Combined Impacts of Climate and Land-Use/Land-Cover Changes on the Flow Regime and Ecological Diversity in the Songkhram River Basin, A Tributary of the Mekong River

Abstract Climate change (CC) and land-use/land-cover (LU/LC) alterations are among the most pervasive drivers of hydrological and ecological change in river basins worldwide, yet their combined impacts on fluvial ecological diversity remain insufficiently quantified, particularly in data-scarce tropical regions. This study presents a novel integrated framework that jointly evaluates the individual and combined effects of projected CC and LU/LC change scenarios on flow regimes and aquatic ecological diversity in the Songkhram River Basin a critical tributary of the Mekong River in northeast Thailand. The Soil and Water Assessment Tool (SWAT) was calibrated and validated on a daily timescale for streamflow simulation. Bias-corrected and downscaled CMIP6 climate projections (2026–2055) under two Shared Socioeconomic Pathways (SSP2-4.5 and SSP5-8.5) were combined with scenario-based LU/LC projections to simulate future flow regimes. Ecologically relevant flow alterations were assessed using hydrological alteration indices, and fluvial ecological diversity was quantified through the Shannon Diversity Index (SI). Results reveal that CC alone will significantly reduce annual streamflow (up to 22%), shift seasonal water availability, and intensify hydrological extremes - reducing low flows by up to 75% and amplifying short-duration flood events by up to 59%. CC substantially reduces ecological diversity, with the SI declining by 23.04% under SSP2-4.5 and 30.55% under SSP5-8.5. In contrast, conservation-oriented LU/LC scenarios support biodiversity recovery, increasing the SI by up to 20.66%; however, these gains are insufficient to offset CC-driven losses. When stressors are combined, CC effects dominate, resulting in net declines in ecological diversity of 18.62%-23.87% relative to the baseline. These findings provide robust scientific evidence that underscores the urgency of integrated land and water management strategies to build landscape resilience and safeguard riverine biodiversity across the Mekong region in a rapidly changing climate. Graphical Abstract This study develops an integrated modeling framework to assess the combined impacts of climate change (CC) and land-use/land-cover (LU/LC) change on river hydrology and ecological diversity. Using comprehensive datasets, including spatial information (land use, digital elevation model, soil, rivers, and population density), hydro-meteorological variables (rainfall, temperature), and future climate projections from multiple Global Climate Models under SSP2-4.5 and SSP5-8.5 scenarios, the research evaluates both natural and anthropogenic drivers of LU/LC change. Land-use dynamics were simulated using the Dyna-CLUE model, incorporating key driving factors such as topography, climate, and proximity to infrastructure, with logistic regression and policy-based scenario constraints. Hydrological processes were modeled using the SWAT model, calibrated and validated. The graphical abstract further depicts the impact assessment phase, where hydrological alterations are quantified and ecological responses are evaluated through the Shannon Diversity Index (SI). Scenario-based comparisons clearly demonstrate that both individual and combined effects of CC and LU/LC change influence streamflow regimes and ecological conditions. Overall, the graphical abstract succinctly depicts how integrated approaches that combine CC, SI, Dyna-CLUE, and SWAT provide a comprehensive and reliable tool for assessing ecological diversity under future changes, thereby supporting sustainable river basin management, ecological conservation, climate adaptation strategies, and informed policy decision-making.