Drought Assessment Using Tritium River Water Measurements for Existing Dam Infrastructure in the Ishikari River basin, Japan

Abstract:

A proposed methodology is based on estimated groundwater volumes from tritium river water measurements in the Ishikari River basin of Hokkaido Island, Japan. In our drought assessment, we characterize a groundwater storage that is available and can be used for the water supply during prolonged droughts. For the groundwater storage estimation, we utilized tritium river water measurements obtained during baseflows to estimate water mean transit times (MTTs). Tritium is ideally suited for characterization of the catchment’s responses in river water samples with MTTs times up to 200 years. Tritium is a component of meteoric water, decays with a half-life of 12.32 years, and is inert in the subsurface. In Hokkaido, river water samples were collected in June, July and October 2014 at selected river gauging stations operated by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT). These stations record hourly water levels, have catchment areas between 45 and 377 km² and are located upstream of MLIT dams at altitudes between 36 m and 860 m MSL. The measured tritium ranged between 4.065 TU (±0.07) and 5.290 TU (±0.09) with both lowest and highest tritium values analysed in June river samples at Tougeshita and Okukatsura stations, respectively. For the MTT estimation, we selected exponential(80%)-piston(20%) Lumped Parameter Model (LPM) with constructed tritium in Hokkaido precipitation and obtained a non-unique fit of young (1-11 years) and old (16-98 years) groundwater MTTs. This result indicates that the bomb-peak tritium is still present in Japanese groundwater and may take several years to flush out. From the MTTs and baseflow discharges, the calculated groundwater volume ranges between 13 MCM and 12500 MCM and indicates potentially available groundwater storage during prolonged droughts in the Hokkaido headwater catchments. In the future studies, the accuracy of the estimated groundwater volume can be increased by conducting another tritium sampling at baseflows after 3 years from now. Having tritium series measurements with 3 year interval allows us to choose either young or old MTT value and to reduce ambiguity of LPM mixing parameters. In addition, tritium in precipitation will reach natural levels and one tritium river water sample will be sufficient to estimate a robust groundwater storage volume.