Diurnal Fluctuation in Stream Flow and in Water Quality by Evapotransplration During Summer Drought Periods

在日本北部被北方森林覆盖的源头盆地，观测到冬季和夏季的干旱流。在夏季干旱中，白天的蒸散发引起了水流量和电导率的显著日波动。结果表明，随着河流流量的逐日减少，HCO_3^-离子浓度也随之降低。假定干旱流是地表流和下游浅层流的混合流，将干旱水文曲线分为两个部分。(1)由于夏季降水不足，从融雪期到夏季干旱期，流域流量呈指数递减。(2)夏季低潮期暴雨流量减少速度快于融雪期，暴雨径流后2 ~ 3 d流量恢复到长期衰退趋势。(3)用HCO_3^-、Cl^-、SO_4^=、NO_3^-、Na^+、K^+等分析了水的化学成分。在相同浓度下，HCO_3^-约占阴离子总数的2/3，Cl^-约占阴离子总数的1/3。发现水系电导的日波动是由HCO_3离子浓度的波动引起的。(4)水流分量、地下水和浅水的波动是由河道附近的蒸散发引起的，从而导致HCO_3^-离子浓度的日波动。

In a headwater basin covered with boreal forest in northern Japan, drought flow was observed in winter and in summer. In summer drought it was found that remarkable diurnal fluctuation of stream flow and of electric conductance is caused by daytime evapotranspiration. It was found that the daily reduction in stream flow, was accompanied by a reduction in the HCO_3^- ion concentration.The drought hydrograph was separated into two components, assuming the drought flow to be a mixture of surface flow and of shallow flow entering the stream in its lower reaches.(1) Caused by the deficency in rainfalls in summer in this basin, flow rate decreases exponentially from the snowmelt period to the summer drought period.(2) Stormflow in summer low-flow period reduces more rapidly than that in snowmelt period and flow comes back onto the long-term recession trend in 2 or 3 days after storm runoff.(3) The chemical composition of water was analyzed on HCO_3^-, Cl^-, SO_4^=, NO_3^-, Na^+ and K^+. HCO_3^- occupied about 2/3 of the total anions and Cl^- did about 1/3 in equivalent concentration. The diurnal fluctation in the stream conductance was found to be caused by the fluctuation in HCO_3^-ion concentration.(4) The fluctuation in flow components, subsurface and shallow water was estimated to be caused by evapotranspiration near the stream channel, resulting in the diurnal fluctuation in HCO_3^- ion concentration.