Assessing Water Source Stability in the Three River Source Region: A Comprehensive 35-Year Analysis

Terrestrial ecosystem water balance refers to the state between precipitation inputs, and outputs via evapotranspiration, groundwater recharge and runoff. The Three River Source Region (TRSR) is known as the "water tower of Asia" and is an important water source region for much of southeast Asia. Spatial and temporal distributions of water balance across the TRSR have been affected by recent changing climatic conditions. However, the status and variation of water balance under the influence of environmental factors remain unclear. We employed SOILWAT2, a daily time step ecosystem water model, along with 35 years of climate data from 29 meteorological stations to characterize the hydrological dynamics of the region. Our results revealed that 15 of the sites exhibited evidence of a net water source (precipitation > actual evapotranspiration) between 1981 and 2015. These stations were predominantly located at intermediate elevations (3,400-4,100 m) and low latitudes (<34 degrees N), while stations at lower and higher elevations as well as higher latitudes were not sources because of moisture limitations by a dry climate. In contrast to a strong spatial pattern, we found no obvious temporal trend of the net water source. The interannual and seasonal variability of the net water source is dependent on the temporal fluctuations in potential evapotranspiration and precipitation. We therefore conclude that there is an elevation and latitude dominated spatial pattern of the water balance on the TRSR, with important implications for the availability of water resources.