Impacts Of Driving Factors Of Runoff And Sediment Transportations Changes In Yanhe Basin

During the past several decades, the rapid development of climate and human activities have intensively caused changes in hydrological characteristics, which affects the basin runoff and sediment yielding process.Since the implement of the Project of “Grain for Green” have obvious improved the conditions of serious soil erosion on the Loess Plateau. The study of this research is Yanhe River Basin which located in the Yellow river watershed also showed the same characteristics.The researches data which included meteorological and hydrological data from 1960 to 2010 and NDVI(May~October) during 1980-2010 in the Yanhe Basin were used to analysis the impact of precipitation and human activities on the change of runoff and sediment yielding. Many statistical methods have been established to analyze the dynamic variational characteristic and relationships of these datas. The linear trend method was used to get theirchanging trends. The stages of the natural annual meteorological and hydrological process are clear when a method with accumulated deviation from average value as its parameter was used. The result of quantitative analysis on the runoff and erosion responses to frequent land use change and climate change were obtained with no interceptdouble mass curve prediction model.With a field experiment on runoff plots, the result-assisted of water and sediment reduction under slope vegetation measures were revealed by analysed the precipitation, runoff and sediment in flood season.The main conclusions are as follows:The trend of runoff and sediment yielding showing a oscillating and declining by the time series. The annual runoff and sediment variation rate reaches-0.19×108m3/10 a and-0.09×108 t/10 a. It changed sharply at 1971 and 1996 which caused it divided into three stages, and found it has an increasing number of low flow years. There was positive correlation between runoff and sediment yielding. The runoff amount from July to September in 2013 is the highest over the years and runoff amount of July nearly 4.1 times higher than the average for many years of the observe data. The generation trait of runoff and sediment erosion that fewer times and rate of runoff generations, less runoff amount and sediment near-zeroof flow slope were observed on forage grass.Average annual precipitation showed a declining trend which was similar variation history to runoff and sediment yielding. The annual precipitation rate reaches-15.5mm/10a; basin precipitation fluctuations are large with coefficients of variation(CV) being 0.202. Precipitation in the Yanhe Basin were not exist obvious change in phase, and it can be simply divided into three stages included 1960~1971, 1972~1996 and 1997~2010. Normal rainfall occurred in Yanhe River Basin reached 78.4% during past 50-year. Precipitation in flood season of 2013 throughout the year accounted for 91%; extreme precipitation occured in July to present obvious characteristics that high rainfall, precipitation lasted long amd short interval of each precipitation.Since 60’s in 20 centuries, a lot of measures of soil and water conservation were launched in Yanhe Basin, and obvious benefits of sediment deduction are achieved. However, the forcused content and importance are different in each stage. The implementation of soil and water conservation area of 2 698.42km2 including an area of vegetation 2 370.05 km2 until to 2006, and its management level was 53%. The situations of vegetation cover during 1980~2010 accelerate improve year by year, and its increased rate with 0.258/10 a have become more prominent. Till the end of 2008, Yan’an city have completed the national plan of “Grain for Green” which accumulatively over an area of 58.8l×105 hm2.The linear fitting correlation between runoff, sediment and precipitation is better with value fit well over 0.97. Due to high-frequent human activity, the impacts of climate change on runoff and sediment yield in Yanhe Basin has been gradually weakened by the change of the underlying surface condition therein which have dominated in 1960 ~ 2010. Change in the rainfall is not the case, the situation between runoff and sediment yield has altered fundamentally which manifested as a significant decrease. Vegetation coverage in Yanhe watershed on the rise in general which were different from the trends of runoff and sediment yield after 1999. The coefficient of partial correlation between runoff, sediment and precipitation were 0.705 and 0.414; the coefficient of partial correlation between runoff, sediment and vegetation coverage rate were-0.705 and-0.563.The relation between the precipitation amount of watershed with the amount of runoff deepth and sediment yield of Ganguyi Station could be described with no intercept of the linear Double accumulation equation as xyr?0723.0 and xys?0018.0(ry is runoff deepth, sy is sediment yield, x is precipitation amount). The linear model of main hydrological data of base period of 1960- 1971 based on the double mass curve have a higher fitting degree(R2>0.9). The estimation via the double mass curve indicated that soil and water conservation dominated by vegetation restoration contributed 12.87% and 52.29% to the runoff and sediment load decline, respectively.The characteristic of slope-runoff in plots varies with different type of land use purposes. The benefit of runoff and sediment reductionunder covered by tree-grass planting measures is higher than that of the bare land. July and August are sensitive periods of surface flow and sediment slope. Reducing of sediment of vegetation is close to 100%, while its reduction of runoff is accounted for over 85%.