Methodologies for the Calculation of Actual Evaporation in Geotechnical Engineering

The prediction of actual evaporation (AE) is required when calculating the net moisture flux from the ground surface to the atmosphere. The proposed Wilson-Penman equation appeared to yield reasonable predictions of AE from saturated clayey soils but overpredicted AE from coarse-grained soils in arid regions. In recent years, two distinct approaches have emerged for the prediction of AE from unsaturated soil surfaces. Both approaches are based on the realization that evaporation tends to “shut off” as the natural water content approaches residual water content conditions. The first approach to calculate AE from an unsaturated soil involves the adjustment of the total suction at the ground surface. The second approach to calculate AE involves the determination of the “evaporation-rate reduction point” from the drying soil-water characteristic curve. A vapor pressure reduction factor is then applied to calculate AE. Both approaches are attempts to take the effects of “surface resistance” into consideration. The paper presents the results of a comparative study in which both approaches are used to calculate AE from drying sand column tests. These results provide the geotechnical engineer with improved computational procedures for the calculation of AE from a wide range of soil types.