The relationship between actual (Eact) and potential (Ep) transpiration above a grass-covered forest clearing was investigated numerically and experimentally from simultaneous measurements of soil moisture content profiles, mean meteorological conditions, turbulent heat and water vapor fluxes in the atmospheric surface layer, and soil hydraulic properties for two drying periods. The relationship between Eact/Ep was found to be approximately constant and insensitive to variability in near-surface soil moisture content. To explore this near-constant Eact/Ep, a model that relates potential and actual transpiration and accounts for root-uptake efficiency, potential transpiration rate, and root-density distribution was proposed and field-tested. The total amount of water consumed by the root system was integrated and compared with eddy-correlation latent heat flux measurements (field scale) and total water storage changes (local scale). Model calculations suggested that the deeper and more efficient roots are primarily responsible for the total water loss within the root zone when the near-surface soil layer approaches their wilting point.
