The Science of the Total Environment 313 (2003) 141–152 0048-9697/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0048-9697(03)00268-7 Estimating falling rate evaporation from finite soil columns Ernest K. Yanful*, S. Morteza Mousavi Geotechnical Research Centre, Department of Civil and Environmental Engineering, The University of Western Ontario, London, Ont., Canada N6A 5B9 Received 6 June 2002; accepted 12 April 2003 Abstract Estimating evaporation is very important in many fields including agriculture and geo-environmental engineering. For example evaporation has a definitive role in the performance of soil covers widely used in mining and landfill applications for environmental protection. The ability to estimate evaporation from different soils without performing detailed experiments or modeling could be very useful especially during the initial stages of design. Gardner and Hillel developed an equation for calculating evaporation from finite soil columns involving the soil moisture diffusivity. The capability of the equation to predict falling rate evaporation from finite coarse sand and fine sand was studied. The studied cases were also modeled using the finite element model SoilCover. The results from the model and the equation were compared to experimental data and found to agree reasonably. The agreement indicates that the analytical solution could be used to estimate falling rate evaporation from finite columns of coarse sand and fine sand. It can facilitate the estimate of actual evaporation from soils especially in the design of engineered covers. The necessary data required are soil–water characteristic curve, the hydraulic conductivity-suction function and initial water content. The analytical method relies heavily on the soil diffusivity; hence the concepts of diffusivity and its role in water movement in unsaturated soil are also discussed.  2003 Elsevier Science B.V. All rights reserved. Keywords: Evaporation; Diffusivity; Unsaturated; Experimental; Modeling; Sand 1. Introduction Evaporation is an important process that controls the moisture regime and the moisture availability to vegetation. In engineered soil caps and covers used for environmental protection, evaporation and evapotranspiration rates can exert significant influ- ence on suction, unsaturated hydraulic conductivity *Corresponding author. Tel.: q1-519-661-4069; fax: q1- 519-661-3942. E-mail address: eyanful@eng.uwo.ca (E.K. Yanful). and the overall water flux (Yang and Yanful, 2002). The design of such covers requires a prediction of the balance of precipitation, storage, runoff, evaporation or evapotranspiration. Evapo- ration is the most difficult part of this balance to obtain. In general, evaporation occurs in two forms, namely from free water surface (potential evaporation) and from the soil surface (actual evaporation). The drying process occurs in two fairly distinct stages (Fisher, 1923; Pearse et al., 1949): (1) constant rate stage and (2) falling rate