In order to apply an infiltration model to determine excess rain-fall in long term runoff analysis or water balance analysis, it is necessary to examine whether the model is suitable to describe not only the decreasing of infiltration capacity due to rainfall but also the recovery due to evapotranspiration. In this study, we used a soil column with impermeable bottom to investigate the change of infiltration capacity for various antecedent evaporation under ponding condition. The observed data showed that the infiltration capacity decreased abruptly when the wetting front reached the bottom. Green-Ampt and Horton's infiltr-ation model were used to fit these experimental data. The results indicated that, in every instance, Green-Ampt equation gives better results than Horton's equation.

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A soil model is presented for estimating the seasonal variations of evaporation, ground surface temperature and soil water content. In the model the surface heat budget and diffusion of water vapor in the soil pores are taken into consideration. The resistance of water vapor diffusion in the soil pores, the ground surface albedo, and soil thermometric parameters are functions of the soil water content. Required meteorological data to operate the model are the daily amount of precipitation, daily maximum and minimum air temperatures, and the daily means of wind speed, solar radiation, specific humidity and air temperature.

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Experimental verification of the soil model as reported in the previous paper was made by long-time observations of evaporation, ground surface temperature and soil water content. The first observation of evaporation from a sand was performed using a lysimeter with a diameter of 0.46 m, for a period of 416 days at Sendai. Total amount of observed evaporation is 583 mm and that of calculated one is 616 mm. The difference between the calculated and observed result is ±0.3 mm d-1 for a five-days mean evaporation, and that is ±1°C for a daily mean ground surface temperature. The second observation was carried out at a field of bare soil ground consisting of a fine sand called Narita sand, for a period of 23 days at Chiba. The soil model well simulates the observed evapotation and ground surface temperature. Total amount of observed evaporation is 45 mm, and that of calculated one is 51 mm.

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Surface soil moisture content, surface heat fluxes and meteorological data were measured simultaneously on a bare field to investigate the effect of surface soil moisture on the heat balance at a bare soil surface. A simple heat balance model for a bare soil surface was constructed, including empirical expressions of parameters (albedo, atmospheric emissivity, ratio G to Rn, aerodynamic resistance and soil resistance) which were determined from the measured data at 9 a.m., as it is difficult to evaluate the effect directly from the measured data. By calculation using this model, the relationship between the volumetric soil moisture content within the top 2cm layer and the surface heat balance under a given meteorological condition was clarified.

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The forest evapotranspiration is estimated using the short-term period water-budget method with correction of the amount of forest soil-water storage calculated by “exponential tank model”. Forest evapotranspiration is also calculated using heat balance method in which seasonal variation of evaporation efficiency β is assumed. With comparing the both values of evapotranspiration, it is found that forest evaporation efficiency in Japan is rarely restrained by the forest soil-water content.

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The application of radar rainfall measurements to hydrologic practices involves many difficulties in identifying radar constants B and β, and calibrating radar information via use of ground measurements and so forth. It is of practical importance to take into account the space scale difference for comparing radar measurements with ground information precisely. The present study converts ground point rainfalls to the average areal rainfall, using the Kriging method, and shows its effect on the identification of B and β, and calibration. The Kriging method enables one to estimate the average rainfall over a given catchment area, quantify the accuracy of areal estimates, provide data input for automatic contouring and design the raingauge networks.

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For the purpose of estimating the evapotranspiration on an irrigated land in arid district, the budget of radiation, the budget of energy and meteorological parameters were measured at an alfalfa field in an irrigated farm. In the daytime, the latent heat flux increased to 450 W/m2 which was about 4.5 times greater than the sensible heat flux. The bulk transfer coefficient of sensible heat flux was estimated to be 0.0088. The canopy resistance (rc) on the alfalfa field was equal to zero before 10 o'clock because of the evaporation of dew. The rc values were about in the range of 40∼60 sec/m from 12 till 17 o'clock then increased rapidly. A model for rc which contains solar radiation as variable was adopted. On the basis of these results, a model for estimating the evapotranspiration was developed. Then sensible heat flux, latent heat flux, reflectivity, amount of dew and transpiration were estimated from air temperature, vapor pressure of air, wind speed, solar radiation, long wave radiation, net radiation and soil heat flux which were measured for 17 days at the alfalfa field. Computed diurnal change of reflectivity was about the same as measured one. There was a positive correlation between estimated daily amount of transpiration and measured daily soil water decrease.

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