デルタ稲作農業の自然環境とデルタの開発構図(<小特集>メコンデルタの自然と農業)

Abstract

The Vietnamese part of the Mekong Delta is divided into seven physiographic divisions in relation to rice culture. They are Trans-Bassac Horst, Floodplain, Modern Delta, Coastal Complex, Broad Depression, Plain of Reeds and Western Coastal Zone. Some of them are further subdivided. Floodplain is a graben filled up by the Mekong and the Bassac's fluviation. Trans-Bassac Horst is a slightly uplifted geologic block with flat ground surface. Modern Delta is the downward extension of Floodplain and it has deltaic networks of rivers of great tidal range. Broad Depression is a blocked-in swamp behind Coastal Comples. These five divisions form the important rice growing area of the delta. The last two, Plain of Reeds and Western Coastal Zone, are swampy areas on the margin of the delta still waiting possible reclamation. The demarcation of the divisions and their subdivisions are given in Fig. 2. The delta is a wet terrain, being signified by the plentiful monsoonal rainfall with even distribution over the rainy months (Fig. 3), and by low-lying topography. The delta is, however, divided into two parts with respect to the hydrologic conditions, in which the alternation of dry and wet cycles are more pronounced in one region, and less significant in the other. The two classes are closely related to the physiographic divisions; Trans-Bassac Horst, Floodplain, and Coastal Flat comprise the former region, and Modern Delta, Broad Depression and Lagoonal Swale belong to the latter. An adaptation of the rice growing methods to the given hydrologic conditions is the character that defines the rice culture of the delta. Rice growing does not rely on river water, if not at all, but it depends more on rainfall and its localized run-off which is controlled by micro-relief. The above characteristics of the present hydrology are summarized in Table 1 corresponding to the physiography and typical types of rice culture. The river water, however, will have to be tapped to meet the requirement of water for TN rice (new high-yielding varieties of rice) which is rapidly extending its area as the early rainy season's crop as well as the dry season's crop. Abundant flow of rivers, creeks and canals is readily usable as it is often perennial being induced by the significant tidal fluctuation in the sea and in major rivers. The possible types of river water control and utilization are described and summarized in the same table, in view of giving the favorable conditions to the extended TN rice and possible diversification of the delta agriculture. The natural environment is one which will allow further modification by the efforts of individual farmers and of local communities once infrastructual and institutional facilities are provided by the Government.