Experimental Consideration on the Effect of Different Soil Types on Soil Moisture Dependence on Evaporation Efficiency in Unsaturated Sandy Soil

In risk assessments of slopes after rainfall, to quantify the amount of fluid outflow from the system, it is essential to obtain the amount of evaporation from the slope surface. The amount of evaporation can be estimated from the general meteorological data and the soil surface temperature using the bulk method (β-method). Evaporation efficiency, an essential parameter of the bulk method, expresses the ease with which evaporation occurs and is dependent on soil moisture and surface soil type. However, the effect of different surface soil types on the soil moisture associated with evaporation efficiency has not yet been quantitatively evaluated using soil parameters. This study discusses the effect of different soil types on the soil moisture dependence on evaporation efficiency based on the results of the laboratory experiments on two types of sandy soil. Results showed that the effect of different soil types can be evaluated by focusing on the range where evaporation efficiency is valued at less than 1.0. Furthermore, the experimental results confirmed that the evaporation efficiency was slightly smaller for samples with higher fine fraction content in the range of shallow soil moisture content. The result showed consistency with the estimations since the higher the fine fraction content, the smaller and more structurally complex the void becomes. These findings suggested that the effect of soil conditions on the soil moisture dependence on evaporation efficiency can be evaluated using the void size distribution.