Development of a Simplified Water Storage Operation Curve at Irrigation Ponds for Flood Mitigation -A Case from Tamatsu District in Tottori, Japan-

Abstract

In recent years, Japan has been receiving heavy rainfall, and the flooding of irrigation ponds is a major concern. The irrigation ponds in the Tamatsu district in Tottori, Japan, are small and overflow easily during heavy rainfall. Pre-releasing water from these ponds was suggested as a simple, safe, and low-cost solution. However, local governments have prepared no clear guidelines or instructions on the amount of water that should be pre- released or regarding the desired water level after pre-release. Currently, the water volume that is pre-released from the ponds depends entirely on the pond manager’s experience. To mitigate this situation, we first observed the water level and discharge of the irrigation ponds and collected rainfall data from the nearest meteorological station. We then assessed the water storage operations of pond managers at three irrigation ponds in the Tamatsu district in 2018. Subsequently, we prepared the height-volume curve (H-V curve) of each pond by level survey and 3D image analysis, and a cumulative water demand curve based on the volume of water released from each pond during no rainfall periods. Considering the cumulative water demand curve and the height of the intake tap of each pond, we prepared water storage operation curves for the irrigation ponds. This will assist pond managers to determine the appropriate water level reduction during heavy rainfall events without causing an irrigation water shortage. We confirmed the extent to which the pond management curve could cope with rainfall. Using the developed water storage operation curve, managers can easily understand the drop of the water level in the pond. Moreover, the curves are easily constructed from the information on the beneficial area, precipitation, water level fluctuations of the irrigation pond during dry days, and the H-V curves of ponds. Finally, the effectiveness of the water storage operation curve is evaluated for different rainfall events.