A simple method has been developed on the basis of the Watanabe's new technique for measuring evaporation, to calculate both the bulk resistance and the aerodynamic resistance of the soil to vapor diffusion. The aerodynamic resistance was used for isolating the real surface resistance from the bulk resistance. Also it was used for calculating surface, momentum and sensible heat roughness lengths for the bare sand soil. Laboratory tests were carried out on fine and coarse sand under a constant temperature and humidity conditions, to compare the results of steady evaporation, surface resistance, aerodynamic resistance and roughness lengths. It was found that, the steady evaporation from fine sand is much larger than that from coarse sand under the same conditions of relative humidity, temperature, depth of the ground water table, etc. The surface resistance for both fine and coarse sand was modeled as a power function of the top 0 to 0.50 cm soil moisture. The results indicated that, the surface resistance of the fine sand is much smaller than that of the coarse sand when the two types of sand were put under the same conditions. The two types of sand have almost the same aerodynamic resistance under the same wind conditions. A proposed iterative technique was used to calculate surface, momentum, and sensible heat roughness lengths for the two types of sand. Measurements were done and back analysis technique was used for determining the unsaturated hydraulic properties on the basis of van-Genuchten's model. The Finite Element Method with Galerkin's technique has been used to analyze the moisture movement in the unsaturated zone to get the top soil moisture content at any time during the evaporation process.

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The unsaturated hydraulic properties are very important parameter in any quantitative description of partially saturated flow equation. Currently, most in-situ methods are estimating the unsaturated hydraulic conductivity k (θ) on basis of analysis that require estimates of the soil water flux and capillary pressure head gradient. These analysis typically involve differencing of field measured capillary pressure head h (θ) and saturation ratio θ data, so that this process can significantly amplify instrumental and measurement errors. The k (θ) values have additional errors from differencing in θ and h data. More over, reliable technique result is not easy to be carried out. One such method is based on the estimates of the unsaturated hydraulic properties k (θ), h (θ) of soil which may be computed inversely from the measured saturation through the evaporation process. This technique is mainly based on the inverse solution of the water flow in unsaturated soils. The van-Genuchten model (1980) was adopted in this technique for representing the unsaturated hydraulic properties. The applicability of this technique was successfully checked by an in-situ test and it is used for analysing the saturation distribution of layered soils. A new evaporation measurement equipment is used for the in-situ test instead of the difficulty in measuring the flow rate through the unsaturated soils. The obtained results are as follows; 1) Differencing of field data can be avoided by using this new measurement technique, 2) The hydraulic properties of unsaturated layered structure soils can be estimated by the inverse technique of sonic intensive in-situ test results, 3) The saturation distribution and capillary pressure can be controlled by using some different properties of multilayer soils with the help of the proposed technique.

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