A method for predicting evaporation process in unsaturated soil surface is presented based on physical properties of soil and meteorological data. The soil-water characteristic curve (SWCC) is estimated by physical properties of soil (i.e. particle size distribution

specific gravity and dry density). Then

the relationship between relative humidity of soil surface and water content is obtained based on the SWCC. Finally

the Penman-Wilson model is applied to predict evaporation curve of unsaturated soil surface. In this method

only in situ soil physical properties and meteorological data are needed to get the predicted evaporation curve. The evaporation rate of soil surface can be obtained with given water content at that time. Laboratory evaporation tests of thin layer soils are conducted in a designed evaporation measuring system. The measured evaporation curves are compared with those predicted. The results show that the measured and prediction evaporation curves have same processes and slight difference in evaporation rate. Three evaporation stages

i.e. stable rate stage

reducing rate stage and residual stage

are evidently shown in both observed and predicted curves. The stages are divided by as called critical moisture content and air-dried moisture content. The suggested method is accurate and practical for engineers to predict unsaturated soil surface evaporation rate and determine the boundary flux.