Variation of streamwater chemistry during rainfall event and the fitness for the L-Q method in a mountainous headwater catchment in the Shimanto River basin

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  • 四万十川源流部の森林流域における降雨イベント時の渓流水質変動とL-Q法への適合度
  • シマントガワ ゲンリュウブ ノ シンリン リュウイキ ニ オケル コウウ イベントジ ノ ケイリュウ スイシツ ヘンドウ ト L Qホウ エノ テキゴウド

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Abstract

The variation pattern of streamwater chemistry during a flood event was classified to compare the fitness for the L-Q method in a forested, mountainous headwater catchment in the Shimanto River basin, Shikoku Island, southwestern Japan. The catchment (18.7ha) underlaid with sedimentary rock has steep slopes and is covered by a natural forest (predominately Abies firma and Tsuga sieboldii). Investigations were conducted for 17 events, including 4 heavy rains which mean rainfall exceeding 100mm. The variation pattern between discharge and concentrations was classified to 4 types: SO_4^<2->, Na^+, Ca^<2+>, Mg^<2+> have always negative correlation; Cl^- and SiO_2 have negative correlation except for large-scale rainfall events; NO_3^--N has usually positive correlation but negative in large-scale rainfall events; K^+ has no correlation. For Cl^-, SiO_2 and NO_3^--N, the relationship between discharge and concentrations on the runoff recession differs from those on the runoff rising. The difference of those becomes clear as the scale of rainfall event is greater. The temporal increase of K^+ was observed on the runoff rising process in some events. Due to the increase and remarkable decrease of NO_3^--N during heavy rains, the fluctuation was greater in NO_3^--N than those in Cl^-, SiO_2 and K^+. It is concluded that the fitness for the L-Q method was SO_4^<2->, Na^+, Ca^<2+>, Mg^<2+>>Cl^-, SiO_2, K^+>NO_3^--N in this catchment.

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