Tropical Hydrology Simulation Model 1 (Trophydsym-1) was applied to the Kali Progo river basin, Central Java, Indonesia, which contains five subbasins. The simulated and recorded hydrographs were in close agreement both at every transfer point of the subbasins and in three different years.Some hydrologic characteristics were found from application of the model. The basin, which is mainly comprised of fresh volcanic formations, is capable of storing much water and releasing it gradually as base flow. Therefore, the basin provides stable low flow for irrigating paddy land, which has expanded to 40 to 60 percent of the whole basin area, with multiple cropping patterns of rice-rice-upland crops or even rice-rice-rice.

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During the last stage of evaporation, complete vaporization occurs within the ground and a dry layer in which water moves exclusively in the vapor phase (DSL) forms at the surface. The authors have developed a model of the evaporation process during the last stage and a method for estimating the upward vapor flux through the DSL in this stage (DSL method). However the daily amount of evaporation cannot be obtained by this method alone, for the last stage of evaporation appears only during the daytime and the DSL method is not applicable to the evaporation process during the nighttime.The soil moisture behavior in the last stage of evaporation has been examined and it has become apparent that the moist layer in which water moves in both phases, liquid and vapor, can be divided into two sublayers; while the moisture conditions of the upper sublayer exhibit definite diurnal tendencies, those of the lower sublayer remain reasonably constant during the diurnal cycle. Since the lower sublayer generally lies deeper than about 5cm, temperature gradients across the layer are smaller than about 1 Kcm-1. Hence, the upward water flux in the lower sublayer (qm) also remains nearly constant throughout the day, because the flux qm is controlled by the gradient of soil water content. It is shown possible to estimate the daily amount of evaporation on the basis of these characteristics from the daytime value of qm obtained using the DSL method.

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When a runoff analysis of the mountainous basin was obtained to use the evapotranspiration which was estimated by the existing potential evapotranspiration, the result disagreed with the runoff discharge. Because the existing potential evapotranspiration did not change with the same seasonal changes as the evapotranspiration at the mountainous basin.Author developed in this paper the estimation equation of the potential evapotranspiration changing with the same seasonal changes as the evapotranspiration of the mountainous basin, and showed the estimated method of the evapotran-spiration using that equation. Effects of the difference of the forest aspect on the evapotranspiration was investigated.Author proposed the estimation equations of the potential evapotranspiration (12) and (13). The result of equation (12) was better than that of (13) at the mountainous basin in Aichi and Miyazaki Prefecture, and it was better than the pan evaporation, Hamon and Penman equation, too.The evapotranspiration in a year of the waste forest has decreased by 37 percent that of the matured forest in the mountainous basin, Aichi Prefecture. That of the sapling forest has decreased by 29 percent of the matured forest in the same basin, Miyazaki Prefecture.

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