Bedrock vadose zone storage dynamics under extreme drought: consequences
for plant water availability, recharge, and runoff
Abstract
Bedrock vadose zone water storage (i.e., rock moisture) dynamics are
sparsely observed but potentially key to understanding drought
responses. Exploiting a borehole network at a Mediterranean blue oak
savanna site-Rancho Venada-we document how water storage capacity in a
deeply weathered bedrock profile regulates woody plant water
availability and groundwater recharge. The site is in the Northern
California Coast Range within steeply dipping turbidites. In a wet year
(water year 2019; 647 mm of precipitation), rock moisture was quickly
replenished to a characteristic storage capacity, recharging groundwater
that emerged at springs to generate streamflow. In the subsequent
rainless summer growing season, rock moisture was depleted by about 93
mm. In two drought years that followed (212 and 121 mm of precipitation)
the total amount of rock moisture gained each winter was about 54 and 20
mm, respectively, and declines were observed exceeding these amounts,
resulting in progressively lower rock moisture content. Oaks, which are
rooted into bedrock, demonstrated signs of water stress in drought,
including reduced transpiration rates and extremely low water
potentials. In the 2020-2021 drought, precipitation did not exceed
storage capacity, resulting in variable belowground water storage,
increased plant water stress, and no recharge or runoff. Rock moisture
deficits (rather than soil moisture deficits) explain these responses.