Climate, soil, and vegetation: 4. The expected value of annual evapotranspiration

The depth of interstorm evapotranspiration from natural surfaces is composed (by proportion to vegetal canopy density) of evaporation from bare soil and transpiration from vegetation. The former is obtained in terms of random variables describing initial soil moisture, time between storms, and potential rate of evapotranspiration from an exfiltration analogy to the Philip infiltration equation modified to incorporate moisture extraction by plant roots. The latter is assumed to occur at the potential rate for natural vegetal systems. In a zeroth‐order approximation the initial soil moisture is fixed at its climatic space and time average whereby using an exponential distribution of time between storms and a constant potential rate of evapotranspiration the expected value of interstorm evapotranspiration is derived. This mean value is used to obtain the annual average point evapotranspiration as a fraction of the potential value and as a function of dimensionless parameters defining the climate‐soil‐vegetation system.