Spatial distribution and controlling factors of surface water stable isotope values (δ¹⁸O and δ²H) across Kazakhstan,...

Climate change is expected to alter hydrological and biogeochemical processes in Central Asia (CA), and surface water stable isotope values (δ¹⁸O and δ²H) can be used to examine these changes. Spatially extensive stable isotope data, however, are sparse, which constrains the understanding of hydrological processes in transboundary rivers across Kazakhstan. Therefore, we conducted a survey of surface water stable isotopes across the region. River and lake water isotope values exhibit spatial variability that was closely associated with isotope values of precipitation, physiographic factors, landscape characteristics, and local hydrological processes, e.g., evaporation and mixing of waters from different sources. River water was characterized by lower δ¹⁸O and δ²H values and higher d-excess relative to lake water, suggesting evaporative enrichment of lake water. Analysis of δ¹⁸O versus δ²H for rivers and lakes yielded distinct regressions, (river [RWL], δ²H = 6.08δ¹⁸O - 16.7, r² = 0.837, p < 0.001) and (lake [LWL], δ²H = 6.23δ¹⁸O - 22.1, r² = 0.924, p < 0.001). The slope and intercept of the RWL and LWL were slightly lower than the local meteoric water line [LMWL] (δ²H = 6.96 δ¹⁸O - 1.0, r² = 0.942, p < 0.001). River water δ¹⁸O showed a significant negative correlation with elevation and longitude, but not with latitude. The spatial distributions of δ¹⁸O and d-excess values showed a remarkable gradient from west to east across Kazakhstan that was associated with moisture moving from the Mediterranean region to Kazakhstan. We also found generally higher δ¹⁸O values and lower d-excess values in low-elevation areas because of high evaporation rate in Kazakhstan. These baseline data will be useful for documenting the effects of climate change on the hydrological cycle in Central Asia.