SOME PLANT FACTORS CONTROLLING EVAPOTRANSPIRATION

Stomatal conductance varies within a leaf, between leaves in a canopy and between layers. It depends primarily on light and it is possible to derive a canopy conductance, g(c), by summing the stomatal conductances of leaves of all layers. This may be done by measurements or by using a simple model. Such a calculated canopy conductance can be used successfully in the Penman-Monteith formula to predict the evapotranspiration of a closed canopy well supplied with water. For incomplete canopies, soil and plant evaporation require different treatments. The ratio E(c)/E(o) of canopy transpiration to open water evaporation varies roughly in proportion with the fraction of soil covered by vegetation, while the ratio E(s)/E(o) of soil evaporation to E(o) is controlled both by net radiation at the soil surface and surface soil moisture. Soil water limitation may be modelled by making E(c)/E(o) a function of soil water reserve or through a variable stomatal conductance, g(s). In the latter case, g(s) may be related to either soil water potential or leaf water potential, which depends on transpiration and resistance to liquid water flow through the plant. Such resistance is shown to be relatively conservative at the canopy level, but its usefulness for predicting evapotranspiration is debatable since a part of stomatal closure is now attributed to hormonal control.