ELSEVIER Agricultural Water Management 28 (1995) 185-200 Agricultural water management Water use for upland irrigation in a humid Japan Atsushi Yomota, George M. Ndegwa ’ region of Fuculty of Environmental Science and Technology, Okayarna University, 2-l-1, Tsushima-naka, Okayama City 700, Japan Accepted 12 January 1995 Abstract The situation of actual water use in a typical upland irrigation district was clarified through a study of irrigation quantity supplied to irrigators on a demand basis in Japan. Three methods were used to determine the district water use during periods with no rainfall. The first method considered this water use to be the maximum 10 days’ average irrigation quantity in each month while the second method considered it to be the constant term in the relation between daily irrigation quantity and the moving average of daily rainfall. The third method considered this water use to be the constant term in the multiple linear regression of daily irrigation quantity and rainfall. All the methods gave almost the same results for the respective months analysed. This water use was about l-2 mm day-’ which was very low when compared with the design irrigation quantity of 3.7-5.3 mm day-’ for the district. Effective rainfall was also determined. A land use survey in a representative sample area indicated that fallow fields occupied about 30% of the total irrigation area during the irrigation season. From the results of this study, it is recommended that the irrigation plan be based on an areal, e.g. district, water use instead of the present crop based consumptive use. Kqwords: Water; Irrigation; Japan 1. Introduction Field irrigation has usually been recognized as a means of preventing water from becom- ing a limiting resource to crop production. In most arid regions where irrigation is practised, all crop water needs are made available for the land area irrigated; hence, rainfall during the irrigation season is not generally considered an essential portion of the water require- ’ On study leave from Department of Agricultural Engineering, Jomo Kenyatta University of Agriculture and Technology, P.O. Box 62000, Nairobi, Kenya 0378.3774/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDlO378-3774(95)01179-X