The purpose of irrigation and drainage is to control the soil water content to the best condition for plant growth. In the humid region such as Japan, the amount of rainfall generally exceeds the potential evapo-transpiration. For example, the mean annual precipitation in Japan is about 1800mm/year, and the mean annual evapotranspiration is estimated as 600-800 mm/year. Therefore, the irrigation for upland field is usually considered to be unnecessary. But, the precipitation is nonuniform in time. A precipitation of over 200 mm sometimes occurs in one day. On the other hand, a drought period extending over 30 days sometimes occurs in summer. Therefore, irrigation is necessary to compensate for the irregular rainfall. In a mountainous area, however, where orchard or tea garden has been reclaimed actively in recent years, the irrigation system has other purposes as well. It is important there to have an uniform soil water content not only in time but also in space. The soil water control becomes the critical condition for system planning or design in either case.
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