Impact of Paddy Field Reservoirs on Flood Management in a Large River Basin of Japan

Abstract

The flood retention capacity of paddy fields is well-recognized in Japan, and all the existing flood control practices via paddy field management achieve reductions in peak flood discharge. However, the previous studies have not assessed the flood management potential of paddy fields in a large river basin with average paddy coverage, and the existing hydrological models are not quite suitable for simulating river discharge from closed-drainage paddy reservoir storage. We herein attempt to improve the watershed-scale version of global hydrological model H08 to simulate a reduction in the peak discharge from paddy reservoirs in the Abukuma River basin of Japan. The NSE and R2 index showed fair reliability of the H08 model during the calibration and validation stages. The simulated results from the improved model show 11% and 6% peak reductions in high paddy coverage areas for a normal year (2018) and a major typhoon year (2019), respectively. The peak-reduction percentage increased with decreasing rainfall, depending on the overflow from the paddy reservoirs. The results indicate that the paddy reservoir is not highly effective in a large river with less than 20% paddy coverage, but the peak discharge reduction capacity shows that paddy reservoirs can make some contribution when used in combination with dam operation.