Seasonal Change of Streamwater Chemistry in Tsukuba Experimental Forested Land

Abstract

This paper describes the seasonal changes of the streamsolute concentration for two years in the Tsukuba Experimental Forest with 67.5 ha. NO₃^--N concentration shows the clear seasonal change according to which it is elevated in summer, but declines in winter. Runoffs of K⁺ and Ca²⁺ seem to have a pattern similar to NO₃^--N behavior. On the other hand, SiO₂ concentration declines occasionally due to the effect of the rainfall runoff, as does Na⁺ concentration.
Two types of runoff can be found in time-dependent changes of the streamsolute concentration during the rainfall runoff event. Most of the solute concentrations decrease in the rising limb, and increase in the recession limb. In the end stage of the recession limb, the solute concentration classified into Type I recovers approximately the baseflow level prior to the rainfall runoff, while that into Type II overshoots the baseflow level. SiO₂ and Na⁺ belong to Type I, and NO₃^--N, K⁺ and Ca²⁺ to Type II. This proves that (1) concentration of Type I remains lowered due to the effect of the rainfall runoff, compared to the baseflow level, and (2) when a new rainfall runoff event occurs, concentration of Type II is raised still more than the older one. Thus, there is a concentration increase of NO₃^--N in summer due to successive rainfall. These runoff features are reliable for interpreting the seasonal changes mentioned above.