Parameterisation 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 modifies the actual air properties around the plants. This last effect is not accounted for in classical approaches used in crop models. Yet Shuttleworth 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 Wallace is proposed, allowing use of the model to calculate maximum transpiration by using daily variables. The adaptation concerns soil evaporation. A potential soil evaporation is calculated assuming that, when the soil surface is wet, total evaporative flux consumes the whole available energy. It is used as an input to a two-staged model to calculate 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 exhibit the same dynamics as microlysimeter measurements, which can be high even when the crop is fully developed. Bulk canopy resistances derived 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 contribution of plants to total evapotranspiration is highly variable as a result of the interactions between direct soil evaporation and plant transpiration.