Sublimation from a Macroscale Central Arizona Watershed: Canopy Effects

The contribution of sublimation to the water budgets of the Salt and Verde River basins in Arizona is examined with macroscale snowpack and water budget simulations for a pair of dry and wet years, 2007 and 2008 respectively. These basins provide approximately 40% of the water to the metropolitan Phoenix, Arizona supply. Meteorological variables from the North American Regional Reanalysis project are adjusted for terrain and vegetation effects to drive the snowpack model. Results suggest that in central Arizona, the presence of forest cover reduces total sublimation due to reduced wind speeds and vapor pressure deficits over ground snow cover. Despite high canopy sublimation rates (e.g., 0.67 mm dy^-1), total canopy sublimation is limited by duration of canopy snow cover due to frequent winter canopy unloading. Canopy effects on sublimation are more evident over the Salt due to more extensive forest cover on the Salt than the Verde. The contribution of sublimation to total ablation (i.e., the sum of melt and sublimation) is increased during the dry year (17.1% and 20.5% of total ablation on the Salt and Verde respectively) relative to the wet year (14.9% and 11.9% of total ablation on the Salt and Verde respectively). Water budget simulations combined with discharge observations to estimate total sublimation over the two years show poor agreement with total sublimation determined from the snowpack model. On the Salt, the water budget estimate of 2007-2008 sublimation (0.630 km³) exceeds sublimation determined from the snowpack model (0.412 km³), with opposite results on the Verde (0.240 km³ and 0.367 km³ total sublimation from the water budget and snow model simulations respectively).

While the conclusions drawn from this study are undoubtedly influenced by the weather conditions specific to 2007 and 2008, useful generalizations can be made when considering the effects of total snowfall and snow cover duration on melt and sublimation totals, along with interbasin climate, terrain, and forest cover differences. During dry years, the reduction in snow cover duration is much less notable than reductions in melt at elevations where most of the snowpack resides in the colder and drier Verde. More extensive forest cover to reduce sublimation and more high elevation land area to produce greater melt on the Salt, leads to a lesser increase in the relative contribution of sublimation to ablation during dry years on the Salt compared to the Verde. This is critical from a water resources perspective, suggesting that during dry years, when sublimation loss is most important, a greater fraction of snowfall is lost to sublimation. This is made more important considering recent and continuing trends toward a drier climate in the southwestern US.