nakata_2006.htm

Bulletin of Glaciological Research 23 (2006) 1-12
©Japanese Society of Snow and Ice

Relationship between seasonal trends in stream water chmistry and forest type in a snowy
temperate region of northwestern Honshu, Japan

Makoto NAKATA
Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan

Abstract

　 Stream water chemistry was compared between neighboring Cryptomeria japonica evergreen　coniferous (CFs)and deciduous broad-leaved forests (BFs)in a snowy temperate region of northwestern Honshu, Japan. The stream water chemistry of the study area was characterized by extremely high concentrations of Cl^- and Na⁺, which were thought to be mostly derived from sea salts. There were negative correlations between runoff and the concentrations of many ions, especially Cl^-, Na⁺, and Mg²⁺, from spring to mid-autumn. However, these correlations became weak in late autumn and winter due to the high influx of sea salt components transported by the northwest monsoon, as well as the acid shock phenomenon. As the runoff increased from late February to mid-March, the electrical conductivity (EC)of stream water rose abruptly, reached a maximum in early March, and then fell abruptly. The fluctuations during this period were extremely sharp in BFs. The pH of stream water in BFs (6.3-6.8) was significantly higher than in CFs (6.1-6.5) from spring to mid-autumn. The pH in late autumn and winter was lower than that from spring to mid-autumn, especially in BFs (5.5-6.3). This may be induced by the decline in the acid-neutralizing capacity (ANC), the supply of sea salts caused by the northwest monsoon, and the increase in NO₃^-concentration in late autumn and winter. Both the snow depth and total of chemical components in snow were greater in BFs than in CFs due to the difference in the accumulation pattern of snow on tree crowns and the ground. Approximately 80% of the chemical components in the snow were estimated to originate from sea salt. These chemical components in the snow dissolved more rapidly in BFs than in CFs from late winter to early spring due to the difference in the quantity of sunlight received at the snow surface and the density of the snowpack. Two patterns of pH drop (as low as 5.4-5.6) in stream water were recognized from late winter to early spring: one induced by an early snowmelt in BFs, and the other by the dilution of the ANC during the peak flow in the snowmelt season in both BFs and CFs. Acid shock was evident when the weather improved following cold snowy days in this snowy temperate region of Japan. This tendency was clearer in BFs than in CFs due to the different snowmelt patterns between the two forest types.