A preliminary estimate of how stream water age is influenced by changing runoff sources in the Nagqu river water shed, Qinghai-Tibet Plateau

Stream water age is an essential indicator of stream water renewal and pollutant transport rates. Recently, the concept of young water fractions, calculated using stable isotopic data, was proposed as a measure of stream water age. However, the current calculation methods do not classify stream water fractions with annual cycles (365 days), which are helpful to understand the sources of runoff and the water renewal rate in the watershed. In this study, we took the Nagqu River Watershed (NRW, with an area of 16 900 km(2)) as an example and proposed the concept of middle-aged water to solve this problem. We used two time thresholds tau yw and tau ow to divide the stream water age into young water, middle-aged water and old water in runoff and calculated middle-aged water using a distributed hydrological model. The results revealed that in the NRW, an average of 23% of the runoff was less than 51.6 days old, whereas an average of 55% of the runoff had ages ranging from 51.6 to 365 days. The spatial change in stream water age is significantly influenced by the sources of streamflow, evaporation, air temperature and soil moisture. The annual average change in old water storage values were -0.33 km(3), suggesting that the mean of old water in the NRW decreased from 2016 to 2019. The results of this study are helpful for the study of sources of streamflow and stream water fractions, as well as providing a more effective understanding of the internal hydrological process in a depopulated alpine zone under the impacts of climate change.

Automated summary

This study proposed the concept of middle-aged water in the Nagqu River Watershed to address the limitations in current stream water age calculations. The spatial change in stream water age is significantly influenced by various factors, and the average change in old water storage values from 2016 to 2019 was negative.