Analysis Of Runoff Generation Mechanism In The Upper Huai River Basin Under Changing Environment

In recent years,with the increasing of human activities and the dual impact of climate change,the hydrological elements of typical watersheds have also changed accordingly,especially in the change of runoff yield.Research on the impact of different influencing factors on runoff changes under changing environment has attracted more and more scholars’ attention.In this context,in order to explore the changes and causes of runoff in typical river basins,and achieve the rational allocation of regional water resources and the coordinated development of regional ecology.Based on the long-term hydrological and meteorological data,the upper Huai River Basin(above Wangjiaba station)was selected as study area,the Mann-Kendall trend analysis and Pettitt mutation test method were examined to test on long-term series of the hydrological elements,and then divided the period according to the mutation test results to carry out phased analysis.The elastic coefficient method based on Budyko hypothesis quantitatively was used to calculate the contribution rate of human activities and climate change to runoff change in the upper Huai River(above Wangjiaba station).Based on the principles of hydrology and various modern statistical methods,this paper reveals the response law of flood characteristics in different stages of a typical watershed,and comprehensively analyzes the dominant runoff generation mechanism and pattern of a typical watershed.GIS-DRP method was used to analyze the spatial distribution map of dominant runoff generation mechanisms in the study area,and the distribution proportion of various dominant runoff generation mechanisms was further calculated which is verified with the previous research results.The main research results are as follows:(1)The characteristics of the underlying surface conditions,rainfall and runoff changes in the upper reaches of the Huai River basin(above Wangjiaba station)were analyzed.From 1990 to 2020,due to the impact of human activities and the continuous influence of national policies such as returning farmland to forests and returning farmland to lakes,the area of arable land in the upper Huai River basin(above Wangjiaba station)has been decreasing.The area of forest land decreased first and then increased.On the contrary,the grass area increased first and then decreased,and the area of water bodies and cities and towns shows an increasing trend.Analysis of trends and abrupt changes in hydrometeorological series data such as rainfall and runoff indicate that the inter-annual variation of rainfall shows an insignificant downward trend and no sudden changes,while the runoff exhibits a decreasing and significant change trend and the year 2007 was detected as the mutation point after abrupt change test.(2)After dividing the base period and the mutation period before and after the mutation year,the contribution rate of human activities and climate change to the runoff reduction was quantitatively calculated by the elasticity coefficient method based on Budyko theory.Compared with the first stage before the mutation point,the contribution rates of climate change and human activities to the runoff changes in the upper Huai River basin(above Wangjiaba station)are 23.6%and 76.4%,respectively.Human activities are the main driving factors that cause runoff changes in the study area.(3)The dominant runoff generation mechanism and runoff pattern of 161 flood event in two stages of the study area were analyzed.There were 82 floods in the first stage,and the floods dominated by the excess-infiltration pattern accounted for 59.8%.The proportions of excess-storage runoff and mixed runoff were 21.9%and 18.3%;the 79 floods in the second stage were also dominated by excess-storage runoff,but compared with the first stage,the proportion has increased by 7.3%to 67.1%,the proportion of mixed runoff was 22.8%,with an increase of 4.5%compared with the first stage,while the proportion of excess-infiltration runoff decreased significantly(11.8%).The results show that the components of flood runoff events in the basin have changed(4)The changes of runoff generation mechanisms in typical watersheds were analyzed.The main influencing factors are rainfall,evaporation and underlying surface conditions.This article has not considered the evaporation factors,and chooses rainfall and underlying surface.Conditions are analyzed as influencing factors.It is concluded that the rainfall change trend isn’t obviously,and it is inconsistent with the change of the flood characteristic value,which indicates that the change of underlying surface conditions is the main factor that causes the change of runoff mechanism.(5)The GIS-DRP method was applied to verify the results,and generate the spatial distribution map of runoff generation mechanism in typical watershed.According to statistics of the proportions of various runoff components,the proportion of underground runoff is the highest,accounting for 72.6%,followed by soil flow,accounting for 15.7%,and the proportions of super-permeable surface runoff and saturated surface runoff are 6.1%and 5.6%.The dominant runoff generation mechanisms in the study area are Rg and Rint,which makes the runoff generation patterns are mainly excess-storage runoff and mixed runoff.