This work refers to the potential evapotranspiration (ETp) considered as a meteorological element according to Thornthwaite and Wilm. Later on Penman and Sanderson have published also pioneer works on ETp. ETp can be considered the opposite of the rainfall. While this one means the water transferred from atmosphere to the surface, the former means the water back to the atmosphere. ETp represents the transformation of the liquid water state to the gaseous state. This process depends on the energy received from the sun and of the energy balance on a vegetated surface.The soil moisture depends on the water balance between the rainfall and the ETp data. The rainfall data can be easily measured by raingages, but the ETp measurement is more difficult: it is required equations based on several meteorological elements. The Thornthwaite model works well under humid climate conditions, but in dry and arid climates it underestimates the ETp because does not consider the saturation deficit of the air, that is very high in these climatic conditions. Recent studies showed that the problem may be reduced by using a modified mean daily temperature model that uses an adjusted mean temperature based on the daily thermometric amplitude (Tmax-Tmin). This amplitude is usually higher in dry climate and lower in superhumid climate, allowing the correction to estimate the ETp values in both cases.
Thornthwaite model; Camargo-71 model; water balance; concept of ETp; accuracy of equation
