BIRDSALL-DREISS DISTINGUISHED LECTURE: HOW DO POROUS TERRESTRIAL SURFACES CONTROL EVAPORATION INTO THE ATMOSPHERE?

Globally, evaporation consumes about 25% of solar energy input, and it drives the hydrological cycle by sending more than 60% of terrestrial precipitation back to the atmosphere via evapotranspiration. Quantifying evaporation is important for assessing changes in hydrologic reservoirs and surface energy balance, and for many industrial and engineering applications. Key interactions of evaporating surfaces with internal transport mechanisms and with environmental conditions remain largely empirical. Evaporation dynamics from porous media is significantly different than from free water surfaces due to liquid withdrawal from internal pore spaces, and nonlinearities arising from gradual drying of surfaces. Porous media properties are responsible for the often abrupt transition from initially high evaporation rate (stage-1) to a slower diffusion-controlled stage-2. This phenomenon is attributed to disruption of capillary liquid continuity needed to supply surface evaporation. The nonlinear response of evaporation rate to surface water content is attributed to enhanced vapor fluxes from active pores that become increasingly isolated as the surface gradually dries. Increased spacing between evaporating pores for thick boundary layer (atmospheric demand < 5 mm/day) result in an increase in evaporative flux per pore that, in turn, may fully compensate for reduced evaporative surface area sustain a constant evaporation rate. Implications of the findings for estimates of evaporative losses used in hydrological and climate models will be discussed