Simulating stream discharge and temperature in the Horsefly watershed: evaluating model performance, future scenarios under climate change, land cover and riparian vegetation composition and effects on aquatic life

Abstract

This study assesses the impacts of climate change, land cover, and riparian vegetation on stream discharge and temperature in British Columbia’s Horsefly watershed, a key habitat for Pacific salmon. Using coupled Distributed Hydrology Soil Vegetation Model and River Basin Model, we validated simulations across three basins (Moffat, McKinley, and McKusky/McKay) for the summer-fall period. DHSVM performed well for unmanaged flow (adjusted R²: 0.50–0.54) but struggled with managed flows, while RBM accurately predicted temperatures (NNSE: 0.63–0.79). Subsequently, we applied these models to project stream discharge and temperature under two Climate Change Scenarios (CCS) i.e. SSP 1-2.6 (CCS-I) and SSP 3-7.0(CCS-II), for 2040 and 2060, Landcover Scenario (LCS) under agriculture and harvesting practices and Riparian Vegetation Scenarios (RVS) under buffer width and tree height variations. Results suggest that stream discharge declines, most pronounced in July (reduction of more than 3.4 m³/s in Moffat) for CCS. LCS reduced summer discharge. Whereas, stream temperatures are projected to increase in October (Moffat > 1°C, McKinley > 10°C, McKusky/McKay > 4°C) for CCS, exceeding the optimal thermal ranges for salmonids. Riparian vegetation provided thermal buffering (up to 2.3°C), though its efficacy was limited. Findings highlight salmon habitat vulnerability to combined climate and land-use pressures.