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DSSS: How do Porous Terrestrial Surfaces Control Evaporation into the Atmosphere?


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Dani Or is a professor of Soil and Terrestrial Environmental Physics and Director of the Institute of Terrestrial Ecosystems (ITES) in the Department of Environmental Systems Science at the Swiss Federal Institute of Technology (ETH) Zurich in Switzerland. His research focuses on mass and energy transport in porous media, on mechanics of abrupt landslides and avalanches, and on linking physical processes and biological activity in porous media. Dr. Or has authored over 180 refereed publications, co-authored a book, and over 270 proceeding papers and abstracts. Dr. Or is the outgoing Editor in Chief of the Vadose Zone Journal, recipient of the Kirkham Soil Physics Award (2001), 2004 Fellow of the Soil Science Society of America, chair of the 2008 Gordon Research Conference on Flow and Transport (Oxford, UK), and 2010 Fellow of the American Geophysical Union. He is the 2013 Birdsall-Dreiss distinguished lecturer, and the recipient of the 2013 Helmholtz International Fellow Award. For more information visit:


Globally, evaporation consumes about 25% of solar energy input, and it drives the hydrological cycle by sending about 60% of terrestrial precipitation back to the atmosphere. Quantifying evaporation is important for assessing changes in hydrologic reservoirs, 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 the gradual drying of evaporating surfaces. Porous media properties determine the often abrupt transition from initially high evaporation rate (stage-1) to slower and diffusion-controlled stage-2 evaporation. The transition is marked by disruption of capillary liquid pathways that supply evaporation during stage-1 (vaporization plane at the surface). The nonlinear relationship between surface water content and evaporation rate is related to enhancement of diffusion fluxes from evaporating pores that become increasingly isolated with surface drying. The increased spacing between remaining active pores for low atmospheric demand (low wind speeds and thick boundary layer, typically < 5 mm/day) result in an increase in evaporative flux per pore that may fully compensate for the reduced surface water content during drying and thus sustain a constant evaporation rate. Implications for formulation of rigorous boundary conditions, and for constraining estimates of evaporative losses by hydrological and climate models will be discussed.