Study on Characteristics of Water Resources in Western Japan (1st Report) : Statistical Rainfall Characteristics of Two Rainy Seasons(Baiu and Shurin)

Baiu and Shurin which are the representative precipitation periods of summer half year of Japan and very important for water resources of our country are compared mainly by their rainfall amounts. We found quite differences between them with their rainfall distribution of 30 year average ten day or five day rainfall amounts, and these differences seemed to be caused by upper air moisture transfer of each season. Moreover the precise distribution of Baiu and Shurin's rainfall amounts reported from more than 2500 rainfall observing stations in Japan shows the characteristic distribution probably due to the terrain effect or the local scale precipitation mechanism. The dependency of river flow on each precipitation season is also considered to some extent. Obtained results are as follows : 1) According to the distribution maps of the ten and five day rainfall amounts, the area of the maximum rainfall amount moves from Ryukyu region (Baiu season) to Hachijo-Zima region (Shurin season). (Fig. 1) 2) Rainfall amount of Shurin season is distributed homogeneously over Japan Islands and generally less than Baiu. 3) Rainfall band moves northward parallel to latitude line and returns east southward parallel to the orientation of Japan Islands according to the distribution maps of the 5 day rainfall amounts. 4) The distribution of the appearance of 5 day rainfall maximum of summer half year, and the ratio of Baiu season's rainfall amounts (P_B) to Shurin season's (P_S) show that in Western Part of Japan they have more rainfall amounts in Baiu season, and in Eastern Part of Japan, on the contrary, they have more rainfall amounts in Shurin season. 5) Some situation of upper air moisture transfer intimately corresponds to the distribution of rainfall amounts of each season. (Fig. 5,6) 6) Predominant relation of each precipitation season appears not only in separating Japan Islands into two parts, but depends on the local scale terrain effect. (Fig. 7) 7) Generally in plain parts of Japan Islands is found the predominance of Shurin, and on the other hand in terrain region is found the predominance of Baiu. (Fig. 8) 8) Discharge is more abundant in Baiu in any parts of Japan Islands. (Table 2) This corresponds to the predominance of Baiu in mountaineous regions.

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