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

利用氚、氘和氧-18等环境同位素示踪剂，研究了日本中部山区森林盆地的水停留时间和水溶质浓度的均匀过程。研究结果总结如下：1)土壤水和地下水弥散流模型估算的停留时间分别为4个月和19年左右。2)溪水是土壤水和地下水的混合物，土壤水停留时间短，地下水停留时间较长。根据一个简单的混合模型，地下水在河流混合物中的比例平均每年约为30%。(3)土壤水和地下水的混合地点在坡脚和河谷底部称为河岸带，两种水的水系在这里集中，供给溪流排放水。4) 1.0 m以下土壤水分稳定同位素比值相对于1.0 m以上土壤水分稳定同位素比值较为均匀，其数值受输入水量的影响较大。5)土壤水分稳定同位素比例的上述均一化是由于土壤水分在蒸发和入渗过程中发散型零通量面和收敛型零通量面的发展和消失等动态行为所致。

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.