Runoff Source Analysis And Nitrate Output Characteristics Under Different Rainfall Regime

Nitrate is the main form of non-point source nitrogen pollution in watershed,which has high solubility and easily migrates with runoff into rivers,lakes and other receiving water bodies.Excess nitrate can cause eutrophication in water,threatening water ecological health and water environmental security.Rainfall-runoff is an important driver and carrier of nitrate migration to receiving water bodies.The high variability in antecedent soil moisture and rainfall characteristics complex the generation and concentration of runoff in the watershed,resulting in variability and uncertainty in nitrate export under different types of storm events,limiting the understanding of the hydrological driving mechanisms of nitrate export from the watershed.Therefore,clarifying the watershed runoff response and nitrate output patterns under different antecedent soil moisture and rainfall characteristics can help reveal the hydrological driving mechanism of nitrate transport in the watershed can help to reveal the hydrological driving mechanism of nitrate transport in the watershed,which is of great scientific significance and practical value for the development of watershed nitrate pollution prevention and control strategies.Given this,we conducted this study in Wulongchi catchment,located in Danjiangkou reservoir,which is the water source area of the middle route of South-to-North Water Diversion Project.The storm events during the study period were classified into four types based on the antecedent soil moisture and rainfall intensity:events that were occurred under wet antecedent soil moisture conditions and dominated by volume（Wet-Vol）,events that were occurred under wet antecedent soil moisture conditions and dominated by intensity（Wet-Int）,events that were occurred under dry antecedent soil moisture conditions and dominated by volume（Dry-Vol）,events that were occurred under dry antecedent soil moisture conditions and dominated by intensity（Dry-Int）.To understand the runoff response characteristics of different types of storm events.We quantified the proportion of pre-event water and event water in the runoff of different types of storm events using the Mix SIAR model with hydrogen and oxygen stable isotope techniques,then analyzed the changes in runoff sources in the watershed.The concentration-discharge relationship（C-Q）of nitrate was used to determine the nitrate export patterns of different types of storm events,and the influence of each environmental factor on the nitrate export of the watershed was analyzed by correlation analysis.The main results are as follows:（1）The antecedent soil moisture and rainfall intensity affected the generation and concentration of runoff in a watershed.The Wet-Int events had an advantage in watershed runoff generation and runoff path activation,with the highest runoff,peak and average flows.The Dry-Int and Wet-Vol events had intermediate runoff production capacity due to a lack of effective runoff paths and rainfall inputs.The Dry-Vol event was dominated by runoff yield under saturated storage and shallow subsurface flow,its runoff path activation was restricted,the total runoff was the lowest,and the event average discharge was 26.6%less than the average value of discharge during the monitoring period.（2）Pre-event water was the main source of runoff in the watershed,and the average contribution of pre-event in runoff was 67.29%.The wetter substrate of the watershed and higher degree of hydrological connectivity leads to the richer subsurface runoff paths for prior water transport.The runoff yield under excess infiltration under heavy rainfall intensity conditions increased the proportion of event water.The largest percentage of pre-event water was made by the facilitated transport of pre-event water and the restricted input of event water in Wet-Vol events,reached 69.49%.Storm during the Wet-Int events increased the proportion of event water（32.68%）.The Dry-Vol and Dry-Int events showed a common constraint and common increase in the transport of pre-event water and event water,so their runoff sources were relatively similar.（3）Nitrate export was controlled by antecedent soil moisture and rainfall intensity by altering the hydrologic connectivity of the watershed and activating the nitrate source.The highest median nitrate concentration was 4.51 mg/L in the Wet-Vol events,and the highest median nitrate load was 5.53 kg/h in the Wet-Int events.The lowest median nitrate concentration,and the median nitrate load occurred in the Dry-Vol events.The variability of the nitrate export pattern decreased with increasing rainfall intensity and gradually converged to the chemostasis pattern.As the proportion of pre-event water increased,the nitrate export pattern showed a change in the dilution-chemostasis-mobilization pattern.A large number of runoff pathways were activated during rain intensity dominated events,and the uncertainty of activation in the nitrate source region was reduced to a chemostasis pattern and a dilution pattern.The nitrate export pattern of the Wet-Vol events was dominated by the mobilization pattern and the chemostasis pattern,and the Dry-Vol events were dominated by the dilution pattern.（4）The results of the correlation analysis between nitrate concentration and each environmental variable showed that,for all rainfall events,initial nitrate concentration,and precipitation 3 days before the event（AP₃）were important influencing factors.In the Wet-Vol events,nitrate concentration was increased with the initial nitrate concentration,t precipitation 3 days before the event（AP₃）,and the proportion of pre-event water.The rainfall duration,the duration of peak attainment,and the average flow of Dry-Vol events were positively correlated with the nitrate concentration.In summary,different antecedent soil moisture and rainfall intensity affected the runoff generation and the establishment of hydrological connectivity in the watershed,and then altered the activation of runoff sources and nitrate source areas in the watershed,resulting in variability in nitrate export patterns.The results of the study will help to further understand the hydrological driving mechanism of nitrate export in the watershed,and provide the scientific basis for safeguarding water quality and protecting the water ecological environment.