Effects Of Optimized Land Use Allocation On Phosphorus Loss In Small Watershed During Rainfall Events

The Three Gorges Reservoir area has a unique geographical location,complex relationship between man and land,and its water environment safety is of great concern.Soil erosion and non-point source pollution are important threats to the water environment safety of the Reservoir area.There are many hills and mountains in the Three Gorges Reservoir area with abundant rainfall,and there are many small watersheds.During the rainy season,a large amount of runoff is collected from each small watershed into the water body in the reservoir.Elements are an important part of the non-point source pollutants in the Three Gorges Reservoir area.Phosphorus is often lost through runoff and sediment as the carrier.At present,most studies on non-point source pollution in small watersheds focus on the loss of phosphorus in daily monitoring of single or multiple land use types or during the simulated rainfall process.Quantitative research on the effect of land use optimization on phosphorus loss in the runoff process needs to be further studied.This study selects the Shipanqiu small watershed in Zhongxian County of the Three Gorges Reservoir area and two comparative watersheds in the watershed:the traditional agricultural model watershed（CG）and the land-use-optimized watershed（EG）as the research objects.Monitor the changes in runoff,sediment,and different forms of phosphorus concentration and output during the secondary rainfall process,and explore the law of phosphorus loss in small watersheds during the secondary rainfall process and the impact of land use optimization on phosphorus loss during the rainfall runoff process.It provides a reference basis for the control and management measures to be taken to control the concentration and load of phosphorus loss in small watersheds.The main findings are as follows:（1）At the small watershed scale,the average concentrations of total phosphorus（TP）,total dissolved phosphorus（TDP）,dissolved inorganic phosphorus（DIP）and particulate phosphorus（PP）in the 10 monitored rainfall events are 0.249,0.169,0.092and 0.080 mg·L^-1,respectively,the TDP/TP concentration ratio ranged from 0.48 to0.81,and the field is 0.67.In the first 40 minutes,the main form of loss was PP,and then TDP gradually dominated.The peak concentrations of TP and PP were significantly correlated with the peak runoff（P<0.05）.41.07,17.17 and 28.73 g·hm^-2,in which the TDP/TP output ratio was 58.84%,indicating that the main phosphorus loss form during the secondary rainfall in Shipanqiu small watershed was soluble phosphorus.（2）In the small watershed of Shipanqiu,the cumulative phosphorus flux and cumulative runoff have a quadratic function relationship in the process of secondary rainfall and runoff.It can better predict phosphorus output;according to the initial scour effect,the efficiency of reducing phosphorus loss is higher when intercepting 30%to 40%of the runoff sediment in the early stage of runoff,and the interception measures can be comprehensively formulated according to the economic cost.（3）During the monitored 10 rainfall-runoff processes,the TP concentration of EG was lower than that of CG,and the two ranges were 0.09-0.75 mg·L^-1 and 0.13-2.82mg·L^-1 respectively;Reduced peak TP concentration.The average EMC concentrations of TP,TDP,DIP and PP of EG were lower than those of CG during the rainfall-runoff process,and EG and CG showed significant differences in the EMC of TP,TDP and DIP（P<0.05）.The main form of phosphorus loss in both catchments during rainfall runoff was TDP,but the TDP/TP in the EG catchment was larger and DIP/TDP was smaller.（4）The output loads of TP,TDP,DIP and PP of EG were 45%,43%,57%and 47%lower than that of CG.The output loads of phosphorus in various forms of EG and CG were significantly correlated with the total runoff（P<0.01）,the slope of the linear fitting of CG of various forms of phosphorus and the total runoff is 1.66-1.75 times that of EG,the output load of CG of various forms of phosphorus is more sensitive to the response to runoff than EG,and the sediment yield per unit area in the process of rainfall and runoff is The difference in phosphorus concentration and output load can be more caused by the quantity than the yield per unit area.Optimizing land use allocation can effectively reduce the loss of various forms of phosphorus,and provide a reference for the prevention and control of phosphorus loss in small watersheds in the Three Gorges Reservoir area.