Trends of bulk precipitation and Streamwater Chemistry in a Small Mountainous Watershed on the Shikoku Island of Japan

Abstract

For this study, characteristics and trends of the chemical constituents in bulk precipitation and streamwater were observed in a small mountainous watershed on the Shikoku Island of Japan, which covered an area of 27.4 hectares. Bulk precipitation and streamwater chemistry data spans from May 1997 to October 2004, and January 1996 to October 2004, respectively. The data were tested for two types of trends: (1) a monotonic trend to determine if concentrations of the chemical constituents were generally decreasing, increasing, or stable during the study period, and (2) a step trend to determine if a change occurred following the December 1999-January 2000 forest thinning. Both parametric and non-parametric statistical analyses were carried out in this study. Although the study area is only 35 km away from the Pacific Ocean, bulk precipitation chemistry was also influenced by terrestrial sources to a large extent. Streamwater chemistry was influenced by bedrock weathering, which was dominated by Ca2+ and HCO3−, and was not strongly related top recipitation chemistry.Non−parametric Seasonal Kendall Test(SKT)showedadeceasingtr end of Ca2+and anincreasing tr end of K+in bulk precipitation.Despite the decreasing trend of Mg2+, an increasing trend of pH was found in the stream water.Non−paramet ric Mann−Whit ney−Wilcoxon Rank Sum test showed statistically sign if icantin creases of NO3−and Ca2+ in stream water followed by a moderate thinning operation.