Our Science in Action








Much of the western United States depends on high elevation snowpack in forested watersheds for water supply, wildlife habitat, and recreation in both adjacent and downstream communities. The forest-snow-water relationship is well studied in areas with stable – that is, cold – seasonal snowpacks, however much less is known about this relationship in the growing number of areas with ephemeral snowpacks. In this study, we evaluate how forest cover and climate regulate the seasonality of water inputs, soil moisture, root zone water stress, and percolation at contrasting seasonal vs. ephemeral snowpack sites, in Arizona, USA. We use a soil moisture model (Hydrus-1D) and three years of continuous soil moisture measurements at 6 sites distributed across gradients of forest canopy cover. Compared to the seasonal snowpack site, the ephemeral site has wetter soils during winters due to frequent mid-winter melt events, leading to greater annual percolation. However, earlier snowmelt at the ephemeral snowpack site also prolongs the duration of root zone water stress. At both sites, dense forest cover is associated with less percolation and longer duration of soil drought, due to increased transpiration. These effects are amplified during an abnormally dry winter. This study develops key steps towards using spatially distributed snowpack and soil moisture measurements to assess how changing climate and forest cover jointly regulate water availability for future ecosystems and downstream use.