Effects of particle characteristics on the evaporation rate from unsaturated medium were examined by experiments. Also, three models for estimating the temporal change of evaporation rate were tested about their applicability and parameter values for each medium.
The evaporation rates were determined for the unsaturated media consisted of glass-beads or Akadama Soil of uniform diameter. During the experiments, the potential evaporation was kept constant by maintaining constant temperatures and relative humidities (23°C, 40% and 28°C, 36%). The experimental results showed that the volumetric water content was the most explanatory parameter of the evaporation rate.
The observed temporal changes of evaporation rates were analyzed using three models. The first model was based on the assumption that the cumulative evaporation is proportional to the square root of time (Gardner (1959)). The second model was improved one of the first model for its early stage of evaporation. The third model assumed that the temporal evaporation rate is proportional to the reciprocal of cumulative evaporation. The results showed that the second and third models could present better estimation than the first one. Under the conditions of higher potential evaporation, the early evaporation rate, E_o in the third model decreased as the particle size increased.

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