森林立地学会誌　森林立地49(2), 2007, 133〜144

Jpn.J.For.Environment 49(2), 133−144  2007

 

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

 

篠宮佳樹^＊・山田　毅^＊*・鳥居厚志^＊

^＊森林総合研究所四国支所・^＊*森林総合研究所

 

国内でも有数の多雨地域である高知県西部，四万十川源流域に位置する森林流域において，大雨(総降雨量100mm以上のイベントとする)を含む降雨イベント時の渓流水質(Cl^-，SO₄^2-，NO₃^--N，Na⁺，K⁺，Ca²⁺，Mg²⁺，SiO₂)の変動パターンを分類し，L-Q法に対する適合度を比較した。試験流域の面積は18.7ha，平均傾斜37°，植生はモミ･ツガ天然林，地質は堆積岩，調査は総雨量18〜289mmの17降雨イベント（2時間毎に採水）で行った。各降雨イベントの流量−濃度関係を基に，�@SO₄^2-，Na⁺，Ca²⁺，Mg²⁺：常に負の相関，�ACl^-，SiO₂：負の相関だが，降雨規模が大きくなると相関低い，�BNO₃^--N：正の相関だが，降雨規模が大きくなると負の相関，�CK⁺：相関無しの4タイプに分類した。SO₄^2-，Na⁺，Ca²⁺，Mg²⁺の流量−濃度関係はCl^-，SiO₂，K⁺，NO₃^--Nより対数上の直線性が高かった。Cl^-，SiO₂は流量増加過程と流量逓減過程で流量−濃度関係が異なり，規模の大きな降雨イベントほどその違いが明瞭になった。大雨の前半に流量増加に伴ってNO₃^--N濃度は上昇し，後半にNO₃^--N濃度は顕著に低下したため，大雨時のNO₃^--N濃度の変動はCl^-，SiO₂より大きかった。以上より，本流域のL-Q法の適合度はSO₄^2-，Na⁺，Ca²⁺，Mg²⁺＞Cl^-，SiO₂，K⁺＞NO₃^--Nであると考えられた。

 

Shinomiya Yoshiki, Yamada Tsuyoshi and Torii Atsushi：Variation of streamwater chemistry during rainfall and the fitness for the L-Q method in a mountainous headwater catchment in the Shimanto River basin

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⁺, Ca²⁺, Mg²⁺ have always negative correlation; Cl^- and SiO₂ have negative correlation　except　for　large-scale rainfall events; NO₃^--N has usually positive correlation but negative in　 large-scale rainfall events; K⁺ has no correlation. For Cl^-, SiO₂ and NO₃^--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 NO₃^--N during heavy rains, the fluctuation was greater in NO₃^--N than those in Cl^-, SiO₂ and K⁺. It is concluded that the fitness for the L-Q method was SO_４^2-, Na⁺, Ca²⁺, Mg²⁺ > Cl^- , SiO₂ , K⁺ > NO₃^--N in this catchment.

 
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