It is very important that for a drought preventing measure, water utilization and control plans of storages are worked out reasonably and properly. First of all, variations and distributed types characteristics of precipitation and a capable period of low precipitation duration must be investigated in any places. In this paper, correlations between precipitation variations characteristics shown with a coefficient of variation and droughty places are compared and investigated, and so a capable period of low precipitation duration is estimated with them. Regions with an inclination of drought occurrence are similar to ones with a large coefficient of variation. A relation between a magnitude of low precipitation and a period of its duration is the same as a depth-duration relation of low precipitation, and is approximately shown as an exponential equation. And with utilizations of this equation, P=αe^βT, where P : an occurrence probability of low precipitation, T : a duration, α and β : coefficients, a capable period of low precipitation is estimated.
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