Hashino had already proposed the application of Freund's bivariate exponential density function for a design storm. At present, it is impossible to use this distribution to estimate a design flood because of its nonstationarity (average and variance) and mutual dependency. As a first step to conquer these difficulties, authors assume that the average rainfall is nonstationary and the random component of rainfall is expressed by AR (1) based on the analysis of observed rainfall. Finally, authors adopt the storage function runoff model as rainfall runoff system and propose the differential equations whose solutions provide the first four moments of discharge under the impact of mutually dependent rainfall inputs. The probability density function of discharge is easily derived by the obtained first four moments of discharge.

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The TOPMODEL is a physically based variable contributing area model of basin hydrology. It has two distinct advantages to be applied in developing areas with little hydrological observations. One is that it has both advantages of lumped model and distributed model, i. e., few parameters to be identified and the capability of simulating the impacts of land use changes and water resource system developments. The other is that the soil topographic index that the model greatly depends on may be easily identified through satellite information. In this paper, a distributed hydrological forecasting model by combining the block-wise application of the TOPMODEL with the Muskingum-Cunge flood routing method is proposed. The model is applied to the daily and hourly runoff simulation for the 3500km² Fujikawa basin in Japan and to the daily simulation for the 19000 km² sub-basin of Minjiang catchment in China. The results presented are encouraging to suggest that the proposed model is hopeful to be reasonably applied to large basins with DEM and other GIS oriented data.

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A runoff simulation system for a large river basin is applied to the Shiguang River Basin (6, 000km²) in China. The watershed basin is subdevided into grid boxes with about 225km2 in which rainfall and evapotranspiration are assumed to be obtained by a numerical mesoscale atmospheric model. Runoff from each grid box is calculated and routed along channel networks. The system works well and it will be applied to the Huaihe River Basin (10, 000km²) where HUBEX Project is going.

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This paper describes a computer model for organizing network system and outputting river parameter, for the usage of all kinds of river model. Making use of digital data input, GSMLW develops a “free, seriate tree”structure to handle the complicated river system and generate output data of river parameters. According to requirement, river parameters can vary with the curvature distance of river central lines or straight distance to the outlet.
There are three parts of this model, one is to find central lines of stream network, one is to identify influx points of tributaries, and the other is to compute river parameters and output data files. In the modeling, the critical technique is the “free, seriate tree” structure. This tree structure is adaptable to the random tree structure of a natural river system, and thus is efficient in river system modelling.

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Many research studies have been carried out in the past to develop flood inundation simulation models. However, such models consider only overland and river flows components of hydrologic cycle and are designed to simulate floods in local scale only, not in basin scale.In real situation, infiltration and base flow to river have significant impacts on the occurrence or non-occurrence of floods due to a particular rainfall. Therefore, flows in unsaturated and saturated zones too should be considered for proper flood inundation simulation. In this study, a physically based distributed hydrologic model is developed for flood inundation simulation by combining a newly developed overland flow and river flow simulation models with unsaturated zone and groundwater models. The overland flow and river flow models are validated individually with test data and then, coupled with other models. The model is applied in a river catchment in Japan to simulate a flood event of 1996. Output of the model shows satisfactory agreements with surveyed flood inundation and thus, shows that the model is suitable for basin scale flood inundation simulation.

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In run-off analysis, it is impotant to consider the physical processes, for example, the infiltration of rainfall into soil and the storage of water in a storage layer etc, because the effective rainfall is governed by these processes. In this study, in order to consider the physical process of run-off in a river basin, an infiltration flow equation was used for the run-off analysis. In computation, the finite difference method is used, but if the mesh size was large, results of calculation would not agree with the theoretical solutions. So the conservation of water was taken into account in the infiltration flow equation. By using this model, the computation result agreed with the observed one very well even in the large mesh scale grids.

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The runoff discharge model developed for an urbanized watershed is extensively applied to examine the usefulness of the rainwater infiltration facilities to control the water level during flooding. The rainwater infiltration facilities examined are the permeable pavement and the house infiltration system. From the simulated results the followings are clarified. Reinforcement of permeability covering the pervious ground surface with the permeable pavement is ineffective to lower the water level. The peak water level is reduced by replacing the impermeable pavement with the permeable one in the parking lots. And multiple use of the rainwater infiltration facilities is effective for decreasing the occurrence of inundation by lowering the water level and is effective to delay the time of peak water level.

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A lumped rainfall-runoff model is developed by spatially integrating kinematic wave equation in order to reduce the computational burden of water flow routing in the slope area. To spatially integrate kinematic wave equation, the digital topographic information and steady state assumption are employed. The applications of this lumped model showed that the simulated hydrographs by this model agreed well with those by the distributed kinematic wave model which did not use the steady state assumption, while the computational time of this model was much shorter than that of the distributed model.

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The Shirakawa dam basin, the upper reach of the Mogami River, is one of the heaviest snowfall areas in Japan. The snowpack is very important water resource, but it sometimes brings about snowmelt flood. In this study, therefore, a method using satellite data and ground data on snow depth is developed to evaluate the snowmelt runoff. Snow covered area is obtained by comparing NOAA/AVHRR data with the ground data that also gives snowmelt rate. Addition of snowmelt discharge calculated from the area, melt rate, and the density and the rainfall discharge is compared with the observed discharge flowing into the dam.

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The effect of vegetation recovery on water balance and sediment yield in mountainous region has been discussed for long time, however, the quantitative evaluation of the relation between vegetation recovery and runoff phenomena has not come to an end. In order to evaluate the effects of vegetation recovery, we have started observations of rainfall, discharge and sediment yield at 5 experimental basins located in the upper Watarase River. This paper describes a result of preliminary study concerned to the relationship between water stage and discharge of the sabo dam at which we installed water gauges. Sabo dams may be the most suited structures for measurement of water stages in these devastated upper mountain basins.
The results obtained are as follows. By the experiments, it was confirmed that the discharge from sabo dam is evaluated by Govinda-Rao's equation for broad crested weir in the range of 0.1<h/L, in which h is head and L is crest length. For the range of h/L<0.1, the characteristics of flow over crest becomes open channel type. But it was also confirmed by field observations that it is possible to use Govinda-Rao's equation instead of Mannning type equation for this h/L<0.1 range. If the depth of crest of prototype becomes smaller than 0.05m, then no equation can be used commonly for discharge estimation, and it is necessary to evaluate for each sabo dam.

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This study concerns with an integrated simulation of coupled heat and mass transfer between soil and atmosphere. A model is constructed in one-dimension from ground water table up to upper atmospheric boundary layer. Numerical solutions of momentum, heat and mass transport equations are available for prediction of physical processes occurring in atmospheric boundary layer and in porous bodies. The processes in the atmosphere and in the porous bodies are linked each other through the equations of heat and mass conservation. The water mass in the soil is treated as a two-phase mixture of liquid water and vapour. The simulated results are supported by the observation data recorded with a meteorological station placed in the experimental field in Hanno new resident town, Saitama prefecture. A good agreement indicates that the model is reliable for predicting a water budget dynamics in a zone near ground surface and atmosphere.

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The difference of transpiration process between coniferous and broad-leaved trees is one of the most interesting topics in forest hydrology and other study fields. Direct measurement of transpiration is indispensable for detailed discussion on the topics. The lysimeter is the most effective way to directly measure tree transpiration. In this study, large weighing lysimeters were constructed. Zelkova serrata (Japanese zelkova) and cryptomeria japonica (Japanese cedar) were planted in containers of the lisimeters, and transpiration was measured. The relationship between transpiration and heat pulse velocity which is an reliable index of transpiration rate and actual transpiration rate was investigated. Apparent differences of transpiration rate and process could not be found between two species as far as the results of the measurements.

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In case that interception evaporation and transpiration of forested watersheds can not be directly observed, a method of separately estimating annual interception evaporation and transpiration is studied and proposed. The 365 days in a year are mainly divided into two groups: rainy days having interception evaporation and non-rainy clays having transpiration. Then daily interception evaporation is calculated from rainfall and potential evaporation determined by the temperature, and effect of annual change of forest condition is also considered. Three parameters of the method are determined by using observed multi-year data of precipitation, runoff and temperature in four watersheds in Japan, giving acceptable results of annual evapotranspiration. Finally the changing patterns of interception evaporation and evapotranspiration are analyzed, demonstrating the sensitivity to rainfall and forest change. This method is applicable to the separation of interception and transpiration from the whole evapotranspiration.

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In order to grasp the process of hydrologic cycle in river basin, it is necessary to estimate the evapotranspiration correctly. But, it is difficult to observe the meteorological data for the evapotranspiration, because of the extent of observation area. The purpose of this work is to determine the ratio of actual evapotranspiration and potential one in each land use to estimate the actual evapotranspiration from the potential one which can be calculated easily. I carried out the observations of actual evapotranspiration at the Bibi swamp experimental station and the Komasato grassland to determine the ratio of paddy and grassland area. In this study, the actual evapotranspiration are calculated by the profile method at the Bibi site, the Bowen ratio method at the Komasato site, the potential one are calculated by penman method. The ratio of forest area are determine using the evapotranspiration from water balance method.

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This paper deals with estimation of actual evapotranspiration in order to evaluate the hydrologic cycle in a catchment area. Estimation of the hydrologic cycle is required for effective water use, flood control, and evaluation of water environment. However, it is difficult to estimate evapotranspiration in a wide area because of a scarcity of measured data.
A method for using remote sensing data to estimate evapotranspirasion is proposed. In previous studies, NDVI values were applied to estimate the evapotranspiration. However, estimated results did not consider water balance in the catchment area scale.
We calculate the evapotranspiration several dam catchmment areas in a snowy and cold region. The study areas are covered by mountains with steep slopes and its land use is entirely forested. In these areas, topographic effects on rainfall as well as snowfall must be taken into account. Rainfall and snowfall are accurately estimated by taking into account topographic factors such as elevation and direction of slope. As a result, actual evapotranspiration is calculated by applying the water balance method based on precipitation data. Finally, actual evapotranspiration is related to NDVI values so that it can be generally estimated in a wide catchment area.

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Three types of experiments on the measurements of spatially mean heat flux were performed by using two sets of scintillometer. In the Case-1, two sets of scintillometer are located adjacently and parallel, in the Case-2, their propagation paths are crossed, and in the Case-3 they are located parallel at the different height. The following results are obtained; 1) the fluctuations of the sensible heat fluxes measured by closely located scintillometer agree with each other, 2) the crossed scintillometer agree well when they are averaged every 30min, but their 1 min-averaged data don't agree, 3) the method to estimate the regional zero-displacement height “d” by using two sets of scintillometer is checked and the proper height of d is derived.

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Spatial and temporal variability of soil moisture is considered to have significant effects on energy and water transfer between land surface and atmosphere. One of the key effects on soil moisture distribution is topographic effect, that is, wet valley and dry hill top affected by surface and sub-surface flow along slope. Then, upward and downward heat and moisture fluxes controlled by Bowen ratio and soil heat transfer parameters determined by soil moisture also vary according to topography. In the permafrost regions, the effect of topography on heat and water transfer can be observed more clearly during melting season.
The purpose of this study is to understand the topographic effect on heat and water transfer in permafrost regions. The topographic effects on permafrost hydrological conditions were examined by the pit observations which were carried out on the Tibetan Plateau as a part of the GAME-Tibet project.

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To understand the effect of the Tibetan Plateau on the seasonal and interannual variation of the Asian summer monsoon, the spatial and temporal variation of land surface soil moisture and its effects on spatially averaged fluxes from land surface to atmosphere should be estimated correctly by using models. In this study, the characteristics of the surface soil moisture distributions on the Tibetan Plateau are introduced based on the observational data obtained as one of field activities of the GAME-Tibet project. To express distribution of surface moisture in permafrost regions, an 1D energy and water flow model including surface water storage function is proposed and applied for the simulation of the effect of the surface water storage on surface moisture by using the observed atmospheric forcing data.

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This paper examines an effect of stochastic and scale-dependent hydraulic conductivity on the infiltration in a site respectively. These are evaluated using the aggregation criteria of the infiltration derived from Philip's equations. The values of hydraulic conductivity in a forest hill slope measured from 100cc-7000cc sampler are distributed according to the Log-normal distribution Ones in a bare kanto loam soil measured from 100cc are also distributed according to the Log-normal distribution, and the others measured by means of trenches are distributed according to the normal distribution. All variations in both sites can be aggregated by means of parameter mean method respectively, but their mean vales in the same sites are found to be scale-dependent from statistical test, and they can produce a significant effect on the infiltration calculation.

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Water and heat budgets in 1993 in the Shutoken Area of about 7800 km2 are studied by using a distributed model. The land use data and DEM are based on the Saimitsu Suuchi Joho of Japan Geography Institute. The meteorological data are based on the 46 AMeDAS stations in the area. In addition to the analysis of annual budgets, the comparison of seasonal evapotranspiration is also conducted.
Influences of grid size are shown by comparison of simulation result by a grid size of 1 km and that by 5km. Influences of subgrid heterogeneity of land use are shown by comparison of simulation result of uniform land use scheme with that of mosaic land use scheme. It is found that subgrid heterogeneity of land use gives stronger influence on annual water and heat budgets in urbanized area than grid size does

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This paper describes the applicability of probability distributions with both upper- and lower-bounds to hydrologic frequency analysis. Two distributions are applied to some data sets of extreme-value precipitation and river discharge: the Slade-type four-parameter log-normal distribution and the extremevalue distribution with upper- and lower-bounds (EVLUB) proposed by Kanda (1981). Their goodness of fit to the data sets is assessed in terms of the standard least-square criterion (SLSC). The analysis using the bootstrap method indicates that these distributions with the upper-bound give less variability in quantile estimates than the three-parameter log-normal distribution with the infinite upper-bound. Finally, this paper discusses incorporation of the probable maximum precipitation (PMP) or probable maximum floods (PMF) into the frequency analysis models as the upper bounds.

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This paper describes the results obtained from flood frequency analysis in river basins in Japan. Using annual maximum discharge data at ninety-nine locations on the major rivers, the authors fitted a number of frequency analysis models (probability distributions) and evaluated them in terms of not only goodness of fit and but also stability of quantile estimates. The standard least-square criterion (SLSC) is used as a goodness-of-fit criterion. The stability of quantile estimates is assessed in terms of estimation error obtained by the jackknife method, which can be used for bias correction and quantification of estimation error. Based on the application results for ninety-nine samples, this paper has revealed:(1) goodness of fit of various models;(2) that SLSC =0.04 could be a threshold for good-fitted models; and (3) that the jackknife estimation error can be an index to evaluate the stability of the models. Importance of graphical analysis using probability paper is stressed.

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When the systematic observed hydrological record is not long enough, the historical flood information is useful to increase the precision of flood-quantile estimation. Historical flood data, which can be used in flood frequency analysis by adjusted-moment method or maximum likelihood method, often contain much larger error than systematic observed data. Thus it is important to evaluate the effect of the error on the precision of flood-quantile estimators. Monte Carlo simulation by using Gumbel distribution shows that flood-quantile estimator contains positive bias when the standard error exceeds a certain level (in this study, approximately 1/10 to 1/8 of the average for total sample). Moreover, when it exceeds 1/5 to 1/4 of the average, the improvement of the precision cannot be confirmed for the calculated flood-quantile estimators.

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In river planning, a design flood discharge has been decided based on runoff analyses of design storm hyetographs with a given return period. However, the design return period of the design flood discharge seldom coincides with that estimated statistically based on data of observed flood discharges. In this paper, the difference between the probabilities of the design flood discharges and design storm rainfalls is evaluated using a bi-variate probability theory.

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Hydrologic frequency analysis often uses the annual maximum series (AMS) of rainfall and river discharge. However, sometimes the use of AMS is insufficient because AMS may include non-flood annual maximum discharges. This paper describes the use of the partial duration series (PDS) as well as the AMS, giving the theoretical background of the relationship between the generalized Pareto (GP) distribution for the PDS and the generalized extreme-value (GEV) distribution for the AMS. These distributions are evaluated for seventy-nine river discharge samples in Japan from the view point of both goodness of fit and stability of quantile estimates obtained by the jackknife resampling method.

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A global river discharge data set was developed from various sources. Intensive quality controls were applied for the data, and climatological mean runoff for 30 years from 1961 through 1990 at 431 stations over the globe were estimated based on monthly discharge. The data set covers approximately 40% of land surface excluding Antarctica, and approximately 50% of total discharge from land to sea. Linear trends for the period and the interannual variations associated with El-Nirio Southern Oscillation are investigated and their geographical distributions were presented. The data set will be effectively used for validations of global water balance estimates and investigations of global hydrologic cycles.

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The 1994/95 tdrought posed many challenges for water resources management in Fukuoka City, western Japan, and surrounding communities. During the drought, much effort was expended to maximize available resources and to reduce demand. In this paper, a decision support system is developed for drought operation of the Fukuoka City water supply system. The methodology applied incorporates a genetic algorithm procedure to derive some optimal daily release policies in order to minimize long-term drought damages and water shortage threats. The application of the DSS to the operation of the water supply system of Fukuoka City, reached very promising results.

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Flood control plans by reservoir in small urban basins generally assume a case of occurring uniform rainfall in the catchment. However, the spatial distributions of rainfalls are extremely different especially for thunderstorms, and it is impossible to specify the area of concentration for each precipitation. The reservoir connecting catchments makes areal deviation averaged so that more effect of flood control is expected. Such reservoir could achieve not only the same effect of flood control as a bigger single reservoir but also more effect than a single reservoir having the same capacity.

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Recently, the flood control facilities are provided positively in each place. Consequently the incidence of flood disaster decreases. However, the improvement of flood control facilities obscures the inhabitant's consciousness of flood disaster. In this study, we take notice of the effects of improvement of flood control facilities on individual recognition, and analyse from the viewpoints of next two. The first is the estimation for the present condition of the flood control by inhabitants, and the dependence of inhabitant on the flood control facilities against flood hazard. The second is inhabitant's recognition of the incidence of flood disaster and the extend of submergence damage.

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Field observation of flood damages caused by heavy rainfall associated with Typhoon 9719 occurred on September 16, 1997 was carried out along the Yasaka River in Oita Prefecture as well as aerial photographing of inundated areas. In this flood, maximum discharge of around 1, 500m³/s, which exceeds planned flood discharge of 1, 250m³/s of this river, was measured. Inundation height was measured at 66 points and relation between distribution of submerged houses and morphology around the river was studied. The direction of flood flow was read from the falling direction of paddy field and deposition of sand was observed behind dense bamboo forest due to the roughness effect of the forest. Aerial photographs show that flood flow went straight while causing inundation and scouring just behind the dike at meandering areas.

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A ground water flow model is developed with a special attention to a boundary between the unsaturated region and saturated region, and the combination of the each formula of the 1-dimensional motion, The vertical flow in the unsaturated region and the horizontal flow in the saturated region. The proposed model is apllied to the analysis of the seepage flow into the river embankment and it is verified by the result of the experiment on a large scale model.
Finally, the horizontal seepage flow in unsaturated region is estimated based on the distribution of the suction simulated by the proposal model. The horizontal seepage flux in the unsaturated region is small enough to omit against that in the saturated region.

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This paper desribes the method and result of investigation of saline water intrusion in aquifer using electrical sounding. Vertical soundings were carried out in coastal area of Haruno Town, Kochi Pref. and vertical resistivity distributions were obtained. A deep layer of low resisvity was found near the coast, and it was concluded to be the layer of saline water. Horizontal distributions of its depth were estimated by conventional curve matching method and newly proposed method with numerical analysis and Standard Powell's method. Its result and validity is shown and discussed.

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The air sparging coupled with the soil vapor extraction as the physical remediation technologies has been applied to a site contaminated with volatile substances like tetrachloroethylene. In order to implement these technologies, it is indispensable to understand the mechanism of air migration injected in groundwater and the change of groundwater concentration. In this paper, we presented the results of full scale field experiment implemented to estimate the change of groundwater concentration caused by long term air injection in groundwater. As a result, it was clarified that the change of groundwater concentration within the radius-ofinfluence was caused by volatilization into injected air and stirring of groundwater by air injection.

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A practical system for using terrestrial heat in shallow ground has been proposed by the authors' research group to control pavement temperature. This system comprises a water tank and a pavement incorporating a heat exchanger pipe system that was built on the campus of Fukui University.
A field experiment has been ongoing since July 1995. Water and ground temperatures have been measured continuously in and around the tank along with the heat carrier fluid flow rates.
The heat energy budget of the tank, obtained from the field experiments, confirmed our past result that terrestrial heat across the tank walls is effective in not only melting snow on roads in winter but also suppressing the rise in pavement temperatures in summer.
The proposed numerical model was able to reproduce the behavior of the water temperature in the tank including the effects of the inflow of cold water and the return flow to the pavement heat exchanger in winter.

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Sources of water and soluble substances in the river and groundwater were studied. As the δ D andδ¹⁸O values of river and groundwater were on the meteoric line and these values varied with sampling altitudes, these waters were thought to have arisen from precipitation. CO₂ pressure in river and groundwater were estimated to be high from pH values and HCO _3- and were equilibrated with CO₂ pressure in soil. The δ ¹³C values of river and groundwater were in agreement with that of soil. High CO₂ pressure derived from soil was thought to cause weathering of feldspar in rock and to increase HCO_3- and Ca²⁺ concentration in waters. As the δ ¹⁵N values of NO_3- in river water varied with land use of sampling pints, the source of nitrogen contamination could be estimated from δ ¹⁵N values of NO_3- in waters.

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In this study, a new algorithm for snow is developed based on microwave radiative transfer theory and evaluated using the measured snow at the 100 ground-based stations which locate widely in the northern hemisphere. At about the half of the evaluated stations, the estimation errors are very low. Further, the reasons for over- or underestimation are discussed in relation to snow particle size and vegetation cover. Using the proposed algorithm, 6 years data sets of snow in the Eurasian continent are generated from SSM/I. The characteristics of seasonal variation of snow in the Eurasian continent and inter-annual variation of maximum snow on the Tibetan Plateau are demonstrated by analyzing the generated data sets.

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The objective of this study is to develop algorithms for estimation of soil and snow hydrological parameters by using Synthetic Aperture Radar (SAR) based on microwave radiative transfer theory. The backscattering coefficient of the land surface obtained by SAR is affected not only by soil moisture or snow depth but also by surface roughness and other factors. The ground-based measurement data obtained under the various conditions of the bare soil and snow was examined for understanding the characteristics of the distributions of two surface roughness parameters, that is, the standard deviation of surface height and the correlation length of surface. Multi-parameter SAR algorithms for estimation of soil moisture and snow surface parameters were developed based on numerical simulation by using the scattering model that includes Integral Equation Method (IEM) and first-order radiative transfer solution.

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A preliminary analysis of TRMM-PR (Ku band scatterometer) data is conducted in this paper. Research area is mainly Indo-China peninsula including the surround seas. The incident angle's dependences of the backscattering coefficients at different land covers is found among ten land covers respectively. Their characteristics can be explained from the surface scattering theory, and the backscattering coefficients at shallow incident angle are largest from agricultural field. We compare their incident angle's dependences with ones calculated from surface scattering theoretical models for sea surface and bare soil.

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The vertical profiles of wind velocity under the pressure distribution typical in the winter season have been investigated on the basis of the data acquired from December, 1996 to February, 1997. The observations in the lower layer were made above Fukuoka City by using a Doppler sodar. The data which Fukuoka and Kagoshima meteorological observatories obtained by releasing rawinsondes were used as ones in the upper layer of the atmosphere. A strongly stratified layer is formed between the Ekman layer and the free atmosphere. The existence of the layer makes the vertical profiles of wind velocity complicated. The geostrophic wind velocity in the free atmosphere varies linearly because of the horizontal gradient of temperature. The wind profiles have been analyzed by using a onedimensional model in which the eddy diffusivitiy and the geostrophic wind vary with the height. The analyzed results agree well with the observed ones.

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Field observation was performed on two islands located in Tokyo Bay to investigate the atmospheric environment over Tokyo Bay by using Radio Sonde, Dopper Sodar and airplane in summer 1998.
The following results were obtained; 1) The atmospheric boundary layer (ABL) over Tokyo Bay was composed of multiple layers.Especially, it was composed of three layers on August 10. 2) These multiple layers were formed by the effect of various kinds of return flows from the land. The upper boundary layer about 1250-2000m over Tokyo Bay was hotter and drier than the one over the land. 3) The airplane observation suggests that the air pollutant might be more concentrated over Tokyo Bay rather than over the land due to the difference of the boundary layer structure.

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It was pointed out that a prefabricated classroom building for school children which had been constructed before summer turned out to create a very hot environment in summer. The galvanized sheet iron roof of the building was covered with a water-retentive ceramic tile and water was sprayed on the tile in an attempt to cool the classroom by evaporative cooling. Because of evaporating cooling, the surface temperature of the wetted covering tile was 15 °C lower than that of the original roof under similar meteorological conditions, and the room temperature was lowered by 2 to 3 °C

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Cloud droplet formation is an important process in water cycle between the earth and the atmosphere. In order to investigate the nonsteady growth of cloud droplets due to the condensation of the atmopheric water vapor on (NH₄) ₂ SO₄ particles, a mathematical model has been constructed and the nonsteady growth has been simulated numerically with use of the mathematical model. The mathematical model is constituted by the conservation laws of water mass and heat energy and the state equation of ideal gas. As the speed of time variation of droplet heat Q_W is very fast compared with that of droplet mass m_w, droplet temperature T_a can be treated as in steady state. The equilibrium droplet size a_e is dependent on the 3/2 power of the initial radius a_so of cloud condensation nucleus (NH₄) ₂SO₄. The larger a_s0 is, the more the droplet grows in its equilibrium size a_e. It takes much time for large condensation nucleus to attain the equilibrium size a_e. It also has been cleared that Kelvin's equation is not always applicable to estimate the cloud droplet siz

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The severe rainfall prediction method using artificial intelligence which simulates the act of weather forecast experts who are well versed in the local weather is proposed in order to support the flood control.
Under the condition that the system does not take the place of the numerical weather forecast, the objectives of development of the system are as follows; i) to forecast severe rainfall in fine grid scale by consideration of subgrid phenomena which are difficult to be expressed in numerical models, and ii) to make a real-time explanation of the important causes of severe rainfall to river managers.
Then, the subsystems of the SRAI are developed and discussed in terms of their performance. The accuracy of the system was 80% for forecast of two hours ahead rainfall over 10mm/hr. The system shows the important initial conditions for severe rainfall effectively using backward reasoning.

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In Japan, there are a lot of mountains that have great slopes and they have an effect on a wind field, rainfall and snowfall. The effect is very complicated because the atmosphere is stratified. As one of the effects of the stratification, internal waves are given, and these motions cause the shear layer in front of the great and long slope in case that humidity is very low. On the contrary, if humidity is very high and cloud microphysical processes are taken into account, air releases latent heat and stratification was changed into neutral state. In this study, our purpose is to know the effect of the sin-curved mountain in front of the great and long slope, and as a result, the mountain has an effect to decline the area of snowfall.

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Downscaling of large-scale atmospheric circulation characteristics to estimate local hydrometeorological processes is important, e. g., for assessing the local impact of a possible future climate change. In the present study, this possibility was investigated for Kyushu Island, Japan. Six months of 12-h values of two large-scale air flow indices, wind flow f and vorticity z, estimated using different spatial resolutions, were correlated with rainfall observed at 128 stations on Kyushu. For both f and z, the correlations increased with increasing resolution. For the highest resolution, the mean correlation for f; was 0.198 and the maximum correlation 0.306. Forz, the corresponding values were 0.482 and 0.764. Firstly, the results show that vorticity has a potential to be used as a predictor in downscaling of GCM output to hydrologically relevant scales, as well as a variable in rainfall time series modeling. Secondly, they indicate that the output of GCM models need to have a spatial resolution of at least 2.5°×2.5° in order for meaningful statistical downscaling to be feasible.

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To utilize data of Radar rain gauge in prediction for snowfall disttibution a model was proposed In this model, a calculation of wind field on arbitrary terrain is included. The accuracy of the prediction model are examined by comparisons with field data of hourly measurements of snow fall at seven sites, maximum snow depth distribution in dam depth basin and dam inflow.

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A turbulent velocity field was measured in a vegetated compound open channel flow where vegetation density changed abruptly from low to high and high to low in the downstream direction. The changing process of mean velocity, secondary currents and Reynolds stress fields were explained with three dimensional mixing structure caused by large horizontal eddies. As a result, three dimensional structure of turbulent flow, and relation between mean velocity field and turbulent structure were clarified experimentally in an open channel flow with abrupt change in the vegetation density in the downstream direction.

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It is important for river engineers to know the hydraulic roles of spur dikes. A series of laboratory tests for horizontal structure of open channel flow with spur dikes was performed by varying theinterval of spur dikes. The results indicate that the structure of flow is different according to the in The reattachment length is found to be a function of the interval of spur dikes. The momentum transport rate between the main flow region and spur dike region in the most upstream spur dike region takes a peak when the interval is 4. 0 times of the length of a spur dike. The flow near the tip of the subsequent spur dike plays an important role in the momentum transport rate in the first spur dike region.

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In a strait where geometric characteristics enhance vortex pairing and stretching, strong vortexes can be observed in a quite stable manner. In this paper vortex behavior in the wake of a flat plate set up perpendicular to the flow is studied by flow visualization and velocity measurements. The experiments are conducted in three boundary conditions, i.e. a rectangular channel, a compound channel and a rectangular channel with being locally shallow around the flat plate. The results show that vortex paring and stretching are more stable in the last case than the others. The results also indicate that secondary flow structure brought by the basic boundary condition strongly affects the local behavior of the vortexes, especially in the stretching process.

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There are many submersible and non-submersible structures having large-roughness for the purpose of bank protection, fish way and so on It is very important to clarify the resistance of large-roughness and the structure of flow. The resistance estimation by large-roughness has been made by equivalent roughness and Manning'sroughness coefficient But it is difficult to estimate hydrodynamic forces acting on the individual large-roughness and local velocity distributionear by these methods.
The water flow near non-submersible large-roughness was clarified in previous study. In this paper, we measured hydrodynamic forces acting on submersible large-roughness directly. The purpose of this study is to develop twodimensional numexical method that can reproduce water flow with submersible and non-submersible largeroughness.

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