Assessing streamflow and sediment responses to future climate change over the Upper Mekong River Basin: A comparison between CMIP5 and CMIP6 models

Study region: The Upper Mekong River Basin (UMRB), Southwest China. Study focus: With climate change unfolding and climate change knowledge evolving over time, it is imperative to investigate whether the latest CMIP6 models offer enhanced utility in climate change impact studies compared to their predecessors. This study strengthens the comparison between CMIP5 and CMIP6 models in assessing hydrological responses to future climate change. This was achieved utilizing the Soil and Water Assessment Tool, driven by downscaled CMIP5/ CMIP6 model outputs under RCP8.5/SSP5-8.5. Both streamflow and sediment responses, encompassing the spatial and temporal changes, and the relationships between streamflow and sediment loads, were carefully evaluated and compared between CMIP5 and CMIP6. New hydrological insights for the region: CMIP6 indicates a stronger warming in 2071-2100 over the UMRB compared to CMIP5. Mean annual precipitation/streamflow is projected to increase by 22.7%/26.3% using CMIP5 and 28.4%/34.4% using CMIP6. Mean annual sediment load changes, however, show a discrepancy between CMIP5 (-3.7%) and CMIP6 (+13.8%). CMIP6 exhibits larger inter-model variability in both climate and hydrological projections. Regarding future spatial distributions of mean annual water and sediment yields, a considerable agreement is demonstrated between CMIP5 and CMIP6, despite CMIP6 showing larger projections over most subbasins. Additionally, both ensembles exhibit approximate relationships between streamflow and sediment loads, indicating a comparable decline in watershed sediment generation and transport capacity under future climate change. Overall, CMIP6 suggests more severe climate change impacts on streamflow and sediment loads in the UMRB than CMIP5, highlighting the need to update climate change adaptation and mitigation policies based on the latest insights derived from CMIP6.