Hydrological imbalance in Nam Co Lake, the third-largest lake on the Tibetan Plateau

Lakes on the Tibetan Plateau (TP), as key components of the Asian Water Tower, play a vital role in regional water resources and hydrologic cycle. Groundwater is an important but often ignored factor in the lake’s hydrologic cycle, largely due to challenges in quantifying its contribution. In the case of the Nam Co Lake, the third-largest lake on the TP, research on basin groundwater remains notably limited. This paper presents, for the first time, an analysis of groundwater in the water balance of Nam Co Lake, based on a comprehensive hydrogeological investigation. Results revealed lacustrine groundwater discharge into the lake was about 10.31 ± 0.31 × 108 m3 during May to October 2018. Daily lake level change showed that the rising lake level resulted in a volume increase of 13.06 ± 1.53 × 108 m3 during the same period. The hydrometeorological observations revealed that during the observation period, precipitation over the lake, recorded by the automatic weather station, totaled 7.51 ± 0.75 × 108 m3, while evaporation from the lake, measured by the eddy covariance system, amounted to 9.56 ± 0.12 × 108 m3. Additionally, runoff of surrounding rivers into the lake reached 22.83 ± 2.28 × 108 m3. Thus, a lake water balance analysis revealed a surplus input of 18.03 ± 2.42 × 108 m3 compared to the output during the water balance duration. The only plausible explanation for the water imbalance is seepage, most likely occurring along the region’s known subsurface fault system. These findings underscore the significant role of groundwater and highlight the magnitude of lake seepage, offering new insights into the hydrological cycle of Nam Co Lake and the broader TP region.