Transport Characteristics Of Phosphorus Driven By Rainfall In The Wulongchi Agricultural Watershed

Phosphorus is not only an indispensable nutrient element for organisms in the ecosystem,but also a limiting factor that leads to eutrophication of water bodies.Runoff deduced by rainfall is a significant driving force for phosphorus migration.However,due to the randomness of rainfall-runoff process,the difference and complexity of phosphorus pollution occurrence and migration process,lead to the uncertainty of phosphorus output time,frequency and composition.In addition,it is impossible to capture the variation characteristics of phosphorus output flux in each stage of the rainfall-runoff process due to the lack of high-frequency monitoring data.At present,the output law of phosphorus and its components with runoff remains to be further clarified.It is of great scientific significance and practical value to identify the migration law of phosphorus and its components in rainfall runoff for understanding the hydrological driving mechanism of phosphorus transport and formulating the control strategy of watershed non-point source pollution.To fill these knowledge gaps,this study was conducted in Wulongchi watershed,which is a typical agricultural small watershed.The objects of this study were to:(i)analyze the response characteristics of runoff to different rainfall events by monitoring the variation characteristics of rainfall and flow;(ii)clarify the output characteristics of each phosphorus component under different rainfall conditions,combining with high-frequency monitoring of phosphorus concentration data;(iii)determine the phosphorus concentration-flow hysteresis modes of each phosphorus component using hysteresis index and flushing indexes,and the driving factors of different hysteresis modes were clarified by redundancy analysis and hierarchical segmentation.The main results are as follows:(1)The rainfall in the basin was mainly short-duration light rain,accounting for67.5%of the rainfall in the whole monitoring period.The previous rainfall has a great influence on the peak flow,average flow and runoff duration in heavy rain events.The shorter the flood peak delay time.In addition,there was an obvious threshold effect between runoff and humidity in the watershed.When the watershed humidity was greater than 15 mm,a watershed runoff event occurs.(2)Phosphorus components showed initial flushing effect during rainfall events,and showed dynamic changes at different rainfall stages.In the early stage of rainfall,the concentrations of TP,TDP and PP increased with the increase of discharge,and gradually decreased in the middle and late stage of rainfall,and the peak concentration mostly appeared in the early stage of rainfall.Overall,the change trend of TP,TDP and PP concentrations was consistent with the change trend of discharge.Under different rainfall events,the PP output flux accounted for 48%-80%of the TP flux,and the PP output flux increased from 52%in light rain events to 76%in heavy rain events.The ratio of PP output flux to TP flux increased with rain intensity,and the two showed a significant linear relationship(R~2=0.73,p<0.05).(3)The hysteresis patterns of phosphorus concentration-discharge varied with rainfall type.Most of the 8-figure hysteresis patterns appeared under light rain events,and the hysteresis of moderate rain and heavy rain was complex,while the hysteresis of phosphorus concentration and flow was clockwise or counterclockwise under heavy rain.The results of redundancy analysis show that the characteristics of rainfall and runoff could explain the hysteresis pattern of concentration-discharge in phosphorus components,and the cumulative explaining degree of the first two axes of TDP hysteresis mode was 68.6%,followed by TP(46.7%)and PP(40.9%).The hierarchical segmentation results showed that the factors with the highest explanatory power for the hysteresis patterns of concentration-discharge in TP,TDP,and PP were average flow(21.8%),runoff duration(17.4%),and previous rainfall(23.2%).This study takes a typical agricultural small watershed—Wulongchi watershed as the research object,analyzes the response characteristics of runoff to different rainfall events,and the output characteristics of typical rainfall events to phosphorus components,and then reveals the phosphorus concentration-flow of each component in the rainfall process.The hysteresis relationship and its driving factors.The trend and persistence of rainfall and runoff sequences in watersheds are consistent;each component of phosphorus exhibits an initial scour effect during rainfall events,and exhibits dynamic changes in different stages of rainfall;the hysteresis pattern of phosphorus concentration-flow is closely related to rainfall types related.The research results are helpful to deeply understand the hydrological driving mechanism of non-point source pollution,and provide a scientific basis for the formulation of non-point source pollution strategies in agricultural watersheds in China.