Mixture of Time Scales in Evaporation: Desorption and Self-Similarity of Energy Fluxes

The time evolution of evaporation from a bare soil, over a 9-d period following irrigation, is described by a combination of daily and hourly drying patterns. From the second day, the daily evaporation shows a second stage of drying that can be described as a desorptive process (evaporation proportional to (t - to)-1/2, where t is time in days and to is the day when the second stage starts). The short time (hourly) evaporation rate can be modeled on the basis of a type of self-similarity in the energy balance components. Combining the evaporative flux behavior at the two time scales, desorption at the daily timescale and self-similarity for the diurnal variations, a robust description of evaporation for drying land surfaces is obtained. This approach is tested using accurate measurements of the different com-ponents of the energy balance at the soil surface, obtained at 20-min intervals. The model accurately describes the time evolution of the evaporative flux and could be used for the disaggregation of daily or weekly evaporation into hourly values.