Characteristics Of Multi-scale Phosphorus Migration In Typical Small Watershed In Hilly Area Of Central Sichuan Basin

The hilly area of Middle Sichuan is an important farming area in the upper reaches of the Yangtze River.The phosphorus enriched in the soil enters the water body through runoff,slope erosion and leaching,which can accelerate the eutrophication of the receiving water,and bring great pressure to the sustainable development of the Three Gorges reservoir ecosystem.However,the research on non-point source phosphorus pollution in agriculture is mainly focused on single land or small watershed scale,and less research on the scale of land and small watershed at the same time.It is very important to carry out multi-scale research on non-point source phosphorus pollution in agriculture.Through the 3 years?2015-2017?field location monitoring of natural rainfall runoff,the paper selects four typical rainfall events of 20170816 rainfall events?medium rain?,20160731 rainfall events?heavy rain?,20160713 rainfall events?rainstorm?and20160718 rainfall events?downpour?for studying the influence of different scales on runoff coefficient,sediment concentration and loss of load,phosphorus concentration and migration load,and discussing the mechanism of land use types and different scales on agricultural non-point source phosphorus pollution,and revealing the sources,transport processes and migration loads of phosphorus pollution at different scales and in order to provide references for the prevention and control of agricultural non-point source phosphorus pollution.The main results and conclusions are as follows:?1?There are differences in runoff coefficient between plot scale and small watershed scale.The runoff coefficient of 6.5°plot?100m²?is generally the largest,and the runoff coefficient of Dayangdi plot?2200m²?is the smallest in plot scales and Surong small watershed based on residential area?3.0hm²?>Jieliu small watershed based on cultivated land?34.6hm²?>Daxing small watershed based on cultivated land and woodland?480.3hm²?>Wanan small watershed based on woodland?1236.4hm²?in small watershed scales,and the runoff coefficient of block scale is not necessarily larger than that of small watershed scale under four rainfall types.?2?Phosphorus is mainly transported by particulate phosphorus?PP?,and the average concentration and migration load of dissolved phosphorus?DP?and phosphate(PO₄^3-)are all small and PP migration load generally accounts for more than 70%of total phosphorus?TP?migration load under four rainfall types.?3?Sediment loss load and phosphorus migration load have obvious spatial and temporal scale effect.On the spatial scale,the sediment loss load and the phosphorus migration load are larger in the smaller scale and small watershed,which shows the spatial scale effect.On the time scale,the sediment loss load and the phosphorus migration load in the rainy season?6-9 months?are significantly greater than the other months,indicating the time scale effect and rainfall is the main driving factor of agricultural non-point source phosphorus pollution.?4?The sediment load and phosphorus transport load in the 6.5°and Surong small watershed are found to be the highest by studying the transport law of rainfall runoff-sediment and phosphorus under the different scales and rainfall types.The scale of plots which the sediment loss load and the phosphorus migration load is 6.5°?100m²?>ST60?7.5m²?>Changpodi?1500m²?>Dayangdi?2200m²?and the small watershed is Surong small watershed based on residential area?3.0hm²?>Jieliu small watershed based on cultivated land?34.6hm²?>Daxing small watershed based on cultivated land and woodland?480.3hm²?>Wanan small watershed based on woodland?1236.4hm²?.That is the 6.5°plot?100m2?and the Surong small watershed?mainly residential area?is the key sources of sediment loss and phosphorus migration.Therefore,selecting large plots for agricultural production activities is an important measure to reduce the pollution of non point source phosphorus in agriculture,controlling the loss of sediment and phosphorus at residential area is an effective way to reduce phosphorus into the water body,and the increase of the proportion of woodland is an important measure to reduce the loss of sediment and phosphorus.