How Do Forests Affect Snowpack Density: Mass, Delivery, or Energy Effects?

The effect of forest canopy processes on snow water equivalent (SWE) beneath trees has been broadly studied. In contrast, little effort has been made to understand how forest canopy processes affect snow density. Yet, density is a fundamental property of snow, and often a critical element in SWE retrievals from remotely sensed data.

To investigate the relative importance of different forest canopy processes on snow density and SWE, we applied the SUMMA (Structure for Unifying Multiple Modeling Alternatives) model at three sites representing diverse snow climates in Colorado (USA), Oregon (USA), and Alberta (Canada) for five years. First, control simulations were run for open and forest sites, indicating greater density beneath forest canopies (by 10's of kg m^-3) in all three climates. Then, experiments were completed to isolate different forest canopy processes: mass reduction due to interception loss (mass effects), changes in the phase and state of water delivered from the canopy to the underlying snow (delivery effects), and modification of the energy budget (energy effects). Delivery effects increased forest snowpack density relative to open areas, often more than 100 kg m^-3, whereas mass and energy effects had little effect on density. In contrast, mass and energy effects had the greatest (and opposite) effects on SWE. Delivery effects were greatest at the warmest times in the season and at the warmest site (Oregon). Higher temperatures melt intercepted snow which then drains to the underlying snowpack.

A comparison to observations at the CO site revealed a discrepancy: SUMMA simulated higher bulk density in the forest, but observations showed higher density in open areas. This suggested errors in the model representation of delivery effects and/or in the snow sampling strategy in the complex sub-canopy environment. New types of observations are needed to characterize how canopies influence the flux of water to the snow surface.