Divergent hydrologic regimes of mega-rivers originated from High Mountain Asia uncovered by satellite virtual station-densified water levels

Abstract

High Mountain Asia (HMA) is experiencing accelerated warming and moistening, whereas sparse monitoring networks hinder comprehensive characterization of mega-rivers’ divergent hydrologic regimes. We establish an easy-implementing and effective framework for densifying water level series using satellite virtual stations (VSs). Virtual station-derived water level models (VS-WLs) were established based on the empirical relationship between water level records from HYDROWEB or DAHITI and near-synchronous water area values derived from Sentinel-1 SAR images. All VS-WLs demonstrated good fit, with an average R² of 0.7858, and the densified water levels had an average root-mean-square error of 1.23 m. For 63 % of the VSs, temporal coverage was extended to encompass the entire 2017–2022 period, increasing the coverage by an average factor of 1.47. Additionally, observation frequency increased by an average of 7.26 times across all VSs, and the seasonal amplitude of water level fluctuations improved by an average of 1.22 times. In general, rivers exhibit stronger water level fluctuations in the southeast than that in the northwest, corresponding to the climatic wet-to-dry transition conditions. The Salween River had the greatest water level fluctuations, averaging 17.34 m, followed by the Mekong River at 10.76 m and the Yangtze River at 10.32 m. Peak water levels varied across basins, but with the peak generally occurring in the wet season (from April to September). Snowmelt, rainfall runoff, and glacier melt are primary contributors to these fluctuations. Additionally, tributary inflows exacerbate river level fluctuations downstream, although these impacts can be moderated by terrain and human control.