Improved Understanding of the Link Between Catchment‐Scale Vegetation Accessible Storage and Satellite‐Derived Soil Water Index

The spatiotemporal dynamics of water volumes stored in the unsaturated root zone are a key control on the response of terrestrial hydrological systems. Robust, catchment‐scale root‐zone soil moisture estimates are thus critical for reliable predictions of river flow, groundwater recharge, or evaporation. Satellites provide estimates of near‐surface soil moisture that can be used to approximate the moisture content in the entire unsaturated root zone through the Soil Water Index (SWI). The characteristic time length (T, in days), as only parameter in the SWI approach, characterizes the temporal variability of soil moisture. The factors controlling T are typically assumed to be related to soil properties and climate; however, no clear link has so far been established. In this study, we hypothesize that optimal T values (Topt) are linked to the interplay of precipitation and evaporation during dry periods, thus to catchment‐scale vegetation accessible water storage capacities in the unsaturated root zone. We identify Topt by matching modeled time series of root‐zone soil moisture from a calibrated process‐based hydrological model to SWI from several satellite‐based near‐surface soil moisture products in 16 contrasting catchments in the Meuse river basin. Topt values are strongly and positively correlated with vegetation accessible water volumes that can be stored in the root zone, here estimated for each study catchment both as model calibration parameter and from a water‐balance approach. Differences in Topt across catchments are also explained by land cover (% agriculture), soil texture (% silt), and runoff signatures (flashiness index). Plain Language Summary The amount of water in the soil accessible to roots of plants for growth is a key element to understand and predict short‐ and long‐term dynamics of the hydrological cycle in a river basin. Satellites provide worldwide estimates of water amounts in the first few centimeters of the soil. If the time scale of water transport from the surface to the root zone is known, this near‐surface water amount can be used to estimate the water amount in the entire root zone of vegetation. We hypothesize that this time scale depends on the maximum amount of water in the soil that is accessible to roots. We show that using river discharge, rainfall, and evaporation data, we can estimate the maximum amount of water that is available to roots and, therefore, the time scale needed to estimate water amounts in the root z