Groundwater Flow Pattern And Regulation In The Lhasa River Basin

Groundwater is a significant part of water resources and the global groundwater system is altering due to climate warming.However,the groundwater flow pattern and the characteristics of groundwater recharge and discharge on the Qinghai-Tibet Plateau（QTP）are rarely studied.In this study,a three-dimensional groundwater flow model was built for the Lhasa River Basin（LRB）,which is a typical alpine basin undergoing climate warming.The groundwater flow model was calibrated based on the observed baseflow data from the four hydrological stations of Pangdo,Tangga,Lhasa,and Yangbajing.The land use data,the glaciers and permafrost distribution data were input to the model to investigate the groundwater flow pattern,calculate the water budget,and forecast water table and baseflow changes under different climate scenarios in the future.Results show that annually～13.6%of precipitation recharges groundwater,which account for more than 90%of total groundwater recharge.Glacier melt water recharge takes up 7.1%of total groundwater recharge,with a magnitude about 5.4 mm/yr.The main groundwater discharges are in the form of baseflow,evapotranspiration,and lateral outflow through the southern boundary,accounting for 93.4%,5.5%and 1.1%in total groundwater discharge,respectively.The discharge of groundwater to river water was found on the majority of river reaches in the LRB,with magnitudes of 0～28.8 m³/s,while river water feeding groundwater was only found on some of the first-order tributaries at high altitude areas.The groundwater particle tracking distances are less than 43.6 km,and the groundwater particle tracking times range from 67 days to 750 millenniums.To some extent,there is a correlation between particle tracking distances and times.The proportion of baseflow in river runoff is about 10.0～14.0%in the LRB,which is similar to the proportion of winter-dry season runoff to annual runoff.Such a proportion indicating that it is feasible to replace baseflow with winter-dry season runoff during calibration of the steady-state groundwater flow model.The increasing baseflow trends are shown in all tested CMIP6 scenarios in the future,among which the scenario of regular development path is the fastest growing,with an increasing rate of 0.1951 m³·s^-1·year^-1.The higher peak values of both baseflow and water table were found in future simulations when compared to that of the past year（1998～2015）simulation.The variation of groundwater flow pattern is more sensitive in the upper mountain area and the high altitude area such as the Nyenchen Tanglha Mountains,where the rising range of groundwater level is larger.With the deepening degree of social development,the high water table and the high baseflow caused by extreme precipitation in the LRB will become more frequent,and the variation in groundwater flow system will be intensified.The results of this study will help sustain water resources in the LRB and forecast the groundwater flow pattern changes in response to future climate change.Besides,it improves the understanding of groundwater flow systems in a changing environment and helps construct numerical groundwater flow models in river basins which geological and hydrogeological characteristics similar to the LRB on the QTP and other regions of the world.