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

在日本北部一个被北方森林覆盖的源头盆地，冬季和夏季都观察到了干旱流动。夏季干旱时，白天蒸散量引起的径流量和电导率日波动较大。研究发现，河流流量的逐日减少伴随着HCO_3~-离子浓度的减少。假定干旱径流是地表径流和下游浅水径流的混合，将干旱过程线分为两部分。(1)由于夏季降水不足，造成了该流域的干旱过程。从融雪期到夏干期，流量呈指数递减。(2)夏季枯水期暴雨流量比融雪期减少更快，暴雨径流后2~3天流量恢复到长期衰退趋势。(3)分析了水体中HCO3-、Cl2-、SO4+、NO3-、Na+和K+的化学组成。在相同浓度下，HCO3-约占总阴离子的2/3，Cl3-约占1/3。水系电导的日波动是由HCO3-离子浓度的波动引起的。(4)水流成分、地下和浅水的波动估计是由河道附近的蒸散引起的，从而导致HCO3-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.