Forest density and snowpack stability regulate root zone water stress and percolation differently at two sites with contrasting ephemeral vs. stable seasonal snowpacks

Forested, snow-dominated watersheds provide a range of ecosystem services for adjacent and downstream communities including water supply, wildlife habitat, and recreation. While forest-snow-water relations have been well studied in areas with stable seasonal snowpacks, less is known about these interactions in the growing number of areas with ephemeral snowpacks. Here, we evaluate how forest cover and climate variability regulate the seasonality of water inputs, soil moisture, root zone water stress, and percolation at two 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 two depths (0–10 cm and 0–30 cm) for 6 locations/site distributed across gradients of forest cover. Each soil moisture measurement location has a unique daily moisture input time series previously developed from local snow/rain observations and modeling. 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 examines two sites over three years and 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.