The ability of the soil to deliver water to plant roots is known as the water-supplying power (WSP). Tanaka et al. ( 1993 ) defined the water retentivity index (Θ ) and hydraulic conductivity index ( K ̅ ) to evaluate WSP and proposed a simple measurement method. We conducted evapotranspiration experiments on soybeans and peanuts in four soils using different indices. The evapotranspiration experiments were used to obtain results specific to the four types of soil (Tanaka and Cho, 1993 ). However, the relationship between the abovementioned indices and the experimental results was not analyzed. Therefore, the nonlinear differential equation representing the dynamics of soil moisture was linearized using these indices, and an analytical solution was obtained and compared with the experimental results. This model is called the Θ - K ̅ model. A major feature of the Θ - K ̅ model is that it is linearized using only the Θ and K variables, which are essential properties that represent the water characteristics of soil. This is the simplest linearization method among all of the traditional methods. The analytical solutions obtained under the initial conditions as well as the boundary conditions corresponding to the evapotranspiration experiments were used to substantiate the experimental results. The results clearly indicate that there is a physical basis for these indices in the analysis of soil water dynamics. It was concluded that the WSP measured using the soil point method was dependent on these indices.

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