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 com bines a non-steady-state hydraulic model with a transpiration model. The hy draulic model includes a reservoir represented by a capacitance, a soil-pla nt hydraulic resistance and a storage hydraulic resistance connected to the capacitance. It simulates the diurnal variation of water uptake and storag e flow from the diurnal course of transpiration used as input. The transpir ation model is based upon the Penman-Monteith equation and a Jarvis-type re presentation of the stomatal resistance (i.e., a minimum stomatal resistanc e multiplied by the product of independent stress functions). Simultaneous measurements of canopy evaporation by an eddy covariance system and water u ptake 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)), gen erating a time lag of about 1 h between the transpiration rate and the wate r uptake from the soil. The hydraulic model correctly represents the experi mental data. The transpiration model provides reasonable estimates, but wit h a significant scatter. The combined model simulates the diurnal variation of water uptake, storage flow and transpiration rate directly from environ mental variables, but in this latter case, the storage flow is estimated wi th a rather poor accuracy. (C) 2001 Elsevier Science B.V. All rights reserv ed.