Root Water Uptake Resolved by Distributed Moisture Storage Changes Through Soil and Weathered Bedrock

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.