Root water uptake resolved by distributed moisture storage changes through soil and weathered bedrock

Authors: Lapides, Dana; DRALLE, DAVID - Us Forest Service (FS); HAHM, W. - Simon Frasier University; DIETRICH, WILLIAM - University Of California Berkeley; REMPE, DENIELLA - University Of Texas At Austin

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/12/2025
Publication Date: 10/15/2025
Citation: Lapides, D.A., Dralle, D.N., Hahm, W.J., Dietrich, W.E., Rempe, D.M. 2025. Root water uptake resolved by distributed moisture storage changes through soil and weathered bedrock. Water Resources Research. 61(10). Article e2025WR040778. https://doi.org/10.1029/2025WR040778.
DOI: https://doi.org/10.1029/2025WR040778

Interpretive Summary: Plant behavior is a major unknown for water resources management and earth systems modeling. Our knowledge limitations stem both from observational limitations (it's hard to study things happening deep underground) and methods for processing data where they do exist. In this study, we present a unique dataset of depth-distributed moisture dynamics through a 12 m deep unsaturated zone in Northern California (Rivendell) and a new method for processing these data to obtain timeseries of depth-distribution drainage and evapotranspiration. We test the new method against simulation data from the model HYDRUS and compare its performance to existing methods in the literature. Our results demonstrate that the new method is the only method capable of capturing depth-distributed evapotranspiration signals. By applying this new method to the Rivendell dataset, we find that plants primarily use water stored in soils during the cool, wet winters. Entering the dry season, water use spreads over a progressively deeper profile until the start of the following wet season. Our findings emphasize the importance of water stored below soils for sustaining transpiration through the dry season. Further, we find that deep drainage continues throughout the dry season, suggesting that root zone dynamics may play an important role in regulating low flows.

Technical Abstract: Understanding how plants access water is critical to biosphere-atmosphere interactions. However, it remains challenging to resolve root water uptake in space and time. Here, we introduce (a) a mass balance method that uses depth-distributed moisture changes in the vadose zone to spatially resolve patterns of evapotranspiration (ET) and (b) an application of this method to a unique data set of continuous moisture dynamics across a deeply weathered root zone in a seasonally dry forest in coastal California. These observations are made possible by a Vadose-zone Monitoring System on a steep hillslope (“Rivendell”) in the Angelo Coast Range Reserve. The new mass balance method accurately distinguishes between numerically generated vertically distributed ET and drainage fluxes. Synthetic tests across nine climate types show that the new method is broadly applicable in arid and Mediterranean regions. By applying the new mass balance method to the Rivendell data set, we determined spatiotemporal water fluxes in the deep root-zone at daily temporal resolution. Layers of the subsurface wet up simultaneously in the wet season. In the wet season, plant moisture for root water uptake was derived primarily from the soil. As the dry summer progresses, water uptake spreads to successively deeper depths until it occurs nearly equivalently across all depths. Water uptake at all depths across years is essentially the same, except in soil where water use patterns follow wet season precipitation patterns. Our results demonstrate that dry season unsaturated zone dynamics mediate the timing and magnitude of recharge to groundwater, with potential implications for summer streamflow.