The Sensitivity of Soil Moisture in Western U.S. Mountains to Changes in Snowmelt

Abstract:

Snowmelt is the primary water source for human needs and ecosystems services in much of the Western U.S. Regional warming is expected to hasten snow disappearance and reduce snowpacks. The soil water budget strongly mediates the effects of changing snowmelt patterns by storing water and altering is partitioning to evaporation, transpiration, and runoff. This study therefore asked the research question, “Under what conditions was soil water availability coupled to snowmelt magnitudes and timing across Western U.S. mountains?” We posed three potential hypotheses to explain decoupling between soil water availability and snowmelt: 1. Contributions from post-snowmelt rainfall, 2. Longer growing season length and/or greater water demand, and/or 3. Insufficient soil water storage. Using 259 Snow Telemetry (SNOTEL) stations, we showed that the timing of Peak Soil Moisture (PSM) was strongly explained by snow disappearance (Pearson r-value of 0.62). However, differences in the coupling of PSM with DSD were dependent on soil and bedrock type, with well-drained areas having earlier PSM relative to DSD. A second analysis focused on 48 SNOTEL and Soil Climate Analysis Network (SCAN) stations in the Northwest and Intermountain Western U.S. where detailed soil hydraulic properties existed. We found the timing of snow disappearance was a strong influence (p<0.01) on the number of days per year that soil moisture was below wilting point at individual stations, whereas summer precipitation was a weaker predictor. We develop a framework to classify stations into three classes: 1. stations that were not subject to water stress from changing snowmelt patterns over the historical records, 2. stations subject to water stress during poor snowmelt years, and 3. stations that relied on rainfall to avoid water stress across historical records. Our combined results demonstrate that snow disappearance timing is a first-order control on soil water availability across many Western U.S. mountain ecosystems. However, soils properties could make areas more/less sensitive to changing snowpacks depending on seasonal precipitation patterns. This type of simple framework could be used to identify areas at risk of changing snowpacks and help constrain vegetation distributions as a consequence of climate change.