Our Changing Mountain Watersheds: Declining Snowpacks, Drying forests, and Post-fire Impacts on Snow Hydrology and Streamflow

Snowpacks are the main source of soil moisture and streamflow for mountain watersheds. Across the western US, warmer winters are responsible for declining mountain snowpacks and when combined with longer, hotter dry seasons, these conditions lead to water-stressed and fire-vulnerable forests. Since 2000, we observe a significant browning trend in the montane evergreen forests of western US ecoregions and this trend is correlated with an earlier snow disappearance date over the same period. In looking at recent large fires, we note that this correlation was particularly strong prior to the fire. Following wildfire, snowpacks are impacted by the combined loss of forest canopy and decreased snow albedo, as black carbon from the burned trees is shed onto the snowpack surface. Previous work has shown that snow melts off days to weeks earlier in burned areas compared to areas outside the fire zone. After a fire, the watershed function changes depending on the fraction of the seasonal snow zone that is burned. The spring freshet becomes more intense, with more runoff occurring over a shorter time. Summer low flows are also affected but only in those watersheds where fire has burned over 50% of the seasonal snow zone. In the post-fire vegetation recovery stage, snow remains important, especially in the lower portions of the seasonal snow zone. Satellite observations show that post-fire summer greening is correlated with the previous winter's snow cover. Together, these observations point to the value of long-term satellite observations for understanding the complex and complicated relationships between snow, forests, fire, and mountain watershed hydrology. These relationships also indicate the need for a deeper and mechanistic understanding of the linkages and feedbacks in these changing mountain environments.