Study on residence times of discharge water and on uniforming processes of solute concentration of water in a mountainous forested basin

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Water residence times and uniforming processes of solute concentration of water in a mountainous forested basin on the central of Japan were investigated using environmental isotopes such as tritium, deuterium and oxygen-18 as tracers. The results of the study are summariezed as follows.1)Residence times estimated from a dispersive flow model of soil water and groundwater were about 4 months and 19 years, respectively.2)Stream water was a mixture of soil water having a short residence time and groundwater having a relatively long residence time. The fraction of groundwater in the stream mixture averaged about 30 % annualy as determined from a simple mixing model.3)Mixing place of soil water and groundwater was at the slope foot and at the valley bottom called the riparian zone where stream lines of both waters concentrated to feeding stream discharge water.4)Stable isotope ratios of soil water below the depth of 1.0 m were relatively uniform comparing with that of above 1.0 m depth, of which values were affected largely depending on values of the input water.5)Above mentioned homogenization of stable isotope ratios of soil water was attributed to the dynamic behavior of soil water such as the development and the disappearance of both divergent and convegent zero flux planes occurred duringe evaporation and infiltration processes.