Abstract
We have been monitoring rainwater and streamwater chemistry in forests located in the upper reaches of the Shimanto River within Yusuhara-cho, Takaoka-gun, Kochi Prefecture, since 2000. The rainwater was collected at one location, whereas streamwater was sampled at run-off points of seven forested catchment basins. In this study, we analyzed data collected from 2000 to 2015 to examine changes in rainwater and streamwater chemistry. In particular, we investigated the effects of the volume of rainfall on water chemistry and the long-term changes. During periods with lower total rainfall, electrical conductivity (EC) was higher and variation in the range of pH was greater. In addition, no long-term trend was observed for rainwater pH, EC (annual average), and annual input of various ions. Although streamwater EC decreased with the increase in cumulative rainfall for 14 days before the sampling day, only a little effect on pH was observed. No long-term change in streamwater pH was found. However, the concentration of dissolved ions such as Na+, Ca2+, and Mg2+ tended to decrease in all seven forest watersheds from 2007 to 2015; the decrease in Cl− and SO42− was distinct. We recommend continuous monitoring of these decreasing trends to determine how long they continue.
