Forest cover and topography regulate the thin, ephemeral snowpacks of the semiarid Southwest United States

Authors: BROXTON, P.D. - University Of Arizona; VAN LEEUWEN, W.J.D. - University Of Arizona; Biederman, Joel

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2020
Publication Date: 2/21/2020
Citation: Broxton, P., Van Leeuwen, W., Biederman, J.A. 2020. Forest cover and topography regulate the thin, ephemeral snowpacks of the semiarid Southwest United States. Ecohydrology. 13(4). https://doi.org/10.1002/eco.2202.
DOI: https://doi.org/10.1002/eco.2202

Interpretive Summary: In the Southwest US, snowpack is a critical source of water supply for farms, businesses, and cities. Because the Southwest is warm and dry, its snowpacks may accumulate and disappear several times during a given winter, depending upon the weather and shelter from nearby trees. Water managers are therefore concerned about the dual impacts of climate change and several types of forest disturbance including drought, wildfire, insect infestation and forest harvest. In this work, we quantify how forest structure interacts with weather to regulate the accumulation and disappearance of snow. We find that while treeless clearings can accumulate up to 30% more snow than dense forests, the rate of snow disappearance in those clearings depends on the degree of shade from nearby trees. Furthermore, we find that peak snow water content is maximized when forest covers between 30-50% of the land. Forest cover greater than 50% results in more snow being intercepted by the canopy, where it is vulnerable to evaporation, while forest cover < 30% provides insufficient shading to snowpack on the ground. Collectively, these results demonstrate the potential for adaptive forest management to conserve and enhance water supplies.

Technical Abstract: In the semiarid US Southwest, water resources depend heavily on snowpacks, which are temporally and spatially limited in this warm, semiarid region. Snow accumulation and ablation in Southwest are heavily influenced by forest structure. Therefore, water resources managers urgently need to understand the future impacts of unprecedented forest changes now occurring from drought, insect infestation, and forest management. We show, using state of the art maps and time series of SWE, which account for spatial variability of snow depth and snow density over a large range of forest structure and topographies in the highlands of central Arizona, that the degree of forest cover and its geometry largely determine which areas have more/less snow accumulation and faster/slower ablation. Compared to tree covered areas, open areas can have 20-30% more accumulation and ablation rates can vary by 15-30% in sunny areas vs shaded areas. Although SWE response to tree cover is widely variable, depending on how much shading the trees provide vs. how much snow is intercepted and lost through canopy sublimation, thick forests generally have less snow water equivalent (SWE) than sparser forests. In general, SWE is optimized at intermediate levels of tree cover (~30-50%) on flat and north facing slopes. However, on south facing slopes, increasing tree cover generally causes a reduction of SWE, as trees are less effective at reducing radiative forcing at the snow surface due to less efficient shading and increased enhancement of longwave radiation from the warm canopy.