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

Climate change is expected to alter hydrological and biogeochemical processes in Central Asia (CA), and surface water stable isotope values (delta O-18 and delta H-2) 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 delta O-18 and delta H-2 values and higher d-excess relative to lake water, suggesting evaporative enrichment of lake water. Analysis of delta O-18 versus delta H-2 for rivers and lakes yielded distinct regressions, (river (RWL), delta H-2 = 6.086 delta O-18 - 16.7, r(2) = 0.837, p < 0.001) and (lake [LWL], delta H-2 = 6.23 delta O-18 - 22.1, r(2) = 0.924, p < 0.001). The slope and intercept of the RWL and LWL were slightly lower than the local meteoric water line [LMWL] (delta H-2 = 6.96 delta O-18 - 1.0, r(2) = 0.942, p < 0.001). River water delta O-18 showed a significant negative correlation with elevation and longitude, but not with latitude. The spatial distributions of delta O-18 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 delta O-18 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. (C) 2019 Elsevier B.V. All rights reserved.