The Research Of Base Flow Process In Mountain Area Of Hexi Island River Basin

As an important component of runoff, base flow is the most important headwaters in dry season in arid and semi-arid regions. The process of base flow is closely related to climate and cryosphere changes. Base flow has great significance in maintaining runoff and preserving ecological environment of revers and lakes. Accurate understanding of the base flow process is a vital basis of correct understanding of watershed hydrological mechanism. Base flow can be used as a parameter to calibrate hydrological model and quantitative understand the component of water balance. It is also an important basis to fully understand the composition characteristics and the reasonable sustainable utilization of water resources. This article focuses on Shule river basin, Heihe river basin and Shiyanghe river basin in Qilian mountains of the Hexi island. Based on daily runoff data from 1955 to 2009 in the upper reaches of Shule river, from 1957 to 2006 in the upper reaches of Heihe river and from 1957 to 1987 in the upper reaches of Shiyanghe river, the recursive digital filter is used for base flow separation. Before using this method to separate base flow, the parameter and number of filtering should be calibrated. Through the comparison, determine the filter parameter and calculated the base flow, and on this basis, analyzed the changes of base flow, and then the affecting factor on the changes of base flow from two aspects of climate change and cryosphere changes is discussed. Through the above research, following is the main conclusions:(1) The selection of filter parameter is very important when the digital filter method is used to separate base flow in study area of Hexi inland mountain basin. Choosing 0.975 as the filter parameter and filtering one time can obtain good result after comparative analysis. The results of base flow separation show that the value of base flow, in turn, are:Heihe river basin is greater than Shule river basin and Shule river basin is greater than Shiyanghe river basin. That is the spatial variation of base flow value in Hexi island is gradually reduce from midland to west and from west to east. The inter-annual average base flow value of this three basins are respectively 10.95×108m3,6.44×108m3 and 1.22×108 m3. There is little difference between the base flow index of these three basins and their values are respectively 0.69,0.72 and 0.68, which explains that base flow has a great proportion of runoff and is an important component of runoff in the study area. The study found that the annual variation of base flow value in Hexi island is "unimodal type" and the peak value of base flow lags behind that of runoff. The calculation results of coefficient of variation of base flow show that dispersion degree in the eastern and western of Hexi island is greater than the central region.(2) The base flow inter-annual fluctuation of western and central regions are severe and overall shows increasing trend, its rate of change are 0.63×108m3/10aand 0.42×108m3/10a. Their base flow indexes are weakly increased. The changes of base flow in eastern region shows a trend of decrease, which rate of change is -0.15×108m3/10a. Since the 1950s, the decadal variation of base flow at the three river basins have a small fluctuation and overall shows increasing trend. The base flow value achieves the maximum at the early 21st century. The decadal variability of base flow index shows that at decadal scale, the proportion that base flow accounts for the runoff is relatively stable.(3) The seasonal changes of base flow are difference in Hexi island. The base flow at four seasons in Shule river basin show increasing trend, but the range of increasing is different. The base flow fluctuation in summer is large and increased more significantly than other seasons. Base flow change in winter is stable and has a weak increase trend. From the overall, the seasonal change of base flow in Heihe river basin is not large, it has slightly increasing trend in autumn and it is relatively stable in spring, summer and winter. In general, the base flow value in summer and autumn at the western and central region of Hexi island have increasing trend and reduced in eastern. Base flow values are relatively stable in winter and spring.(4) Inter-annual variation trend and wave process of base flow is similar with precipitation, correlation coefficients are respectively 0.64,0.57 and 0.75. In inter-annual scale, the variation of base flow in mountainous area is mainly affected by precipitation. The addition of precipitation directly increases the recharge of runoff which recharges base flow through soil infiltration. Analyzing the relationship between base flow variation and runoff in summer shows that there is significantly correlation between the two. The correlation coefficients are respectively 0.82,0.88 and 0.93. That means base flow will change as the change of runoff in summer. As can be seen in the previous analysis, the inter-annual variability characteristics of the total runoff is consistent with base flow. Base flow has a tendency to increase and base flow index is relatively stable illustrate that total runoff is also increased. That is to say, with the increase of runoff, base flow is also increasing.(5) Precipitation is a major influence factor to the change of base flow in Hexi island and temperature is an indirect factor. In a long term, the change of precipitation will directly affect the recharge of base flow. At the same time, this variation will change the value of base flow index to a certain extent. Under the background of global warming, because modern glaciers and permafrost were distributed in the mountain area in Hexi island, which are sensitive to temperature changes. Rising temperatures will lead to the increase of ice and snow melt and permafrost degradation. The increasing of glacial meltwater is good to base flow recharge. The degradation of permafrost increases activity layer thickness, slow down the recession process of runoff in winter and increases the supply from underground runoff to river. Although the rising temperature enlarges evapotranspiration, but it is not enough to offset the melting ice and permafrost degradation on the influence of base flow.