In the floodplain composed of gravel and/or sand, the groundwater recharge from the river occurs partially and actively. To estimate the active and partial groundwater recharge from the river in the floodplain, this research examined to estimate the groundwater flux through the bypath in Okayama floodplain, using the temperature variation. Water temperature was monitored at the Asahi River and one spring at which such groundwater discharges in the Asahi River floodplain of Okayama Prefecture in

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... Japan. Temperature variation of this spring showed the amplitude damping and the phase lag, compared with the river temperature. Results analyzed by using the one-dimensional heat transport model indicted that the Darcy flux of this groundwater was 2.9-6.5 m/d. In addition, results by inverse analysis of the horizontal temperature distribution suggested that the flow distance from the river is applicable in the range between 80 m to 1500 m. The approach by this research would be expected as the easy and simple application method to estimate groundwater recharge and groundwater resources in floodplains.