EFFECTS OF PADDY FIELDS ON WATER CIRCULATION IN ASO NANGOU-DANI

Abstract

 In this study, water requirement rate, groundwater rechage from paddy fields, water volume in spring and river which can be used for paddy field, and groundwater pumping for flooding were investigated to understand effects of paddy fields on water circulation in Minamiaso and Takamori located in Aso Nangou-dani. Averaged water requirement rates before and after mid-summer drainage were 54.7 mm/day and 67.7 mm/day, respectively, which were higher than averaged values in Japanese paddy fields. Althogh estimated groundwater recharges from 2015 to 2018 have decreased due to the 2016 Kumamoto earthquake, they were higher than 69 million m³ every year, which were comparable to that in Shirakawa mid-stream area. The toal amount of ovserved flow rates in the upstream of Myouken Bridge was 359 thousands m³, indicating groundwater discharge from the riverbed in Shirakawa River and Ryoheigawa River. Groundwater pumpings estimated by water balance were around 38 % of groundwater recharge from paddy fields. Groundwater pumping need not to be done on days with 30 - 40 mm rains.