Title: Soil moisture response to snowmelt and rainfall in a Sierra Nevada mixed-conifer forest
Author: Bales, Roger C.; Hopmans, Jan W.; O’Geen, Anthony T.; Meadows, Matthew; Hartsough, Peter C.; Kirchner, Peter; Hunsaker, Carolyn T.; Beaudette, Dylan.
Source: Vadose Zone J. 10: 786-799
Description: Using data from a water-balance instrument cluster with spatially distributed sensors we
determined the magnitude and within-catchment variability of components of the catchment-scale
water balance, focusing on the relationship of seasonal evapotranspiration to changes
in snowpack and soil moisture storage. Co-located, continuous snow depth and soil moisture
measurements were deployed in a rain–snow transition catchment in the mixed-conifer forest
in the Southern Sierra Nevada. At each elevation sensors were placed in the open, under
the canopy, and at the drip edge on both north- and south-facing slopes. Snow sensors were
placed at 27 locations, with soil moisture and temperature sensors placed at depths of 10, 30,
60, and 90 cm beneath the snow sensor. Soils are weakly developed (Inceptisols and Entisols)
and formed from decomposed granite with properties that change with elevation. The soil–
bedrock interface is hard in upper reaches of the basin (>2000 m) where glaciers have scoured
the parent material approximately 18,000 yr ago. Below an elevation of 2000 m soils have a
paralithic contact (weathered saprolite) that can extend beyond a depth of 1.5 m, facilitating
pathways for deep percolation. Soils are wet and not frozen in winter, and dry out in the weeks
following spring snowmelt and rain. Based on data from two snowmelt seasons, it was found
that soils dry out following snowmelt at relatively uniform rates; however, the timing of drying
at a given site may be offset by up to 4 wk because of heterogeneity in snowmelt at different
elevations and aspects. Spring and summer rainfall mainly affected sites in the open, with drying
after a rain event being faster than following snowmelt. Water loss rates from soil of 0.5 to
1.0 cm d−1 during the winter and snowmelt season reflect a combination of evapotranspiration
and deep drainage, as stream baseflow remains relatively low. About one-third of annual
evapotranspiration comes from water storage below the 1-m depth, that is, below mapped soil.
We speculate that much of the deep drainage is stored locally in the deeper regolith during periods
of high precipitation, being available for tree transpiration during summer and fall months
when shallow soil water storage is limiting. Total
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Bales, Roger C.; Hopmans, Jan W.; O’Geen, Anthony T.; Meadows, Matthew; Hartsough, Peter C.; Kirchner, Peter; Hunsaker, Carolyn T.; Beaudette, Dylan. 2011. Soil moisture response to snowmelt and rainfall in a Sierra Nevada mixed-conifer forest. Vadose Zone J. 10: 786-799.