PROPOSAL OF EVALUATION INDEX FOR THE RAINWATER STORAGE FUNCTION OF AN IRIIGATION POND

Abstract

 We proposed a novel evaluation index on the rainwater storage capacity of irrigation ponds based on the total increase in stored water volume accompanying rainfall during peak inflow. On irrigation ponds in Miyagi Prefecture, we calculated the flood inflow and the quotient (drainage-area ratio) obtained by dividing the drainage area by the reservoir area, and investigated rainwater storage capacity using 14 model irrigation ponds prepared based on the drainage-area ratio distribution. As a result, it was revealed that: 1) rainwater storage capacity was recognized even in irrigation ponds of large drainage-area ratio and minor flood peak-cutting effect, which has conventionally been used to evaluate the flood control performance of irrigation ponds; 2) conversely, rainwater storage capacity was relatively small in ponds having high peakcutting effect and small drainage-area ratio; and 3) both kinds of pond can be enhanced in terms of rainwater storage capacity by lowering the water level before rainfall. In using irrigation ponds for flood control of rivers, we observed that release of water below the maximum storage level before rainfall is effective for enhancing the functions and effects of both kinds of pond.