Emergence from drought: Direct and remote observations of moisture storage deficits, plant water stress, and groundwater reveal ecohydrologic recovery

Plant transpiration, groundwater, and streamflow responses to changes in precipitation rates are moderated by largely unobserved partitioning of unsaturated subsurface water storage. Direct observations of deep root-zone storage dynamics are therefore critical to understanding the processes driving frequently observed surficial responses to drought and drought recovery, namely stream discharge and plant stress. Here we present six years of direct measurements of plant water stress and storage in soil and weathered bedrock (approx. 1-5 m) from our rain-dominated oak savanna study site in the California Coast Range, providing a window into the subsurface during the wet 2019 water year (WY), through three consecutive dry years of severe drought, and concluding with two wet years of drought recovery. During the drought, precipitation was insufficient to replenish plant water uptake from the unsaturated root-zone underlying predominantly north-facing slopes with oaks and south-facing slopes without oaks resulting in the multiyear development of a storage deficit. Increased plant water stress led to dieback, and the persistent deficit did not permit substantial infiltration of precipitation beyond the unsaturated root-zone to recharge groundwater and generate streamflow. Substantial high-intensity rainfall during the wet water years following drought (2023 and 2024) quickly eliminated the accumulated storage deficits, leading to rising water tables and streamflow. Thus, we observe initiation of hydrologic drought recovery requires sufficient precipitation to replenish root-zone storage deficits. Furthermore, we advocate for the presence and magnitude of storage deficits (which can be estimated via distributed evapotranspiration and precipitation products) as a metric of drought presence and severity.