The dynamic role of root-water uptake in coupling potential to actual transpiration

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 pro®les, mean meteorological conditions, turbulent heat and water vapor ̄uxes 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 eciency, potential transpiration rate, and root-density distribution was proposed and ®eld-tested. The total amount of water consumed by the root system was integrated and compared with eddy-correlation latent heat ̄ux measurements (®eld scale) and total water storage changes (local scale). Model calculations suggested that the deeper and more ecient roots are primarily responsible for the total water loss within the root zone when the near-surface soil layer approaches their wilting point. Ó 1999 Elsevier Science Ltd. All rights reserved.