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 rarely 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 deeply weathered bedrock profiles 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 documented 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.

The Age of Evapotranspiration: Lower‐Bound Constraints From Distributed Water Fluxes Across the Continental United States

“…From 2019 to 2021, neutron-probe-based monitoring of water storage changes from below the soil through the weathered bedrock indicated that precipitation was insufficient to replenish previously observed storage magnitudes (Hahm et al, 2022), and water contents reached progressively lower minima at the end of each dry season. The decline of rock moisture is attributed to tree water uptake, as negligible groundwater recharge and streamflow were observed (Hahm et al, 2022). The inferred jump of mean minimum ET ages to >1 yr (Figure 1a) in late 2020 coincided with a depletion of water content in the deep bedrock vadose zone well below levels in prior years (Figure 12c in Hahm et al, (2022)), indicative of the use of previous wet seasons' water.…”

“…The later 2019 storage snapshot has the same relative age structure as the earlier 2019 snapshot but is translated in this plotting space downward Collectively, these findings are consistent with inferences made from in situ observations of water dynamics at this site. From 2019 to 2021, neutron-probe-based monitoring of water storage changes from below the soil through the weathered bedrock indicated that precipitation was insufficient to replenish previously observed storage magnitudes (Hahm et al, 2022), and water contents reached progressively lower minima at the end of each dry season. The decline of rock moisture is attributed to tree water uptake, as negligible groundwater recharge and streamflow were observed (Hahm et al, 2022).…”

Symmetry in Hillslope Steepness and Saprolite Thickness Between Hillslopes With Opposing Aspects

“…Abiotic chemical weathering is strongly influenced by plant water uptake and redistribution, which can alter weathering pathways (Lucas, 2001). As deeply rooted oaks utilize water stored within the saprolite for transpiration (Hahm et al, 2020(Hahm et al, , 2022, depletion in water content within the saprolite during the growing season could slow water residence times and increase the production of solutes.…”

Bedrock mediates responses of ecosystem productivity to climate variability

“…Bedrock lithology can affect the biological productivity of overlying ecosystems through variations in the storage of plantavailable water [7][8][9][10][11][12] . While bedrock usually has low primary porosity and hydraulic conductivity 13,14 , differences in mineral weathering rates can lead to different rates of regolith formation 15 , regolith permeability and porosity 15 , and production of nutrients needed to support terrestrial ecosystems 8,16 .…”

“…In karst regions, limestone dissolution corresponds directly to elevated calcium concentration in bedrock, with which the regolith water loss rate and ecosystem productivity are strongly correlated 9 . Furthermore, although intact regolith generally has a lower porosity than the more highly weathered soil layer above it, its thickness, commonly tens of meters, allows storage of significant amounts of water 7,[10][11][12]16,18 . Recent studies have shown that water storage within the regolith can be an especially important source for deep-rooted plants during droughts or summer dry seasons, long after shallow soils are dry [19][20][21] .…”