Quantifying streamflow and active groundwater storage in response to climate warming in an alpine catchment on the Tibetan Plateau

Abstract. Climate warming is changing streamflow regimes and groundwater storage in cold alpine regions. In this study, a headwater catchment named Yangbajain in the Lhasa River basin on the Tibetan Plateau is adopted as the study area for quantifying streamflow changes and active groundwater storage in response to climate warming. The catchment is characterized by alpine glacier and frozen ground which covers about 11 % and 86 % of the total area, respectively. The changes in streamflow regimes (including quickflow and baseflow) and climate factors are evaluated based on hydro-meteorological observations from 1979 to 2013. Then active groundwater storage in autumn and early winter is quantified by recession flow analysis assuming nonlinearized outflow from aquifers into streams. The results show that annual streamflow increases significantly at a rate of about 12.30 mm/10a during this period. The significant increase of annual air temperature compared with nonsignificant variation of annual precipitation indicates that the climate warming takes responsibilities to the increase of streamflow. It is believed that the increased streamflow is mainly fed by glacier meltwater, which has led to over 25 % loss of the total glacial volume in the past 50 years (1960–2009) in this catchment. Moreover, the significant increase of annual baseflow at a rate of about 10.95 mm/10a is the dominant factor for the increase of the total streamflow. Through recession flow analysis, we find that recession coefficient K and active groundwater storage S in autumn and early winter increase significantly at the rates of about 7.70 (mm^0.79d^−0.21)/10a and 19.32 mm/10a during these years. The increase of active groundwater storage can partly be explained by frozen ground degradation, which lead to the enlargement of groundwater storage capacity and accommodate more summer rainfall and meltwater in the wide and flat valley, and then slowly release them into streams in the following seasons. Thus, it is reasonable to attribute the increase of baseflow and the slowdown of baseflow recession process in autumn and early winter to the enlargement of groundwater storage capacity. Through quantifying streamflow changes and active groundwater storage in response to warming-induced changes, this study provides a perspective to clarify the way of glacial retreat and frozen ground degradation on hydrological processes.