The Hargreaves equation can calculate monthly mean evaporation/evapotranspiration using only air temperature with two coefficients. First, it is confirmed that the Hargreaves equation with general coefficients gives better pan evaporation estimates than estimated results for pan evaporation from 143 meteorological stations in Japan and results generated using the Thornthwaite and the Hamon equations. In a further attempt to improve accuracy, two equations are determined to estimate the values of the two coefficients that can be applied throughout Japan with the exception of the Nansei Islands in the subtropical southwest of Japan. It is shown that the Hargreaves equation plus the two new equations gives an improved estimation accuracy with a root mean square error of 0.65mm ± 0.26mm (Ave.±S.D.) .

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Until now, as the modeling of red soil runoff in Okinawa, the suspended sediment (SS) runoff analysis model has been developed. However, because there are not enough long term observed data about red soil runoff from upland field area in Okinawa, the characteristics of the SS runoff analysis model has not been sufficiently verified yet. So, the slope model to generate the virtual data to verify the SS runoff analysis model was developed in this study. Moreover, in order to examine whether the calculated virtual data can be used to verify the SS runoff analysis model or not, the numerical experiments were done by changing the source of SS, concentration ratio of flow in the slope or the difference of particle size. Then, the SS runoff characteristics of the virtual data were compared with these of the observed data. As the results, it was confirmed that the virtual data represent the same SS runoff characteristics as the observed data from the viewpoints of the deference of the appearance time of SSC (Suspended Sediment Concentration) peaks and the discharge peaks, the hysteresis in the relation between the discharge and the SSC, and the first flush. So, it was recognized that the virtual data can be used for the verification of the SS runoff analysis model.

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The characteristics of throughfall of two forest stands with contrastive canopy structures were evaluated. Gross precipitation partitioning into throughfall in the experimental plantation stand of Cryptomeria japonica (ever-green coniferous tree) and that of Lithocarpus edulis (ever-green broad leaved tree) are studied. In order to compensate the effect of spatial variability of throughfall on the forest floor, the data of 3 tipping bucket raingauges were calibrated by the data of 25 or 30 fixed position funnel type raingauges. Spatial variability of throughfall in the C. japonica stand was larger than that in the L. edulis stand. The average ratio of throughfall to gross precipitation of C. japonica stand (0.64) was larger than the ratio of L. edulis (0.37) . In the L. edulis stand, throughfall started after only 0.9∼1.6mm of rain fallen, but it started after 1.9∼4.0mm of rain fallen in the C. japonica stand. These results suggested that LAI and canopy openness cannot fully explain the characteristics of throughfall and additional information, e.g. canopy structure (morphological factors), are needed.

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A numerical model is developed for simulating floods and inundation in the Lower Mekong River basin. The model includes the groundwater analysis and shows good results about the temporal and special variation of inundation area around Phnom Penh. The results also show the inundation effects on the groundwater charge. The amount of ground water charge from inundated water was 52km³ in usual flood.
We simulate the case of small flood to evaluate the flood control effect on the water resources such as agriculture and groundwater. The simulated results are as follows. (1) Inundated areas are remained despite of flood control. (2) Although the flood control in the upper dams restricts cultivate area in inundation region, larger cultivate area can be gained from water storage. (3) The amount of ground water charge decreases due to flood control, and becomes 30km³ in the case of 44% flood control. However groundwater resource from inundation is sufficient for Cambodian people.

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The groundwater in the Kanto Loam Formation, Tokyo, was influenced the peripheral topography, structures on soil surface, and geological features. It was shown that groundwater conservation system was different by the number of precipitation. Results were follows.
The Kanto Loam Formations includes fine/macro pores with much quantity. In this soil layer, water permeability of rainwater was high. The change of annual groundwater level was low in February of the winter season. It tended to become high during the rainy season with the typhoon and the summer season. The appearance time of minimum/maximum groundwater level was similar, too. There was a property in characteristics of increase/decrease of groundwater level. Then, a method was suggested to estimate groundwater level by precipitation and soil characteristic. A change of rise speed of groundwater level was explained that the precipitation supported soil macropore and the falling speed of groundwater level supported a quantity of evapotranspiration. An annual change of groundwater level was examined and the sign of groundwater level suddenly decreased was not appear. The function of groundwater conservation was maintained constantly. Sudden groundwater level decrease after rain stop. Then it was estimated that groundwater outflow occurred from the groundwater maintenance district to the peripheral non-maintenance district. This investigation district has the soil physical properties to include a characteristic of the Kanto Loam Formations and urban precious groundwater is kept in good condition. These superior conditions must be maintained continuously.

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A simple model to evaluate litter interception losses in the plantation stands of Cryptomeria japonica and Lithocarpus edulis were developed. We applied this model to simulate the litter interception and evaporation rates under the different rainfall conditions and different shapes of the litter layer. Model parameters were estimated from field data using microlysimeter at each stand. The litter interceptions under various rainfall conditions were estimated using throughfall data. Evaporation rates from the litter layer during the dry periods were estimated by the relative water content of the litter layer. The estimated annual litter interception loss of C. japonica stand was 5.1∼11.5% of throughfall, and that of L. edulis stand was 9.5∼18.0% of throughfall. These results suggest that the litter interception process has a significant role in the forest hydrological cycles.

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