Characterising Runoff Source And Groundwater Recharge In A Small Catchment On The Loess Plateau Using Isotope And Hydrochemistry Tracers

The hilly and gully region is the main geomorphic unit on the Loess Plateau.Deep soil layer,low groundwater level and the serious erosion consist a special hydrological process in the world.Hillslopes are redominant land unit on Earth,and despite its importance in maintaince groundwater supplies in many parts of the world,groundwater recharge on hillslopes is poorly understand.The slope partions the precipitiaton into lateral overland flow and vertical infiltration.Water which pass through the root zone can recharge the groundwater,and overland flow,soil water,groundwater joint together to recharge the stream channel.We hypothesize that groundwater recharge in the deep unsaturated zones of hillslopes is facilitated mainly by piston flow,and recharge rates are small,but significant.To test the hypothesis,the distinct post-boom tritium bulges present in the soil profile were used 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.In 2014 and 2015,seven 15 to 20 m-long cores were collected from areas of different land used within the two watersheds.The tritium distribution in each soil profile exhibits a well-defined bell-shape,signifying that piston displacement is the main recharge mechanism.The tritium peak was located between depths of 9.3 m to 11.0 m at the semiarid watershed and 6.3 to 7.2 m at the submid watershed,and recharge rates varied from 22.9 to 31.7 mm yr^-1.These recharge rates account for 4-7% of the long-term precipitation,which is approximately 54% of the recharge typically seen on flat landscapes.Grondwater recharges were mainly controlled by soil texture,and land use changes but exhibited little impact from the climate difference between the two watersheds.Storm runoff generation process and event water contributions to a stream under site conditions that present a very deep layer of loess soil,coupled with poor vegetative conditions,in the Loess Plateau of Northwestern China were characterized for a small watershed in the hilly and gully region,utilizing two tracer methods – stable isotopes of water and chloride/sulfate hydrochemistry.Precipitation and streamflow samples were collected during six hydrologic events and were coupled with measurements of stream baseflow and groundwater pre-event.Tracer results for both methods demonstrated that the stream was comprised of mainly event water in proportions greater than 75%(Cl^-,SO₄^2-based)and up to 100%?isotope based?.Variations between the methods used may be explained by hydrological processes and soil geochemistry.Stream isotope composition fluctuated significantly due to precipitation events,likely due to excess overland dominated surface runoff.Hydrochemistry measurements of excess stream water Cl^-may be explained as a precipitation dissolution reaction with salts on the soil surface.Generally,hydrochemistry tracers underestimated the event water contribution at this study site due to soil buffering.This research provides insight into runoff generation mechanisms on the Loess Plateau and will be a useful tool for further analysis of performance of the eco-restoration project for the area.