Groundwater leakage of an endorheic basin with extensive permafrost coverage in the western Mongolian Plateau

Abstract

Groundwater is critical for social development and ecological environment in the Great Lake Depression, a mountainous endorheic basin characterized by permafrost coverage in western Mongolian Plateau. However, its leakage to surrounding regions and the related impacts on water resource stability have never been studied under climate warming. This study attempts to meticulously examine the dynamics of groundwater leakage and its implications for terrestrial water storage (TWS) and groundwater stability from 2002 to 2022. A modified water balance approach by introducing the term of groundwater exchange, different from traditional equation for endorheic basin, was utilized to calculate groundwater leakage. The findings indicate that the average annual groundwater leakage amounts to 111.87 × 10⁸ m³/year, with approximately 89.67 % of this leakage occurring during the warm season from May to October. The study identifies three critical thresholds of the difference between precipitation (P) and evapotranspiration (ET) (P-ET = 17.39 mm, 33.74 mm, and 58.14 mm) necessary for maintaining TWS stability on an annual basis. Under conditions of climate warming, there is a notable increase in groundwater leakage, with an absolute rate of 0.83 mm/year, which is particularly pronounced during the cold season. Based on the average monthly variation of multi-year P-ET, TWS, air temperature, groundwater level and calculated groundwater leakage, it demonstrates that groundwater leakage is significantly influenced by frozen soil conditions, including both permafrost and seasonally frozen soil, which mediate the interactions between groundwater and surface water/soil water. Soil freezing during the cold season from November to April of next year, coupled with sufficient P-ET, plays a crucial role in facilitating groundwater recharge during the warm season. Additionally, according to the basic permafrost and soil properties and calculated changing rate of TWS, the northern edge of Great Lake Depression is identified as potential pathway for increased groundwater leakage. The study concludes that both TWS and groundwater are at considerable risk of decline. This research underscores the significance of groundwater leakage in the broader content of hydrological studies and practical water resource management, while also elucidating the effects of climate warming on groundwater dynamics and water resource stability in cold regions.