| Summary: | Soil moisture processes in the top soil is a pivotal element in the terrestrial phase of the hydrologic cycle over a wide range of scales. At the plot scale, soil moisture exerts a strong control on runoff, evapotranspiration, and the transfer of water to deeper soil levels and aquifers. It affects the heat capacity of the soil and determines important ecological and geochemical processes such as the distribution of soil microbial communities, mineralization rates or soil respiration. The mechanisms that control soil moisture dynamics are controversial, varying for different regions and scales. Most understanding comes from research on water-limited systems that may not be applicable in snow-driven systems. This study presents a field investigation on the dynamics that control the distribution of soil moisture at the plot scale in a snow-dominated sub-alpine catchment. The research experimentally validates previous modeling studies and determines whether evapotranspiration (extractive, energy-driven controls) is the main generator of soil moisture heterogeneity vs. differential snowmelt (additive control). The field study site, located in the Bitterroot Mountains, Montana, is instrumented with soil moisture/temperature sensors, meteorological station, and sap flux system monitoring an Engelmann Spruce. Results show that the highest heterogeneity in moisture occurs at intermediate water contents resulting from the spring snowmelt driven wetting process while the lowest heterogeneity occurs at the early fall conclusion of the drying season. The presence of the tree canopy alters the spatial distribution of precipitation and snowmelt and results in a dampening of weather induced soil moisture variability under canopies in comparison to open areas. We show that spatial distribution of precipitation and snowmelt (additive processes) are a stronger control on the generation of heterogeneity in soil moisture than evapotranspiration (extractive processes) in a snow dominated environment.
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