四万十川源流部の森林における硝酸態窒素の年間流出負荷量とその流出機構

摘要

国内でも多雨地域にある四万十川源流の森林流域を対象に，大雨時のNO_3~-濃度形成過程について検討するとともに，大規模出水時も考慮して年間の硝酸態窒素(NO_3-N)流出負荷量を評価した。試験流域（堆積岩，18·7ha,モミ•ッガ天然林）で，月2回の定期採水と17回（総雨量18～289mm)の出水調査を行った。NO_3-N累加比負荷量の流量以外の説明変数として降雨前NO_3濃度が有効であることを示した。この変数を加えた修正∑L-∑Q法で求めた年間の硝酸態窒素流出負荷量は3～5kg·ha~(-1)·yr~(-1)となり，年雨量，年流量に依存しなかった。この要因は大規模出水時に渓流水のNCO_3濃度が顕著に低下し，累加比負荷量の増加が頭打ち傾向になるためであり，NO_3~-濃度の顕著な低下は渓流近傍から遠い斜面の下層や深層の地中水（極めて低いNO_3~-濃度）が大規模出水の初期を除き流出に大きく寄与するためと考えられた。%We investigated the annual nitrate nitrogen (NO_3~--N) load for seven years, taking stormy flood events into consideration, in a forested catchment in the headwaters of the Shimanto River, Shikoku, Japan. This area frequently receives heavy rain exceeding 100 mm. The vegetation of the experimental catchment (18.7 ha), underlying sedimentary rock, is a natural forest comprising predominant conifers (Abies firma and Tsuga sieboldii), evergreen and deciduous trees. Streamwater was collected twice a month, adding streamwater of 17 flood events (total rainfall of 18-289 mm). It was revealed that NO_3~- concentration before rainfall was an effective predictor except runoff for evaluating the cumulative specific nitrate load. Based on the modified ∑L - ∑Q equation added this predictor, the annual nitrate load was evaluated 3 to 5 kg·ha~(-1)·yr~(-1) and did not significantly correlate to the annual rainfall or annual runoff. This is due to the remarkable decline in NO_3~- concentration during stormy flood events, suggesting that cumulative specific nitrate load tends to plateau. The remarkable decrease in NO_3~- concentration would be due to that subsurface water on hillslope rather than near stream, which has an extremely low NO_3~- concentration, contributes predominantly to streamwater during stormy flood events, excluding the early stage.