Aspect Differences in Vegetation Type Drive Higher Evapotranspiration on
a Pole-facing Slope in a California Oak Savanna
Abstract
Quantifying evapotranspiration is critical to accurately predict
vegetation health, groundwater recharge, and streamflow generation.
Hillslope aspect, the direction a hillslope faces, results in variable
incoming solar radiation and subsequent vegetation water use that
influence the timing and magnitude of evapotranspiration. Previous work
in forested landscapes has shown that equator-facing slopes have higher
evapotranspiration due to more direct solar radiation and higher
evaporative demand. However, it remains unclear how differences in
vegetation type (i.e., grasses and trees) influence evapotranspiration
and water partitioning between hillslopes with opposing aspects. Here,
we quantified evapotranspiration and subsurface water storage deficits
between a pole- and equator-facing hillslope with contrasting vegetation
types within central coastal California. Our results suggest that cooler
pole-facing slopes with oak trees have higher evapotranspiration than
warmer equator-facing slopes with grasses, which is counter to previous
work in landscapes with singular vegetation types. Our water storage
deficit calculations indicate that the pole-facing slope has a higher
subsurface storage deficit and a larger seasonal dry down than the
equator-facing slope. This aspect difference in subsurface water storage
deficits may influence subsequent deep groundwater recharge and
streamflow generation. In addition, larger root-zone storage deficits on
pole-facing slopes may reduce their ability to serve as hydrologic
refugia for oaks during periods of extended drought. This research
provides a novel integration of field-based and remotely-sensed
estimates of evapotranspiration required to properly quantify
hillslope-scale water balances. These findings emphasize the importance
of resolving hillslope-scale vegetation structure within Earth system
models, especially in landscapes with diverse vegetation types.