Abstract
The Southern part of Osaka Plain is one of the regions which has a great many irrigation ponds for paddy-fields. In many geographical writings, one often comes across the statement saying quite simply that it is because this region is an area within the Inland Sea, or the Seto-uchi region, with the least rainfall, that there are many ponds. But the authors do not agree to such an opinion. They consider the reason for the numerous ponds found in the region to be as follows: All the rivers in this region are short and have small catchment areas where there are no high mountains to cause rainfall. But the total area of paddy-field in the river valleys is large for the size of the rivers; therefore the paddy-fields of the region cannot get enough irrigation water from the river source only.
In Japan, wherever river water is insufflcient, irrigation ponds or wells are made use of. So we made an inquiry as to which of the two facilities is better suited for this region. As a result of our study, we have discoveredt hat since this plain is occupied for the most part by diluvial upland with a slight gradient where the soil has small permeability, and moreover, it is surrounded by hills where there are many sites for ponds, it is easier and more advantageou to construct irrigation ponds than to dig wells. As a matter of fact, it has been found that there is need for such irrigation facilities as ponds in this region, and that the topographical features are also suited for making ponds. Hence the pond irrigation has developed.
In order to work effectively, however, irrigation ponds must be easily supplied with water. But judging from map reading only there are many ponds which seem to be very difflcult to collect water. The means of collecting water seemed to be a most interesting problem for the authors; so they selected and studied three characteristic districts_??_the northern part of the region east of Sakai City, the Makio River Valley, and the Kashii River Valley.
The catchment area of the northern part east of Sakai City is the smallest one on this plain, so the possibility of collecting water for the ponds is greatly limited. As a result, the farmers, when transplanting rice, must wait for the rain of the so-called “bai-u” or the rainy season, which begins from between the 10 th to the 20 th of June and lasts until the 1 st to the 10 th of July.
In the Makio River Valley, the river and ponds co-operate to supply irrigation water.
In the Kashii River Valley, there are three characteristic areas. One is preferentially supplied with water from the ponds in the hill-land which easily gather river water ; the second is supplied with water from shallow, dish-like ponds on the plain where it is difficult to gather water ; and the third utilizes under-ground water and river water in addition to pond water.
