Bedrock vadose zone storage dynamics under extreme drought: consequences for plant water availability, recharge, and runoff

Hahm, W. Jesse; Dralle, David; Sanders, Maryn; Bryk, A. B.; Fauria, Kristen E.; Huang, M. H.; Hudson‐Rasmussen, Berit; Nelson, M. D.; Pedrazas, Michelle A; Schmidt, Logan; Whiting, J. A.; Dietrich, W. E.; Rempe, D. M.

doi:10.1002/essoar.10509661.2

Cited by 3 publications

(6 citation statements)

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“…We also report where woody ecosystems have large enough Smax to bank precipitation for multiple years via carryover storage (Figure 1H). In this case, large Smax may confer drought resilience in the sense that plants can sustain transpiration through years of drought, but it may also lead to the build-up of large root-zone storage deficits that cannot be quickly replenished, resulting in vulnerability, not resilience, to precipitation reductions in larger droughts (11,19). Carryover storage may be the hydrological manifestation of structural overshoot by the plant community, wherein high biomass density generates storage deficits that cannot be replenished during dry years.…”

Section: Discussionmentioning

confidence: 99%

Mechanisms underlying the vulnerability of seasonally dry ecosystems to drought

Rempe¹,

McCormick²,

Hahm³

et al. 2023

Preprint

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Large-scale plant mortality has far-reaching consequences for the water and carbon cycles. The role of belowground root-zone water storage (RWS) on the conditions that lead to mortality remains uncertain. It has been proposed that the RWS capacity, Smax, can determine ecosystem vulnerability to drought (Hahm et al., 2019a, Klos et al., 2018). However, incorporating information about RWS into prediction of vegetation dynamics has been limited due to the challenge of quantifying RWS at large scales (Dawson et al., 2020, Trugman et al., 2021). Here, we present a mass-balance framework for assessing forest resilience to year-to-year variability in precipitation, including megadroughts, by quantifying RWS. We use the relationship between RWS and annual precipitation to evaluate the sensitivity of woody ecosystems to precipitation variability by classifying them as either capacity limited, where RWS is nearly constant annually and set by Smax, or precipitation limited, where RWS varies annually based on precipitation amount. We applied this framework to seasonally dry forests and savannas in California and found that approximately 16-23% of the state's total biomass is found in precipitation-limited locations where plants commonly rely on carryover of moisture from one year to the next. These precipitation-limited areas experienced disproportionately high rates of mortality in recent drought. In contrast, approximately 51-58% of the state's biomass is found in capacity-limited locations and thus experiences annually reliable moisture supply. Using precipitation projections for the next century, the model framework reveals a tipping point by which 5,163 km2 (27 Tg aboveground carbon) of forest and savanna could transition from stable to unstable moisture supply. An additional 11,950 km2 (55 Tg aboveground carbon) where moisture supply is already annually unstable is projected to experience increased water stress, due to additional years where precipitation is not sufficient to refill moisture deficits generated in dry years. This framework provides a novel approach for assessing vulnerability of RWS, and thus woody ecosystems, to climate change.

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Section: Discussionmentioning

confidence: 99%

Mechanisms underlying the vulnerability of seasonally dry ecosystems to drought

Rempe¹,

McCormick²,

Hahm³

et al. 2023

Preprint

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“…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).…”

Section: Resultsmentioning

confidence: 90%

“…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.…”

Section: Resultsmentioning

confidence: 90%

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

Hahm

Lapides

Rempe

et al. 2022

Water Resources Research

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Evapotranspiration (ET) is a large and rapidly changing flux in the global water cycle. Many regions are expected to experience additional water stress due to changes in evaporative demand and water supply associated with warming. These changes can be expected to alter the time that water spends in various reservoirs in the terrestrial water cycle, such as soils and groundwater. The age of evapotranspired water can be defined as the elapsed time between when precipitation falls and when that water returns to the atmosphere as vapor via transpiration or abiotic evaporation (Botter et al., 2011). Thus, the age of ET describes the transit time distribution of water molecules through terrestrial storage (including aboveground reservoirs such as snow or lakes, the subsurface, and intraplant storage) before being incorporated into ET, the primary outflow of the terrestrial hydrologic cycle (Schlesinger & Jasechko, 2014). Due to the large magnitude of ET, ET age distributions may have a dominant impact on water remaining in storage available for other outflows like groundwater recharge or stream discharge. The age of ET can provide information about the origins of plant water sources (Miguez-Macho & Fan, 2021), the sensitivity of those sources to drought , and nutrient supply, which depends on water residence time in reactive belowground environments (Li et al., 2017). For

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“…Managers and researchers have long recognized the importance of subsurface moisture conditions for subsequent runoff (Arkley, 1981; Bales et al., 2011; Goulden & Bales, 2019; Grindley, 1960; Hahm et al., 2021; Jones & Graham, 1993; Klos et al., 2018; Lewis & Burgy, 1964; M. Anderson et al., 1995; McCormick et al., 2021; Miller et al., 2010; Rempe & Dietrich, 2018; Rose et al., 2003; Sayama et al., 2011); however, incorporating root‐zone water storage dynamics into forecasting presents a challenge. This is due to both the limited available data on water storage in weathered bedrock, as well as the challenge of understanding interactions between different drivers of root‐zone dynamics.…”

Section: Discussionmentioning

confidence: 99%

Causes of Missing Snowmelt Following Drought

Lapides

Hahm

Rempe

et al. 2022

Geophysical Research Letters

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Water management in snowy mountainous regions hinges on forecasting snowmelt runoff. However, droughts are altering snowpack‐runoff relationships with ongoing debate about the driving mechanisms. For example, in 2021 in California, less than half of predicted streamflow arrived. Mechanisms proposed for this “missing” streamflow included changes in evapotranspiration (ET), rainfall, and subsurface moisture conditions. Here, we demonstrate that ET in drought years generates dry subsurface conditions that reduce runoff in subsequent years. A model including this legacy of depleted moisture storage reduced median error in 2021 forecasts from 60% to 20% at 15 minimally disturbed basins and from 18% to 2% at 6 water supply basins in the Sierra Nevada (basins range in area from 5 to 23,051 km2 and mean annual precipitation from 814 to 1,549 mm). Our findings indicate that the relationship between snowpack and runoff will evolve as plant ecosystems respond to climate change and alter subsurface water storage dynamics.

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Bedrock vadose zone storage dynamics under extreme drought: consequences for plant water availability, recharge, and runoff

Cited by 3 publications

References 0 publications

Mechanisms underlying the vulnerability of seasonally dry ecosystems to drought

Mechanisms underlying the vulnerability of seasonally dry ecosystems to drought

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

Causes of Missing Snowmelt Following Drought

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