From Snow to Rain: Declining Snowpack, Earlier Dryness, and Changes to Fire Risk in the Oregon Cascades

Globally, many mountain catchments transition from snow-dominated to rain-dominated behavior, which raises the question on how drought and wildfire regimes may be altered by these shifts. In 2023, the Lookout Fire burnt about two thirds of the H. J. Andrews Experimental Forest (HJA) in the Oregon Cascades and motivated a range of hydrological studies. Here, we evaluate how future changes to temperature and precipitation patterns, especially shifts from snow to rain, alter hydrological storages and fluxes across (nested) sub-catchments of the HJA.

To evaluate this, we rely on the conceptual hydrological model HBV-light and regional climate projections. We analyzed five catchments within HJA with differences in elevation. Hydrological changes were simulated under RCP 4.5 and RCP 8.5 emission scenarios for the near future (2021–2050) and far future (2071–2100) and compared against observations. Projections for 2071–2100 under RCP 8.5 indicate a strong temperature increase, resulting in a near-total loss (98–100%) of maximum snow water equivalent (SWE) in the study catchments. While winter precipitation is projected to increase by 30%, summer precipitation will decrease by up to 34%. This seasonal redistribution leads to significantly higher winter runoff but exacerbates summer deficits in soil moisture, groundwater, and streamflow.

These changes pose two interconnected challenges for mountain catchments. First, the pronounced shift toward earlier snowmelt and increased winter runoff substantially reduces streamflow and hydrological storages in summer, advancing the onset and prolonging the duration of seasonal dryness. Second, the earlier depletion of soil moisture and groundwater amplifies the spatial and temporal extent of high forest fire risk, enabling fires to occur earlier in the year, to affect larger areas, and to spread into or to start more likely at higher elevations. These patterns are consistent with recent observations of earlier wildfire occurrence and larger burnt areas in the Pacific Northwest. Our findings highlight that snow-to-rain transitions fundamentally alter water availability and disturbance by wildfire in mountain catchments, with implications that likely extend to many temperate mountain regions worldwide.