The Components of Evapotranspiration

Evapotranspiration is the sum of evaporation from the soil and the plant surfaces, and transpiration. The evaporation from the soil in most cases follows a two-stage process depending on whether the soil surface is wet after rain or irrigation or has already dried up. When the soil surface is wet, the evaporation rate is potentially very high; that is why the rainfall frequency is the main driver of soil evaporation, especially at low ground cover. The core model to quantify evaporation is the combination equation, later applied to crop canopies and for computing plant transpiration known as the Penman–Monteith equation. This equation has two resistance variables (the aerodynamic and canopy resistance) which are hard to quantify as they change constantly with the physical environment and the plant’s physiological state. The Penman–Monteith equation is the established method to analyze the evaporation processes in plants and stands and has been thoroughly verified. The transpiration of trees is heavily dependent on canopy conductance and scales up well with the ground cover or the fraction of intercepted radiation.