Temperature and precipitation control the seasonal patterns of discharge and water isotopic signals of the Nyang River on the southeastern Tibetan Plateau

Abstract

River hydrology on the southeastern Tibetan Plateau (SETP) is particularly sensitive to climate change, through the serious impacts of the decline in Indian summer monsoon (ISM) rainfall and the accelerated melting of snow and glaciers during the recent decades. Investigating the effects of meltwater and rainfall on the seasonality and quantity of river runoff in the region is crucial for improving regional hydrological monitoring and water management, and water stable isotopes can provide an effective method. Based on the event-based data of precipitation and river water isotopes, river discharge and meteorology monitored at a hydrometric station of the Nyang River on the SETP in 2011, we show that river discharge has a strong reliance on temperature and precipitation simultaneously through their controls on the meltwater and rainfall runoff, and river water deuterium excess (d) and δ18O potentially track the meltwater and rainfall effects separately. The seasonal signals of river discharge and isotopic data provide evidence that meltwater is the key factor of river discharge and isotopic changes during the pre-ISM season (low water level period) and ISM rainfall during the early to mid-ISM season (growing and high water level period) but key factors are not clear during the mid to post-ISM season (recession period). These inferred seasonal runoff source relationships are in close agreement with the runoff source contributions estimated using a season-specific δ18O-d mixing model approach. Specifically, the result suggests meltwater contributes ∼28 %, ∼41 % and ∼25 % of the total river runoff, and snowmelt does make a major contribution to the meltwater (compared to glacier melt) during the three individual hydrological periods. This study investigated the event-based river discharge and isotopic responses to temperature and precipitation and the relevance of seasonal meltwater and rainfall effects for a representative basin on the SETP, and we propose that long-term isotopic record of river water in this region may document catchment changes in runoff source relationships (especially snowmelt vs ISM rainfall).