Small rainfall as a factor of drought in Okinawa (Department of Agricultural Engineering)

1.この調査は, 沖縄における干ばつの一つの要因としての少雨が, 1年のうちでいつごろ, どの程度の強さで起っているかを統計的に調べたものである。少雨期間の長さは数日程度のものから90日程度のものまでを扱った。なお, この調査のための資料としては, 琉球気象庁が那覇において観測した1891-1968の降雨量の値を使用した。2.この調査によって次のことが明らかになった。(1)夏と秋には, 降水日数は特に著しく少ないわけではないが, 雨の降り方にムラがあり, 時間的配分がよくないために沖縄では特に渇水が起りやすい。(本文中の第1表ないし第3表, 第1図)(2)1週間程度の長さの干天の続く頻度が, 1年を通じてかなり詳しく明らかになった。このような干天の頻度も夏と秋に大きいが, 特に梅雨明けの7月の初めには大きい。しかし, 8月および9月の台風期には干天の頻度が小さくなる。したがって7月にはじまる渇水は中断されて, 8月までには解消されるものが多い。(第2図)(3)かなり長期間(30日ないし90日間)にわたる少雨の頻度は10月-11月に最も大きく, 7月がこれに次ぐ。このことは, 夏に台風が沖縄に接近しなければ, 夏から秋にかけてかなり長期間の干ばつになりやすいことを示す。(第3図)(4)2月から3月にかけては, かなり長期間にわたる少雨の頻度が小さく, 干ばつになるおそれは比較的に少ない。(第3図)(5)沖縄におけるかなり長期間(30日ないし90日間)の干ばつの示数として, 非超過確率雨量, リターンピリオド, および少雨期間との関係が数量的に求められた。(第5表, 第5図)1. This study was made to look into the small rainfall as a factor of drought in Okinawa. Emphasis was put on finding statistically when and how severely the small rainfall has been received so far in Okinawa with respect to the length of dry period. Data for this study were available from the official file of rainfall records for Naha, Okinawa, prepared by the Ryukyu Meteorological Agency. 2. The results are as follows : (1) Water shortage in Okinawa is apt to occur often in summer and autumn due to unevenly distributed rainfall with time, although the number of rainy days is not remarkably small in the seasons. (2) The frequency of dry weather continuing for about a week is also large in summer and autumn, particularly at the beginning of July, right after theBaiurainy season. The frequency, however, becomes less in August and September, in which typhoons often come close to Okinawa. This means that water shotage in Okinawa that often begins in early July is usually interrupted by the active typhoon season. (3) The frequency of small rainfall for relatively long period of time covering 30 to 90 consecutive days is largest in the October to November period. A secondly large frequency appears in July. These facts indicate that the drought of relatively long period of time occurs often in Okinawa in summer through autumn, unless typhoons come close to Okinawa. (4) The frequency of small rainfall for relatively long period of time is small in February and March, showing that drought is less expected in these months than in any other months. (5) Relations among non-exceedance probability rainfall, return period, and the duration of dry weather in Okinawa have been obtained numerically and graphically for periods ranging from 30 to 90 days