Using stable isotopes paired with tritium analysis to assess thermokarst lake water balances in the Source Area of the Yellow River, northeastern Qinghai-Tibet Plateau, China.

A spatially distributed network of thermokarst lakes undergoing significant environmental changes was sampled in 2014 and 2016 to develop a comprehensive understanding of lake water balances in lakes across a gradient of frozen ground conditions. Frozen ground ranges from seasonally frozen ground (SFG) to sporadic discontinuous permafrost (SDP) to extensive discontinuous permafrost (EDP), and is representative of complex conditions in the Source Area of the Yellow River, northeastern part of Qinghai-Tibet Plateau. Radioactive and stable water isotopes in reference lakes (non-thaw lakes), thermokarst lakes, precipitation, wetlands, ground ice and supra-permafrost groundwater are analyzed to characterize systematic variations and to assess lake water balances using stable isotope mass balance (IMB). IMB, paired with analysis of tritium decay gradients, is shown to be a valid approach for detecting short-term shifts in lake water balance, which allows evaluation of the proportion of precipitation-derived versus permafrost-derived water inputs to lakes. All lakes except EDP thaw lakes are evaporation-dominated (E/I > 0.5). Negative water balances occurred most frequently in reference lakes due to hydrological connectivity with rivers. Precipitation-derived water inputs result in positive water balances in SFG and SDP thermokarst lakes, but negative-trending water balances are found in SDP thermokarst lakes due to substantial reduction in water yield. Increasing contributions from thawing permafrost in EDP thermokarst lakes result in strong positive water balance. Permafrost degradation may also lead to the changes in hydrological connectivity between precipitation and wetlands or thermokarst lakes. Based on these findings, a conceptual model of the hydrological evolution of thermokarst lakes under the influence of permafrost degradation is proposed.