Analysis Of Runoff Change And Contribution Rates Of Its Impacting Factors In Dakeng Small Watershed

The quantitative study of the effects of climate and vegetation changes on the runoff in watersheds is a hot issue in current hydrological studies.Due to the differences in the time and space scale of the study area,whether the effect of vegetation change on runoff is positive or negative under the background of climate change is still controversial.And most studies calculate the impact of various factors on runoff on annual scale,while less research on seasonal scale.Seasonal scale impact analysis has important implications for the reasonable allocation of water resources during the year,especially in the presence of seasonal droughts.Thus taking the Dakeng small watershed in Northwest Jiangxi as example,based on the observed daily precipitation,runoff,and evapotranspiration data from 1966 to2016 at Dakeng hydrological Station,the changes of hydrological and meteorological factors,such as precipitation,runoff depth,evapotranspiration and temperature,were analyzed by the comparison of this watershed itself.Combined with time series method,empirical statistics and GIS technology,the trend of vegetation change in the past years was qualitatively analyzed.The contribution rates of impacting factors such as precipitation,evapotranspiration and vegetation on runoff change were quantitatively calculated by SCRCQ?slope change ratio of cumulative quantities?method.Ultimately,the response of hydrological effect caused by vegetation change to precipitation was initially discussed.The main conclusions are as follows:1)Annual temperature,evapotranspiration,precipitation and runoff depth both share a rising trend.And through Spearman analysis,only temperature rose significantly.The results of the cumulative anomaly test showed that two consistent abrupt points of 1987 and 1999 were found both for annual precipitation and runoff depth.As a result,year 1966 to 2016 were divided into three periods,one baseline period from1966-1987?T₁?with pure effect of climate change when the vegetation condition was ideal,two change periods of 1988-1999?T₂?and 2000-2016?T₃?when runoff was double impacted by climate and vegetation change.2)On annual scale,compared to the baseline period,the contribution rates of precipitation,evapotranspiration and vegetation changes to runoff increases and decreases during the T₂ and T₃ periods were 58.4%,3.2%,38.4%and 52%,12.0%,36%respectively.Compared with the period of T₂,the contribution rates to the reduction of runoff in T₃ were 63.6%,4.4%and 32%,respectively.It can be seen that the main driven factor for annual runoff changes in Dakeng small watershed is precipitation,followed by vegetation and evapotranspiration.On seasonal scale,however,there are large differences in the contribution rates to runoff changes in different seasons,and even the opposite.In general,the main driven factor for runoff in Spring and Summer is precipitation,while in Autumn and Winter with limited rainfall and lower temperature,vegetation is the main factor that regulates runoff in dry seasons.3)The vegetation in Dakeng small watershed has experienced severe deforestation and subsequent restoration for many years.At present,the vegetation coverage in the whole watershed has reached a high level.The hydrological effect of vegetation change has a critical value in response to precipitation.Before and after the critical value of precipitation,the effect of vegetation change on runoff changes is completely opposite.It may be possible to explain the difference in the contributions of vegetation changes to runoff in different watersheds and river basins,and it may be helpful to find out the reasons for the controversy and disagreement about the impact of forests on runoff.