The effects of climate change and extreme wildfire events on runoff erosion over a mountain watershed

Increases in wildfire occurrence and severity under an altered climate can substantially impact terrestrial ecosystems through enhancing runoff erosion. Improved prediction tools that provide high resolution spatial information are necessary for location-specific soil conservation and watershed management. However, quantifying the magnitude of soil erosion and its interactions with climate, hydrological processes, and fire occurrences across a large region (>10,000 km2) is challenging because of the large computational requirements needed to capture the fine-scale complexities of the land surface that govern erosion. We apply the physically-based coupled Variable Capacity Infiltration–Water Erosion Prediction Project (VIC–WEPP) model to study how wildfire occurrences can enhance soil erosion in a future climate over a representative watershed in the northern Rocky Mountains – the Salmon River Basin (SRB) in central Idaho. While the VIC model simulates hydrologic processes at larger scales, the WEPP model simulates erosion at the hillslope scale by sampling representative hillslopes. VIC–WEPP model results indicate that SRB streamflow will have an earlier shift in peak flow by one to two months under future climate scenarios in response to a declining snowpack under warming temperatures. The magnitude of peak flow increases with each higher severity fire scenario; and under the highest fire severity, the peak flow is shifted even earlier, exacerbating the effects of climate change. Similarly, sediment yield also increases with higher fire severities for both historical and future climates. Sediment yield is more sensitive to fire occurrence than to climate change by one to two orders of magnitude, which is not unexpected given that our fire scenarios were applied basin wide as worst case scenarios. In reality, fires only occur over portions of the basin in any given year and subsequent years’ vegetation regrowth reduces erosion. However, the effects of climate change on sediment yield result in greater spatial heterogeneities, primarily because of the spatial differences in precipitation projections, while fire conditions were uniformly applied. The combined effects of climate change and a possible continuation of increasing fire frequency and severity will compound excess sediment issues that already exist in this region of the intermountain West.