TUDY ON RUNOFF GENERATION PROCESSES BASED ON WATER STABLE ISOTOPES IN PERMAFROST-INFLUENCED ALPINE MEADOW AREA OF THE TIBETAN PLATEAU

The Tibetan Plateau (TP) is the headwater area of more than ten large Asiatic rivers and the permafrost degradation under global warming may change the hydrological regime of the headwater catchments. In this study, hydrometric observations and water stable isotopic method were applied to investigate the runoff generation processes in alpine meadow area, northeast TP. The following results were obtained: 1) The observed average stable isotope values of various water types was roughly in the order of snowfall ≈ snowmelt < bulk soil water (BSW) < rainfall ≈ stream water ≈ mobile soil water (MSW) ≈ lateral subsurface flow. The depleted spring snowmelt and enriched summer rainfall formed tightly bound soil water and MSW, respectively. The dynamic mixing between tightly bound soil water and MSW resuted in BSW with more depleted and variable stable isotopic feature than MSW. 2) Along with the thawing of the frozen soil, surface runoff and shallowsubsurface flow (SSF) at 30−60 cm was the major flow pathway in the permafrost influenced alpine meadow hillslope during spring snowmelt and summer rainfall period, reapectively. 3) The shallow SSF showed sharp increases after the soil water contents of thawed layers exceeded certain threshold values, and the frozen soil is important to promote the hillslope runoff generation by maintaining supra-permafrost water level to the highly transmissive shallow soil layer. 4) Comparison between two neighouring catchments under similar precipitation conditions indicated that streamflow of the lower catchment with less permafrost proportion and earlier thawing time has larger SSF and higher based flow component, indicating the potential changes of hydrological regims subject to future warming.