PARAMETERIZATION OF THE SHUTTLEWORTH-WALLACE MODEL TO ESTIMATE DAILY MAXIMUM TRANSPIRATION FOR USE IN CROP MODELS

In crop models maximum transpiration is an important component of the computation of water stress factors. It depends on reference climatic variables and leaf area index, and also on soil evaporation which modi fies the actual air properties around the plants. This last effect is not accounted for in classical approaches used in crop models, Yet Shu ttleworth and Wallace theory offers a framework to simulate canopy and soil evaporation fluxes in a coupled way. In this paper an adaptation and a parameterisation of the basic equations from Shuttleworth and W allace is proposed, allowing use of the model to calculate maximum tra nspiration by using daily variables. The adaptation concerns soil evap oration. A potential soil evaporation is calculated assuming that, whe n the soil surface is wet, total evaporative flux consumes the whole a vailable energy. It is used as an input to a two-staged model to calcu late actual soil evaporation. The parameterisation relies on two field experiments performed on well-irrigated soybean, Measurements of net radiation balance show that radiation extinction within the canopy is less than generally admitted. Simulations of daily soil evaporation ex hibit the same dynamics as microlysimeter measurements, which can be h igh even when the crop is fully developed. Bulk canopy resistances der ived from Bowen ratio measurements agree closely with values obtained from classical formulae using a mean stomatal resistance of 250 ms(-1) The modified and properly parameterised model shows that the contribu tion of plants to total evapotranspiration is highly variable as a res ult of the interactions between direct soil evaporation and plant tran spiration. (C) 1998 Elsevier Science B.V. All rights reserved.