Nonmonsoon Precipitation Dominates Groundwater Recharge Beneath a Monsoon-Affected Glacier in Tibetan Plateau

Groundwater recharge is poorly understood beneath the glacier due to the complex sampling conditions, although it is sensitive to the global change. Here we carried out the sampling and isotopic analysis on groundwater, ice-melt water, river, and precipitation through a hydrological year at the lowest glacier of Mingyong in Hengduan Mountains, Southeastern Tibetan Plateau. The precipitation during the monsoon seasons (P-m) are more depleted in heavy isotopes than precipitation during the nonmonsoon seasons (P-nm) due to the influence of monsoon. The ice-melt water and groundwater points are both found located left above the local meteoric water line (delta H-2 = 8.0 delta O-18 + 8.0) and form lines of delta H-2 = 6.3 delta O-18 - 10.0 and delta H-2 = 4.2 delta O-18 - 37.6, respectively. However, their mean isotopic values are both larger than the weighted annual mean of precipitation isotopes. All of the phenomena above points to the occurring of refreezing process after excluding the evaporation process. A theoretical model is built to confirm the refreezing process that causes the lower slope of ice-melt water line. The groundwater line is explained as a mixing line with ice-melt water and precipitation with contributions of 46 +/- 22% and 54 +/- 22%, respectively. For the precipitation contribution, we propose a plausible conceptual model that groundwater comes primarily from P-nm (41 +/- 17%), and minor from P-m (13 +/- 17%). In this context, it is equivalent to 87 +/- 28% groundwater recharged by P-nm because the glacier is mainly accumulated from P-nm. The findings highlight the importance of the P-nm recharging groundwater even in the monsoon-affected glacial regions.