Groundwater recharge in hillslopes on the Chinese Loess Plateau

Study region: Yuanzegou Watershed in Qingjian, Wangdonggou watershed in Changwu, the Chinese Loess Plateau. Study focus: Hillslopes are the predominant land form unit on Earth, and despite its importance in maintaining groundwater supplies in many parts of the world, groundwater recharge on hillslopes is poorly understood. We hypothesize that groundwater recharge in the deep unsaturated zones of hillslopes is facilitated mainly by soil matrix flow, and the recharge rates are much smaller in hillslopes than on flat landscapes. To test the hypothesis, seven 15-20 m-long cores were collected from areas of different land uses to determine the groundwater recharge rates and its controls in deep unsaturated zones of hillslopes at a sub-humid and a semi-arid watershed located on the Chinese Loess Plateau. New hydrologic insights for the region: The tritium distribution in each soil profile exhibits a well-defined bell-shape, signifying that soil matrix displacement is the main recharge mechanism. The tritium peak was located between depths of 9.33 m-11.01 m at the semiarid watershed and 6.29 m-7.22 m at the sub-humid watershed, and the recharge rates varied from 24.5 to 33.8 mm yr(-1). These recharge rates account for 4 %-7 % of the long-term precipitation, which is approximately 56 % of the recharge typically seen on flat landscapes. Groundwater recharges were mainly controlled by soil texture, and land use changes, but exhibited little impact from the climatic difference between the two watersheds.