The impact of forest cover on snowpack in the semi-arid southwestern US

In the semiarid southwestern US, water resources depend heavily on temporally and spatially limited mountain snowpacks that are vulnerable to environmental changes due to the region's aridity, warm temperatures, and plentiful sunshine. Water managers in the region are especially interested in understanding how forest thinning might affect snow water resources because region's forests are undergoing unprecedented changes from drought, insect infestation, and forest management. For the past three winters, we have been monitoring the spatial and temporal characteristics of snowpack at several sites in Arizona's Salt River Basin, an economically important watershed that provides water supply and hydropower to millions of people in central Arizona. Our monitoring, which includes airborne Lidar, multi-angle aerial photography, extensive ground snow surveys, and SnowCam measurements at three research sites, indicates that rates of snow accumulation and ablation in these forests are heavily influenced by forest structure. In particular, areas with less forest cover (e.g. in areas where forest thinning has occurred) generally have more snow accumulation during storms, but can also have faster snow ablation where there is less shading. Snow modelling, aimed at determining the overall impact of forest cover on the snowpack, indicates that the net effect of this is that thick forests generally have less snow water equivalent (SWE) than sparser forests. This is particularly true at high elevations and on south-facing slopes. On flat and north facing slopes at lower-mid elevations, SWE is maximized at intermediate levels of forest cover, especially later in the winter. These results suggest the importance of the interaction between interception and shading, which maximizes SWE at intermediate canopy cover where shading is especially important for preserving substantial snowpack (mid-elevation flat and north facing slopes), but not where the snowpack is resilient to solar exposure (higher elevations) or where trees are less effective at shading the ground (south facing slopes).