Coupling of heat, water vapor, and liquid water fluxes to compute evaporation in bare soils Marco Bittelli a, * , Francesca Ventura a , Gaylon S. Campbell b , Richard L. Snyder c , Fabia Gallegati a , Paola Rossi Pisa a a Department of AgroEnvironmental Science and Technology, University of Bologna, Viale Fanin, 44, 40125 Bologna, Italy b Decagon Devices Inc., Pullman, WA, USA c Department of Air, Land and Water Resources, University of California, Davis, CA, USA Received 7 May 2007; received in revised form 8 January 2008; accepted 15 August 2008 KEYWORDS Evaporation; Coupled fluxes; Soil water content; Aerodynamic resistance; Soil surface resistance Summary The quantification of soil evaporation and of soil water content dynamics near the soil surface are critical in the physics of land-surface processes on regional and global scales, in particular in relation to mass and energy fluxes between the ground and the atmosphere. Although it is widely recognized that both liquid and gaseous water move- ment are fundamental factors in the quantification of soil heat flux and surface evapora- tion, their computation is still rarely considered in most models or practical applications. Moreover, questions remain about the correct computation of key factors such as the soil surface resistance or the soil surface temperature. This study was conducted to: (a) implement a fully coupled numerical model to solve the governing equations for liquid water, water vapor, and heat transport in bare soils, (b) test the numerical model with detailed measurements of soil temperature, heat flux, water content, and evaporation from the surface, and (c) test different formulations for the soil surface resistance param- eter and test their effect on soil evaporation. The code implements a non-isothermal solu- tion of the vapor flux equation that accounts for the thermally driven water vapor transport and phase changes. Simulated soil temperature, heat flux, and water content were in good agreement with measured values. The model showed that vapor transport plays a key role in soil mass and energy transfer and that vapor flow may induce sinusoidal variations in soil water content near the surface. Different results were obtained for evap- oration calculations, depending on the choice of the soil surface resistance equation, which was shown to be a fundamental term in the soil–atmosphere interactions. The 0022-1694/$ - see front matter ª 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2008.08.014 * Corresponding author. Tel.: +39 051 2096656; fax: +39 051 2096241. E-mail address: marco.bittelli@unibo.it (M. Bittelli). Journal of Hydrology (2008) 362, 191– 205 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jhydrol