A number of methods for studying or predicting evapotranspiration are discussed. These include regional water balance studies, the Penman-Monteith equation, the Priestley-Taylor and the Makkink equations, wet canopy evaporation models and models of the Planetary Boundary Layer (PBL). PBL models can be used to predict the saturation vapour deficit after land cover changes. Coupling of PBL models with e.g. the Penman-Monteith equation to estimate the latent heat flux, allows for studying the influence of land cover changes on vapour and heat exchange processes and on the stability of the lower atmosphere. Denudation of once forested land causes an increase of the ratio of sensible heat flux to latent heat flux. The ultimate result of this change in the energy balance is that water originally lost through evapotranspiration, now has to drain superficially. This will cause an increase of the erosion hazard. In addition, less vapour will remain to evaporate into the atmosphere and less vapour will be available for cloud formation. Hence, massive land denudation will provide a positive feedback to any tendency to aridity, it will imbalance local rainfall and the global energy balance.
