Abstract
We continued watching the level of water in Rokudoike, an irrigation pond in Higashi-Hiroshima, Hiroshima Prefecture, for three years between fiscal 2004 and 2006. We studied the feasibility of allocating part of the capacity of the reservoir for catching rainwater to prevent floods. Firstly, upon presenting a formula for calculating the fill-up percentage from the observed water level, we calculated the fill-up percentage. Then, we subsequently examined how the volume of rainfall would affect the fill-up percentage. At times, the percentage dropped close to 50 in fiscal 2005 due to unusually small amounts of precipitation in the earlier half of the rainy season. In the three-year period, however, the fill-up percentage stayed at or above 70 on 86 percent of the days in the rainwater-harvesting seasons. It was obvious that the percentage of the capacity that was actually used for irrigation had decreased with the shrinkage of the size of workable rice-paddies. We decided to calculate what percentage of the capacity of the pond was usable for flood control instead of irrigation. In addition, we decided to apply the calculated percentage as a yardstick to manage the water level on June 1, when the rainy season began. The 73-day period up to August 12 was set for our study of how the fill-up percentage would drop with the discharge of water from the pond for irrigation. In the next step, we simulated changes in the percentage for the study period and decided on the minimum required fill-up percentage that should have been applied on June 1. Furthermore, for the dry ear that occurs once in 10 years, we evaluated the total amount of rainfall and the amount required to recover the initial fill-up percentage, and calculated the initial minimum required fill-up percentage for that hypothetical season. That leads us to the following conclusion. As expected, the pond can retain the minimum required capacity to prevent or ease floods downstream in addition to the capacity necessary to irrigate rice paddies. This conclusion allows for reasonable growth in size of the rice paddies in the future.
