We used co-located observations of snow depth, soil temperature, and moisture and energy fluxes to monitor variability in snowmelt infiltration and vegetation water use at mixed-conifer sub-alpine forest sites in the Valles Caldera, New Mexico (3020 m) and on Niwot Ridge, Colorado (3050 m). At both sites, vegetation structure largely controlled the distribution of snow accumulation with 29% greater accumulation in open versus under-canopy locations. Snow ablation rates were diminished by 39% in under-canopy locations, indicating increases in vegetation density act to extend the duration of the snowmelt season. Similarly, differences in climate altered snow-season duration, snowmelt infiltration and evapotranspiration. Commencement of the growing season was coincident with melt-water input to the soil and lagged behind springtime increases in air temperature by 12 days on average, ranging from 2 to 33 days under warmer and colder conditions, respectively. Similarly, the timing of peak soil moisture was highly variable, lagging behind springtime increases in air temperature by 42 and 31 days on average at the Colorado and New Mexico sites, respectively. Latent heat flux and associated evaporative loss to the atmosphere was 28% greater for the year with earlier onset of snowmelt infiltration. Given the large and variable fraction of precipitation that was partitioned into water vapour loss, the combined effects of changes in vegetation structure, climate and associated changes to the timing and magnitude of snowmelt may have large effects on the partitioning of snowmelt into evapotranspiration, surface runoff and ground water recharge. Copyright © 2009 John Wiley & Sons, Ltd.
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