Impact Of Soil And Water Conservation Measurements And Precipitation On River Runoff And Sediment Based On VAR Model In The Typical Area Of Yellow River

Soil and water loss,water resource shortage,flood hazard and the hanging river in the downstream are the serious water problems in the Yellow River.The water resource shortage and soil erosion coexist in the Yellow River basin.Under the background of climate change,soil and water conservation work has been vigorously promoted,especially the implementation of returning farmland to forest(grass)engineering since 1999 in the Loess Plateau.The utilization of water resources and the regulation of runoff and sediment discharge are becoming a hotspot,and how to evaluate the effects of regional soil and water conservation measures on runoff and sediment discharge has become a focus.Although there have been a lot of studies on the effects of soil and water conservation measures on runoff and sediment discharge in the Yellow river,while many of them is a relative static evaluation method.In this study,Jialu River basin and Tuwei River basin which belong to the Yellow River were selected as the study areas,the changes of runoff and sediment discharge of the rivers were investigated by using Vector Autoregression method(VAR)based on the meteorological and hydrological data from 1959 to 2012.We focus on the evaluation of the effects of soil and water conservation measures and precipitation on the hydrological regimes,and explored the new method to calculate the hydrological benefits.The main results are as follows:(1)Variation of soil and water conservation measures,precipitation,runoff and sediment discharge in the Jialu River and Tuwei River were investigated in this study.In the past 50 years,the area of various soil and water conservation measures(such as terraces,woodlands,grasslands and dams)in the Jialuhe and Tuwei river watersheds increased significantly,especially the increase of woodlands area reached more than 30 times.The annual precipitation in the Jialu river watershed and the Tuwei river watershed showed insignificant trends,which was mainly a random process,and its probability distribution followed the normal distribution.Both the runoff and sediment discharge showed a significant decreasing trend.The annual runoff of Jialuhe River follows skewed chi-square distribution.The distribution of annual runoff in the Tuwei River is normally distributed,while the sediment discharge is skewed chi-square distribution.The change point of annual runoff of Jialuhe River and Tuwei River occurred in 1983 and 1985 respectively,and the annual runoff decreased significantly after the mutation.Similar to runoff discharge,change point of sediment discharge occurred in 1981 and 1999 respectively,and the reduction was more drastic than that of the runoff.(2)The contribution rates of precipitation change and soil and water conservation measures to the reduction of runoff and sediment transport in the Jialu and Tuwei river watersheds were quantified.Soil and water conservation measures are the main causes of runoff and sediment discharge reduction in the two watersheds.Compared with the periods before the change point,the contribution rates of precipitation and soil and water conservation measures to the runoff reduction after the change were 4% and 96% respectively.For sediment discharge reduction,the contribution rates of precipitation change and soil and water conservation measures were 11% and 89%,respectively.Compared with the periods before the abrupt change,the contribution rates of precipitation and soil and water conservation measures to runoff reduction after the abrupt change were 1% and 99% respectively.The contribution rate to sediment transport reduction was 14% and 86%,respectively.The analysis based on the lag model(PDL)shows that the effect of precipitation on runoff cannot last more than one year.There are long-term effects between sediment transport,precipitation and runoff.(3)The dynamic response model of water and sediment discharge to precipitation and various soil and water conservation measures was constructed to further clarify the response between annual runoff,sediment discharge,precipitation and the conservation area of soil and water conservation measures.VAR models based on annual runoff,annual sediment transport,annual precipitation and total area saved by soil and water conservation measures at the end of the year passed the stationarity test in the Jialuhe River and Tuwei River,which could be used for subsequent analysis.Granger causality test showed that precipitation and soil and water conservation measures had one-way influence on runoff and sediment discharge.Moreover,runoff and sediment discharge are Granger causality to each other.There is no correlation between precipitation and soil and water conservation measures.The results of the impulse response function showed that the effects of precipitation and soil and water conservation measures on runoff and sediment discharge in the two watersheds are not consistent.In terms of response intensity,the runoff is more dependent on itself in the Tuwei River watershed,its response to precipitation is weaker than runoff of the Jialu River watershed,and the response to soil and water conservation measures is slower than response of the Jialu River watershed.Variance decomposition showed that the decrease of runoff is the result of the continuous increase of soil and water conservation measures,and the change of sediment discharge is the result of the combined effects of the runoff and sediment discharge.(4)The dynamic relationship among annual precipitation,runoff,and sediment discharge,terrace,forestland,grassland,dam and closure is further analyzed based on VAR method.Annual precipitation,annual runoff,annual sediment discharge,terrace area,forest area,grassland area,dam area and closed area were taken as the explanatory variables.The models of runoff-sediment discharge-precipitation-terrace-forestland-grassland-dam-closed vector autoregressive(VAR)were established respectively in the two watersheds.The VAR method for eight time series of runoff,sediment discharge,precipitation,terrace,forestland,grassland,closure and dam watershed did not pass the unit characteristic root test in the Jialu River,indicating that the VAR method was not stable and the impulse response curve was divergent,indicating the VAR method estimated coefficient was unstable and the VAR model was invalid.VAR model was established for eight time series of runoff,sediment discharge,precipitation,terrace,forestland,grassland,enclosure and dam in the Tuwei River watershed,which passed the stationarity test and could be used for impulse response and variance decomposition analysis.(5)The impulse response function is used to analyze the dynamic relationship among runoff,sediment discharge,precipitation,terrace,forestland,grassland,dam and closure in the Tuwei River watershed.The results showed that the effects of terrace and dam on runoff and sediment discharge are consistent.Woodland increased both runoff and sediment discharge.The hydrological effects of grassland are complex,which increases the flow but decreases the sediment discharge in the short term.Grassland also decreases the runoff and increases the sediment discharge in the long-term.Blocking increases runoff whereas decreases sediment discharge.(6)The contribution rate of each factor to runoff and sediment discharge variation was quantitatively studied.According to the variance decomposition of runoff and sediment discharge in the Tuwei River watershed,the relative contributions of other variables to the change of annual runoff ordered as: Closure < Grassland < Forest < Dam < Sediment discharge< Terrace < Precipitation without considering the contribution of annual runoff.During the lag period of 15 years,the average contribution of Closure,Grass,Forest,Dam,Sediment discharge,Terrace and Precipitation to runoff variation was 1.35%,3.02%,8.02%,8.36%,9.87%,27.09% and 42.36%,respectively.The relative contributions of other variables that affect the change of annual sediment discharge ranked as: Grassland < Dam < Closure < Forest < Terrace < Precipitation < Runoff.During the lag period of 15 years,the average contribution of Grassland,Dam,Closure,Forest,Terrace,Precipitation and Runoff to the variation of sediment discharge was 0.47%,1.36%,2.50%,3.86%,4.35%,12.95%,and 0.47%,1.36%,2.50%,3.86%,4.35%,12.95%,and 74.55%,respectively.Combined with the analysis of impulse response function,the decrease of runoff discharge during the study period is the result of the increases of soil and water conservation measures,while the decrease of sediment discharge during the study period is mainly the result of the decrease of runoff discharge.