How three-dimensional forest structure regulates the amount and timing of snowmelt across a climatic gradient of snow persistence

Authors: Dwivedi, Ravindra; Biederman, Joel; BROXTON, P.D. - University Of Arizona; PEARL, J.K. - Nature Conservancy; LEE, K. - University Of Arizona; SVOMA, B.M. - Salt River Project; VAN LEEUWEN, W.J.D. - University Of Arizona; ROBLES, M.D. - Nature Conservancy

Submitted to: Frontiers in Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/24/2024
Publication Date: 5/16/2024
Citation: Dwivedi, R., Biederman, J.A., Broxton, P., Pearl, J., Lee, K., Svoma, B., van Leeuwen, W., Robles, M. 2024. How three-dimensional forest structure regulates the amount and timing of snowmelt across a climatic gradient of snow persistence. Frontiers in Water. 6. Article 1374961. https://doi.org/10.3389/frwa.2024.1374961.
DOI: https://doi.org/10.3389/frwa.2024.1374961

Interpretive Summary: Western US mountains are a critical source of snowmelt supplying water to millions of agricultural, business, and residential users. The forests in these mountains affect the amount and timing of snowmelt through capturing snow before it lands on the ground and sheltering snowpack from sun and wind. Therefore, rapid changes in western US forests through wildfires, drought and preventive management actions are affecting snow water supplies in unknown ways. Here we measured and modeled snowpack at four field stations in Arizona across a gradient from warm/low mountains to high/cold mountains. We combined data gathered from SNOTEL monitoring stations, airborne laser mapping snapshots of forest structure and snow depths, and daily records of snow depths distributed across gradients of forest structure provided by Snowtography (snow-photography). These datasets were used to train an ultra-high resolution 3D forest snow hydrology model, which was then run over multiple years with a range of warm/dry and cold/wet winter weather. We found that at most sites, cool, shaded snowpack locations to the north of forest canopy had the greatest seasonal peak snowpack water content. Surprisingly, warm, sunny locations, while having less peak snowpack, produced a greater volume of snowmelt than shaded locations, because high radiation exposure rapidly melted snow during the winter rather than leaving snowpack exposed to evaporate into dry air for many months, as occurs in shaded/cold locations. These results contrast with the prevailing wisdom that forests should be managed to develop peak snowpack and suggest instead that forest structure could be managed to enhance peak snowmelt volumes.

Technical Abstract: Across the western US, forest structure is changing rapidly, with uncertain impacts on snowmelt water resources. Snow partitioning is strongly controlled by 3D forest structural effects on interception, wind, and radiation. While models can represent these processes at the scale of individual trees, we often lack snow measurements with sufficiently high spatial and temporal resolution across gradients of forest structure. Here, we utilize four Snowtography (snow-photography) stations with daily measurements over 3-5 years at ~110 positions distributed across gradients of forest structure resulting from wildfires and mechanical thinning. We combine Snowtography with lidar snapshots of forest and snow to train a high-resolution forest snow model and run it for six years to quantify how forest structure regulates snowpack and snowmelt. The four sites represent a climate gradient from lower/warmer sites representative of much of the montane Lower Colorado River Basin to higher/colder sites similar to the more well-studied Upper Basin. Different snow environments created by forest structure (e.g. sunny warm, shaded cool) had greater impacts in warm/dry winters than cold/wet winters. Across sites, forest cover melted snowpack earlier at lower/warmer sites but preserved snowpack longer at higher/colder sites, implying a shorter soil moisture drought before summer rains. Cool, shaded snowpack locations to the north of forest canopy had the greatest peak snow water equivalent (SWE). Surprisingly, sunny/warm locations produced more snowmelt than shaded/cool locations, because high net radiation melted snow during winter rather than leaving snowpack exposed to sublimate into dry air for many months, as occurs in shaded/cool locations. These results highlight that peak SWE is not an ideal proxy for snowmelt volume in the increasing number of watersheds with warm winters and ephemeral snowpack. The results also imply that forest management can influence snow water resources, and that there may be decision trade-offs between enhancing forest resilience through delayed snowmelt and maximizing snowmelt volumes for downstream water resources.