Hydrochemistry and stable isotopes of the Ili River: Deciphering spatiotemporal moisture sources and climatic drivers in Central Asia's wet island

Abstract

The Ili Valley in northwestern China plays a vital role as a green oasis in the large-scale moisture cycle and water utilization in Central Asia's core. To clarify the distinctive hydrochemical characteristics and genesis that differentiate it from the neighboring arid areas, the first large-scale sampling of river water covering the entire Ili Valley was conducted. The results show that the hydrochemical and stable isotopic characteristics vary spatially and seasonally among the tributaries. Rock weathering mainly controls the major ion composition in river water, which gradually varies along the river due to evaporation and snowmelt. The δ²H and δ¹⁸O values range from −103.00‰ to −72.00‰ and −15.00‰ to −9.60‰, respectively. Contrary to what is expected, the δ²H & δ¹⁸O in the Tekes & Ili Rivers are enriched with altitude, showing an “inverse altitude effect" influenced by topography and moisture sources; the Karsh River has lower δ¹⁸O values due to snowmelt; the Kunes River has a U-shaped trend of isotopic concentrations and high d-excess values by precipitation. Both the seasonal deviations in river waterlines (RWL¹ vs RWL²) and the backward trajectory by HYSPLIT 4.0 showed that the origin of spring moisture at low troposphere in the Ili Valley could be traced to the surrounding regions, the summer moisture mixed with frigid air from the Arctic Ocean, while sub-cloud evaporation and surface-groundwater interaction are also the factor internally within the basin. The findings complement the basin-scale hydrochemical monitoring data and help investigate the large-scale hydrological cycle.