Small Soil Storage Capacity Limits Benefit of Winter Snowpack to Upland Vegetation

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In the western United States, the mountain snowpack is an important natural reservoir that extends spring and summer water delivery to downstream users and ecosystems. The importance of winter snow accumulation to upland ecosystems is not as clearly defined. This study investigates the relative contribution of winter precipitation to upland spring and summer soil moisture storage and availability in a semi-arid mountainous watershed. At this site, coarse soil textures and shallow soil depths limit soil storage capacity to 6–16 cm. Winter precipitation exceeds soil storage capacity by 2.5 times. Accordingly, soil moisture profiles at most locations in the watershed reach field capacity in early winter. With soil storage near capacity, water released by snowmelt primarily contributes to deep drainage and makes a limited contribution to the soil moisture reservoir. Water that is retained by the soil after the snowpack melts is lost to evapotranspiration in as little as 10 days. In contrast, spring precipitation extends moist soil conditions by up to 90 days into the warm season, when ecological water demand is highest. These field observations suggest that changes in spring precipitation, not winter snowpack, may have the greater impact on upland ecosystems in this environment. Furthermore, because winter precipitation is in excess compared to the soil storage capacity, soil moisture availability may be fairly insensitive to climate change-induced transitions from snow to rain.