Presentation:
Masami Okamoto (Nihon University)
Contact:
The difficulty of making adjustments to water use in the tropical Asian monsoon region stems from the existence of rainy and dry seasons. While rice fields are irrigated during the rainy season, irrigation is carried out during the dry season too to allow a semiannual crop, making it necessary to manage water resource distribution between the rainy and dry seasons. In the Chao Phraya Delta in Thailand, the use of water for power generation and the expansion of irrigated areas have made the adjustment of water use even more complicated.
The difference between humid and arid areas can be explained by differences between the actual evapotranspiration (actual amount of evapotranspiration) and potential evapotranspiration (amount of evapotranspiration in a situation where the ground surface is sufficiently humid). In a humid area, the difference between the two is small and there is no room for evapotranspiration to increase even if irrigation is introduced. Most of the water drawn from rivers for irrigation eventually flows back to the rivers. On the other hand, in an arid area, the difference between the two is large, and the actual evapotranspiration increases markedly with the introduction of irrigation. The water drawn from rivers for irrigation is discharged into the atmosphere through evapotranspiration and does not return to the rivers. This problem can be seen in the case of the Aral Sea.
The introduction of Participatory Irrigation Management (PIM) is a prerequisite for receiving loans from the World Bank. It is said that irrigation management in Japan served as a model for Participatory Irrigation Management. Farming village society in Japan was built on the premise of water use, and water use organizations were an integral part of the farming village society from the very beginning. Water use organizations in Japan were supported institutionally. The equality of water distribution was made possible largely by the existence of a village responsibility system (murauke seido) where each village as a whole had the duty to pay tributes in the form of rice, and it can be said that the water use organizations were supported also by this system. Thus, Participatory Irrigation Management cannot function in societies without such a background, where water use organizations are set up as a completely separate system from the village community. However strong a village community is, if it is not an organization premised upon water use, it cannot exercise control over use.
In response to the presentation, participants explained cases of water use adjustments in their own study fields. Suggestions were also made. They included one suggestion that the typification of agriculture in various places of the world might be possible using indices such as the number of months with heavy rain and topology or the actual evapotranspiration/potential evapotranspiration and temperature; and another that it might be possible to classify water use forms based on whether there is dry-season rice growing and water use organizations.
(Keisuke Hoshikawa)