Abstract: Using the conception of the infiltration of gravity moisture, a model for estimating infiltration depth is presented, by means of the rainfall and the soil moisture before rain. Comparison with observations made in four stations shows an error of about 11% and average error of 4.6 centimiters. From four factors (characteristics of the soil, the kind of crop, soil motsture. storage and evapotranspiration) an index for estimating the amount of soaking rain is presented. Here "soaking" means enough raix1fa11 for adequate growth of the major crop concerned and an infiltration depth to provede adequate soil moisture, Results of calculation are tested with actual determinations.