-S), the land use change and NPS pollution load are simulated and forecast, the environmental effect of Songjiang wetland caused by the NPS pollution load of the Hu-A watershed are analyzed, to further explore best management practices strategies(BMPs) of land use in Hu-A watershed and put forward a preliminary countermeasures for NPS pollution control and management. Through the analysis of the impact of hydrological processes on aquatic in Songjiang Wetland, the reasonable suggestions for wetland protection and restoration are also given.(1) Single-factor water quality identification indices and comprehensive water quality identification indices are used to quantitative evaluate the water environment quality in Hu-A watershed. The result showed that the comprehensive water quality index was 3.74, the water quality is in Grade III according to the standard of national surface water quality. The results of single-factor water quality identification showed that the main pollutant is TN, TP and Hg. According to the health risk assessment model for heavy metals in study area, the result shows that the total risk of contamination of the assessed value is 1.0939×10-5, the lower harm to human exposure risk.(2) Based on the establishment of watershed spatial and attribute database,according to the status of land use and water quality, the parameters are modified and corrected, runoff and NPS pollution load are simulated. The results showed that the simulation error of runoff is within 10 %, and the runoff change in the range of 2.15×109 m3 to 3.5×109 m3. During the 30 years, total nitrogen increased by 64.56 %; total phosphorus increased 55.48 %; heavy metals(Zn, Cu and Pb) increase the load 34.61 %. Although the agricultural land is the main output of NPS pollution load of land use type, but the construction land area increased and become the largest "contributor" to pollution load.(3) Using SD and CLUE-S models to predict river watershed land use change by three different spatial pattern in 2025. The accuracy results show that the error less than 8 % by using the SD model of watershed land use demand, CLUE-S models that the value of kappa is 0.63, show that the three different scenarios of land use spatial pattern has bigger difference, particularly in the social economy rapid development pattern the degree of conversion of land use change dramatically. Based on L-THIA model for three kinds of land use scenario non-point source nitrogen and phosphorus pollution load results showed that under the condition of social and economic rapid development TN and TP pollution load increase speed than other scene mode.(4) Based on the simulated results of Songjiang wetland pollution load show that TN values from 1980~2010 increased by 51.73 %. The amplitude of TP form 1980 to 2010 increase for 55.56 %. Through the wetland hydrological processes and aquatic biological relevance analysis found that the change of runoff and the aquatic organisms was negatively correlated. Through the results of land use scenario simulation for the two river watershed in the future, and the results of Songjiang Wetland hydrological processes of nitrogen and phosphorus load simulation. In this paper, we discuss the management mode of land use, and try to apply BMPs make a preliminary advice for the reasonable planning of watershed land use change, as well as try to make a preliminary countermeasures for Songjiang Wetland hydrological process control.