Temporal and spatial variations of runoff composition revealed by isotopic signals in Nianchu River catchment, Tibet

Abstract

Abstract Understanding the hydrology of glaciated catchments is an important step in assessing the vulnerability of water resources to a changing climate. Based on multi-isotopes of water (2H, 18O and 3H) and dissolved radon (222Rn), the temporal and spatial variabilities of major hydrological processes along the main flow and tributaries in the Nianchu River catchment were examined and the isotopic response to climate variation was identified. Geographic variation in changes of isotopic composition that differ from other rivers in the Nianchu River catchment was apparent. Along the direction of runoff, river δ18O exhibited more depletion, which was closely related to water mixing and groundwater discharge. End-member mixing analysis using isotopic tracers suggested that annual recharge from summer rainfall and glacial meltwater maintained the surface water resources (their respective contributions rate were 65.9% and 26.5%); groundwater had a significant contribution on runoff in the dry season (about 46.6%). Summer rainfall and meltwater rapidly infiltrated through a series of faults and fissures and were, stored in underground reservoirs and released to runoff in the dry season, thereby ensuring rapid circulation and renewal of water resources (annual renewal proportion was about 40%). It was concluded that rainfall infiltration, meltwater and groundwater storage play important roles in the hydrology of this alpine-cold catchment. Similar to a general alpine-cold catchment, the stable isotopes (δ2H, δ18O) of river runoff will gradually be enriched, while groundwater reserves will increase in the Nianchu River catchment as a result of climate warming and an acceleration of glacial-melting.