Non-steady-state modelling of water transfer in a Mediterranean evergreen canopy

A model simulating the diurnal pattern of water transfer within a Helm oak (Quercus ilex) canopy in Mediterranean conditions has been designed. It combines a non-steady-state hydraulic model with a transpiration model. The hydraulic model includes a reservoir represented by a capacitance, a soil-plant hydraulic resistance and a storage hydraulic resistance connected to the capacitance. It simulates the diurnal variation of water uptake and storage flow from the diurnal course of transpiration used as input. The transpiration model is based upon the Penman-Monteith equation and a Jarvis-type representation of the stomatal resistance (i.e., a minimum stomatal resistance multiplied by the product of independent stress functions). Simultaneous measurements of canopy evaporation by an eddy covariance system and water uptake from the soil by sap flow measurements have allowed one to calibrate and validate the model. The capacitance has been found to be equal to 0.17 mm MPa-1 (with a storage hydraulic resistance of about 2 MPa h mm(-1)), generating a time lag of about 1 h between the transpiration rate and the water uptake from the soil. The hydraulic model correctly represents the experimental data. The transpiration model provides reasonable estimates, but with a significant scatter. The combined model simulates the diurnal variation of water uptake, storage flow and transpiration rate directly from environmental variables, but in this latter case, the storage flow is estimated with a rather poor accuracy. (C) 2001 Elsevier Science B.V. All rights reserved.