Countermeasures against extraordinary droughts include manipulation of reservoirs and construction of reservoirs for water supply in extraordinary droughts. For effective implimentation of these measures, it is important to establish methodology evaluating the effects of such tactics appropriately. In this paper, the drought severity indices are examined before studying evaluation of these measures.

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In order to clarify the various hydrological characteristics of river basins in Japan, the author developed monthly water balance model applicable in general and made analyses for water balance computations during 30 years to each of many river basins and their fluctuations. As the results, extra coefficients to observed precipitation data, snow melting coefficients in relation to the mean basin temperature, tank model types for monthly runoff corresponding to each basin geology, evaluation of the runoff fluctuations through reservior capacities necessary for leveling rates etc. are clarified. The results obtained indicate the specific regional features on water inventory and fluctuation in water balance which are indispensable for water resources planning and management.

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The frequency distribution of precipitation changes from inverse J-shaped type, through the exponential, to normal type with increase in a length of lumped period. The seasonal and regional characteristics of the change with lumped period from 1 hour to 30 days are investigated for AMeDAS precipitation data sets. The gamma distribution is employed as a distribution model fitted to the observed data and the method of maximum-entropy as a method of the parameter estimation. The shape of a frequency distribution can be expressed quantitatively by the information content due to logarithmic geometric mean. The following findings are drawn from the frequency analysis: 1) Lumped period in which a frequency distribution is the exponential one for winter-half-year is rather short on the Japan Sea side and rather long on the Pacific Ocean side. The regional difference is understood by considering that main precipitation-induced disturbance is different for each region and each disturbance has its own time-series characteristics. 2) As for variation of information content due to logarithmic geometric mean in short-term, there is latitudinal difference in the Japanese Islands, which corresponds with difference between precipitation characteristics represented by rain shower and those by continuous rain.

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Neither total rainfall nor rainfall intensity can accout for the temporal change of rainwater quality by itself. This paper shows that the introduction of raindrop size-distribution makes it possible to integrate the effects of total rainfall and rainfall intensity on rainwater quality, and can give the good estimation of electric conductivity and UV absorption due to N03-N concentration of rain water. The analysis was based on the several assumptions for probability density function of raindrop size-distribution, falling velocity of raindrops, and water quality of raindrops. The results showed that the ratio of estimated load factor calculated on the basis of the raindrop size-distribution to the observed load linearly increased as total rainfall increased, and that the best estimation of temporal change of rainwater quality can be gotten in conditions of the raindrop size-distribution of exponential function, the raindrop falling velocity linearly increasing as droplet diameter increases, and the raindrop concentration linealy increasing as the surface area of unit volume water increases and as the falling velocity decreases.

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