More Like this

1. Drought onset and propagation into soil moisture and grassland vegetation responses during the 2012–2019 major drought in Southern California

   https://doi.org/10.5194/hess-25-3713-2021  

   Warter, Maria Magdalena ; Singer, Michael Bliss ; Cuthbert, Mark O. ; Roberts, Dar ; Caylor, Kelly K. ; Sabathier, Romy ; Stella, John ( January 2021 , Hydrology and Earth System Sciences)

   null (Ed.)

   Abstract. Despite clear signals of regional impacts of the recent severe drought inCalifornia, e.g., within Californian Central Valley groundwater storage and Sierra Nevada forests, our understanding of how this drought affected soil moisture and vegetation responses in lowland grasslands is limited. In order to better understand the resulting vulnerability of these landscapes to fire and ecosystem degradation, we aimed to generalize drought-induced changes in subsurface soil moisture and to explore its effects within grassland ecosystems of Southern California. We used a high-resolution in situ dataset of climate and soil moisture from two grassland sites (coastal and inland), alongside greenness (Normalized Difference Vegetation Index) data from Landsat imagery, to explore drought dynamics in environments with similar precipitation but contrasting evaporative demand over the period 2008–2019. We show that negative impacts of prolonged precipitation deficits on vegetation at the coastal site were buffered by fog and moderate temperatures. During the drought, the Santa Barbara region experienced an early onset of the dry season in mid-March instead of April, resulting in premature senescence of grasses by mid-April. We developed a parsimonious soil moisture balance model that captures dynamic vegetation–evapotranspiration feedbacks and analyzed the links between climate, soil moisture, and vegetation greenness over several years of simulated drought conditions, exploring the impacts of plausible climate change scenarios that reflect changes to precipitation amounts, their seasonal distribution, and evaporative demand. The redistribution of precipitation over a shortened rainy season highlighted a strong coupling of evapotranspiration to incoming precipitation at the coastal site, while the lower water-holding capacity of soils at the inland site resulted in additional drainage occurring under this scenario. The loss of spring rains due to a shortening of the rainy season also revealed a greater impact on the inland site, suggesting less resilience to low moisture at a time when plant development is about to start. The results also suggest that the coastal site would suffer disproportionally from extended dry periods, effectively driving these areas into more extreme drought than previously seen. These sensitivities suggest potential future increases in the risk of wildfires under climate change, as well as increased grassland ecosystem vulnerability. 

   more » « less
2. Ecohydrologic Dynamics of Rock Moisture in a Montane Catchment of the Colorado Front Range

   https://doi.org/10.1029/2022WR034117  

   Burns, E. F. ; Rempe, D. M. ; Parsekian, A. D. ; Schmidt, L. M. ; Singha, K. ; Barnard, H. R. ( June 2023 , Water Resources Research)

   Warming across the western United States continues to reduce snowpack, lengthen growing seasons, and increase atmospheric demand, leading to uncertainty about moisture availability in montane forests. As many upland forests have thin soils and extensive rooting into weathered bedrock, deep vadose‐zone water may be a critical late‐season water source for vegetation and mitigate forest water stress. A key impediment to understanding the role of the deep vadose zone as a reservoir is quantifying the plant‐available water held there. We quantify the spatiotemporal dynamics of rock moisture held in the deep vadose zone in a montane catchment of the Rocky Mountains. Direct measurements of rock moisture were accompanied by monitoring of precipitation, transpiration, soil moisture, leaf‐water potentials, and groundwater. Using repeat nuclear magnetic resonance and neutron‐probe measurements, we found depletion of rock moisture among all our monitored plots. The magnitude of growing season depletion in rock moisture mirrored above‐ground vegetation density and transpiration, and depleted rock moisture was from ∼0.3 to 5 m below ground surface. Estimates of storage indicated weathered rock stored at least 4%–12% of mean annual precipitation. Persistent transpiration and discrepancies between estimated soil matric potentials and leaf‐water potentials suggest rock moisture may mitigate drought stress. These findings provide some of the first measurements of rock moisture use in the Rocky Mountains and indicated rock moisture use is not just confined to periods of drought or Mediterranean climates.

    

   more » « less
3. Lithologically Controlled Subsurface Critical Zone Thickness and Water Storage Capacity Determine Regional Plant Community Composition

   https://doi.org/10.1029/2018WR023760  

   Hahm, W. Jesse ; Rempe, Daniella M. ; Dralle, David N. ; Dawson, Todd E. ; Lovill, Sky M. ; Bryk, Alexander B. ; Bish, David L. ; Schieber, Juergen ; Dietrich, William E. ( April 2019 , Water Resources Research)

   Explanations for distinct adjacent ecosystems that extend across hilly landscapes typically point to differences in climate or land use. Here we document—within a similar climate—how contrasting regional plant communities correlate with distinct underlying lithology and reveal how differences in water storage capacity in the critical zone (CZ) explain this relationship. We present observations of subsurface CZ structure and groundwater dynamics from deep boreholes and quantify catchment‐wide dynamic water storage in two Franciscan rock types of the Northern California Coast Ranges. Our field sites have a Mediterranean climate, where rains are out of phase with solar energy, amplifying the importance of subsurface water storage for periods of peak ecosystem productivity in the dry season. In the deeply weathered (~30 m at ridge) Coastal Belt argillite and sandstone, ample, seasonally replenished rock moisture supports an evergreen forest and groundwater drainage sustains baseflow throughout the summer. In the Central Belt argillite‐matrix mélange, a thin CZ (~3 m at ridge) limits total dynamic water storage capacity (100–200 mm) and rapidly sheds winter rainfall via shallow storm and saturation overland flow, resulting in low plant‐available water (inferred from predawn tree water potential) and negligible groundwater storage that can drain to streams in summer. This storage limitation mechanism explains the presence of an oak savanna‐woodland bounded by seasonally ephemeral streams, despite >1,800 mm of average precipitation. Through hydrologic monitoring and subsurface characterization, we reveal a mechanism by which differences in rock type result in distinct regionally extensive plant communities under a similar climate.

    

   more » « less
4. A numerical investigation of bedrock groundwater recharge and exfiltration on soil mantled hillslopes

   https://doi.org/10.1002/hyp.13799  

   Gardner, W. Payton ; Jensco, Kelsey ; H. Hoylman, Zachary ; Livesay, Robert ; P. Maneta, Marco ( June 2020 , Hydrological Processes)

   Here we use Richards Equation models of variably saturated soil and bedrock groundwater flow to investigate first‐order patterns of the coupling between soil and bedrock flow systems. We utilize a Monte Carlo sensitivity analysis to identify important hillslope parameters controlling bedrock recharge and then model the transient response of bedrock and soil flow to seasonal precipitation. Our results suggest that hillslopes can be divided into three conceptual zones of groundwater interaction, (a) the zone of lateral unsaturated soil moisture accumulation (upper portion of hillslope), (b) the zone of soil saturation and bedrock recharge (middle of hillslope) and (c) the zone of saturated‐soil lateral flow and bedrock groundwater exfiltration (bottom of hillslope). Zones of groundwater interaction expand upslope during periods of precipitation and drain downslope during dry periods. The amount of water partitioned to the bedrock groundwater system a can be predicted by the ratio of bedrock to soil saturated hydraulic conductivity across a variety of hillslope configurations. Our modelled processes are qualitatively consistent with observations of shallow subsurface saturation and groundwater fluctuation on hillslopes studied in our two experimental watersheds and support a conceptual model of tightly coupled shallow and deep subsurface circulation where groundwater recharge and discharge continuously stores and releases water from longer residence time storage.

    

   more » « less
5. Diagnosing evapotranspiration responses to water deficit across biomes using deep learning

   https://doi.org/10.1111/nph.19197  

   Giardina, Francesco ; Gentine, Pierre ; Konings, Alexandra G. ; Seneviratne, Sonia I. ; Stocker, Benjamin D. ( August 2023 , New Phytologist)

   Accounting for water limitation is key to determining vegetation sensitivity to drought. Quantifying water limitation effects on evapotranspiration (ET) is challenged by the heterogeneity of vegetation types, climate zones and vertically along the rooting zone.

   Here, we train deep neural networks using flux measurements to study ET responses to progressing drought conditions. We determine a water stress factor (fET) that isolates ET reductions from effects of atmospheric aridity and other covarying drivers. We regress fET against the cumulative water deficit, which reveals the control of whole‐column moisture availability.

   We find a variety of ET responses to water stress. Responses range from rapid declines of fET to 10% of its water‐unlimited rate at several savannah and grassland sites, to mild fET reductions in most forests, despite substantial water deficits. Most sensitive responses are found at the most arid and warm sites.

   A combination of regulation of stomatal and hydraulic conductance and access to belowground water reservoirs, whether in groundwater or deep soil moisture, could explain the different behaviors observed across sites. This variety of responses is not captured by a standard land surface model, likely reflecting simplifications in its representation of belowground water storage.

    

   more » « less