Abstract
In a paddy field area which has some water sources (and/or intake facilities) such as dams, head works, spring ponds, small reservoirs and wells, how and how much water to be used from each source for irrigation is important from the view point of water management. In this paper, temporal water intake volumes from these sources were determined not only at the present irrigation plan, but also at the future, taking the Echi River Fan as a case study. This was carried out by minimizing the total cost of irrigation water using linear programming. The value obtained (optimum water intake volume from each source) was then compared with the actual one. Further, storage of groundwater at the optimum intake was estimated on the basis of a simple water budget. As a result, it was suggested that groundwater had advantages which should not be evaluated only by the cost, and that the upper limit of the water intake volume from the existing dam was about 130 Mm3. In addition, under the optimum intake conditions in the present plan, 65% of the water recharged into groundwater body from the paddy fields during the irrigation period, which was estimated to be 62-63 Mm3, was discharged and 30% of that was stored at the end of the irrigation period.
