Presented in this report are some problems with irrigation projects in Southeast Asia countries, most of which are caused by differences in development of technology between the developed and the developing countries. As the first step in technical cooperation, visiting engineers should understand the actual and existing situation of agricultural technology together with its background and the surrounding conditions in the developing countries. Some ideas are also suggested for better technical cooperation, particularly in the field of wetland paddy cultivation.

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In the vicinity of Nanyo City in southern Yamagata Prefecture, an area of peat land known as Oyachi lies in the flood basin of the Yoshino River. This marshy peat land was converted into rice paddy fields in the seventeenth century and has been cultivated with rice ever since.
However, in the upper basin of the Yoshino River are a number of copper mines, and during and after World War II, there was leakage of cadmium from the huge slag dumps into the river. Ten years ago, the prefectural sanitation department found cadmium traces in rice harvested in Oyachi that were 4 times above the legal limit; the sale of Oyachi rice was thensuspended by law.
Nevertheless, there were no outbreaks of “Itai-itai Disease” such as found in the Jinzu River drainage basin in Toyama Prefecture. The reasons for this may be found in the characteristics of the Oyachi peat soils and in the particular soil improvement practices. Specifically
(1) A thick layer of fresh mineral soil is periodically applied to the paddy fields to reduce soil settling and consolidation and to prevent the excess nitrogen that would be produced by the decomposition of drying peat. This periodic soil application has compacted the underlying soils and prevented the cadmium from rising to the soil surface layer.
(2) While cadmium is very active in oxidation state soil, it is generally inactive in reduction state soils like peat.
These findings are based on a case study of the Oyachi peat lands, but because there are extensive areas of peat soils as well as considerable copper mining in the Tohoku region, they are of general relevance in understanding pollution in Tohoku soils.

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In Japan, mountainous and hilly areas make up about 70 percent of the total area. Therefore, one of the nation's most serious problems is the lack of farm land. The low rate of land consolidation also means that efforts to food self-sufficiency are insufficient. In order to increase the economical potentiality in the district, there is a strong demand for development and improvement of agricultural land.
In Chugoku and Shikoku districts, where the percentages of the mountainous and hilly areas are extremely high, extensive development work has been begun by government. But owing to the unfavorable conditions of topography, geology and climate, there are many factors working against these projects.
(1) Environmental changes: Large amounts of earth work brought by topographical improvement cause changes in hydrological characteristics and result in environmental transfiguration. (2) Land conservation: A large scale earth work makes destruction of the land easier. Therefore, it is necessary to pay much attention to the incidental facilities especial ly in conservation. (3) Construction cost: Owing to these unfavorable conditions for land development, operation costs are necessarily highly expensive. (4) Structure and its effect on agricultural production: Difference in the direction and inclination of slope cause some differences in the land's climatic conditions, especially in solar radiation. Therefore, in farm land development in mountainous district, structure planning of the field has to be considered.

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A multiple regression analysis, one of the statistical numerical estimation methods of water balance in a basin, is introduced with an example basin in this paper. This method can be used easily by means of a digital computer. The mathematical bases of the method are given here. As for the runoff model by this method, the whole basin consists of two kinds of areas. One is a non-irrigated area, the other is an irrigated area. An example of an estimation of water balance of multiple regressional models is given for the Yashiro river basin in Fukushima prefecture. The prominent character of this method is that the models simulate very well the low flow and the return flow of the river at the checking points. Some results of the simulation are shown in several figures. From these results, this method simulates totally water balance well.

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