Abstract
The snowpack regime influences the timing of soil water available for
transpiration and synchrony with the evapotranspiration (ET) energy
demand (air temperature, VPD, and shortwave radiation). Variability of
snowmelt timing, soil water availability, and the energy demand results
in heterogeneous ET rates throughout a watershed. In this study, we
assess how ET and growing season length vary across five sites on an
elevational gradient in the Dry Creek Watershed, ID, USA. We compared
trends of daily and annual ET between 2012 and 2017 to environmental
parameters of soil moisture, air temperature, vapor pressure deficit,
snow cover, and precipitation and evaluate how ET varies between sites
and what influences annual ET at each site. We observed three trends in
ET across the watershed. The first trend is at the low elevation site
where the snow cover is not continuous throughout the winter and rain is
the dominant precipitation form. The first day of the growing season and
ET occurs early in the season when the energy demand is low and soil
water is available. Annual ET at the low elevation site is a balance
between spring precipitation providing soil water into the summer season
and limiting the ET energy demand. The second trend occurs at the middle
elevation site located in the rain-snow transition. At this site, ET
increases with snow depth and spring precipitation extending the soil
water availability into the summer season. At the higher elevation
sites, ET is aligned with the energy demand and limited by growing
season length. At the high elevation sites, decreasing snow depth and
spring precipitation and increasing spring air temperatures result in
greater annual ET rates. The observations from this study highlight the
influence of environmental parameters and the potential sensitivity of
ET to climate change.