Bedrock controls on water and energy partitioning across the western
contiguous United States
Abstract
Across diverse biomes and climate types, plants use water stored in
bedrock to sustain transpiration. Bedrock water storage
($S_{bedrock}$, mm), in addition to soil moisture, thus plays an
important role in water cycling and should be accounted for in the
context of surface energy balances and streamflow generation. Yet, the
extent to which bedrock water storage impacts hydrologic partitioning
and influences latent heat fluxes has yet to be quantified at large
scales. This is particularly important in Mediterranean climates, where
the majority of precipitation is offset from energy delivery and plants
must rely on water retained from the wet season to support summer
growth. Here we present a simple water balance approach and random
forest model to quantify the role of $S_{bedrock}$ on controlling
hydrologic partitioning and land surface energy budgets. Specifically,
we track evapotranspiration in excess of precipitation and mapped soil
water storage capacity ($S_{soil}$, mm) across the western US in
the context of Budyko’s water partitioning framework. Our findings
indicate that $S_{bedrock}$ is necessary to sustain plant growth in
forests in the Sierra Nevada — some of the most productive forests on
Earth — as early as April every year, which is counter to the current
conventional thought that bedrock is exclusively used late in the dry
season under extremely dry conditions. We show that the average latent
heat flux used in evapotranspiration of $S_{bedrock}$ can exceed
100 $W/m^{2}$ during the dry season and the proportion of water
that returns to the atmosphere would decrease dramatically without
access to $S_{bedrock}$.