, 2007 and Xu et al., 2010). Ice-snow melt-water and precipitation in the high mountain regions are the main water resources of the arid areas in northwest China. A significant increasing trend of the precipitation in the upper HRB, especially during the obvious wet period between 2003 and 2012, may be only part of the reason for headwater

increase. Furthermore, increasing air temperature induced more glacier and snow melting during the past decades which contributed significantly to the streamflow increasing in the upper HRB. In the upper HRB, many mountainous terrains are at an elevation of 4000 m or higher, and they are covered in snow or glaciers throughout the year. Melting water of glaciers and snow replenish runoff effectively PR-171 cost (Qin et al., 2013). Numerous studies showed a declination of ice and snow cover areas in the HRB during last several decades (Sakai et al., 2006, Wang et al., 2011a, Wang et al., 2011b, Zhang et al., 2012a and Zhang et al., 2012b). It resulted in the increase of streamflow in upstream mountainous areas of the HRB (Nakawo, 2009). Related studies have also showed that snowmelt runoff increased obviously from 1970 to present (Wang et al., 2010). From monthly changes of average streamflow data, the impact of climate change can be seen more clearly in the HRB. Taking the streamflow of the Yingluoxia (YL) station

as an example (Fig. 13), streamflow increased in the spring, leading to the flood buy Fasudil season in the summer. The spring, March to May, is the snow melting STK38 season, and the rising streamflow can be attributed to higher temperature.

Increasing precipitation for the summer and autumn (see Fig. 10) can explain streamflow rising in the flood season. There is hardly any runoff generated in the middle and lower HRB due to low precipitation and high evapotranspiration, and the change in precipitation does not much affect streamflow. In addition, precipitation increases by a very small amount in the middle and lower HRB, and thus the impact of the precipitation change on the streamflow is negligible. Moreover, the relationships between streamflow and air temperature are different in the middle and lower HRB. Higher air temperature leads to higher actual evapotranspiration which resulted in the decrease of streamflow. Streamflow is the most important water resource that sustains oases and irrigated agriculture in the HRB. During the last several decades, the hydrological regime of the Heihe River has been strongly affected by extensive human activities. They impacted the streamflow by surface water and groundwater exploitation, land reclamation, engineering project development, and new water-related policy implementation. The middle reach region of the HRB is the major water consumer, accounting for about 90% of the total water use from the Heihe River.