The influence of a rainfall event on the chemical changes of forest runoff water

森林流域からの流出水質に降雨イベントが影響を与えるメカニズムを解明するため, 滋賀県南部に位置する桐生水文試験地内の源頭部小流域において降雨イベント時の溶存物質濃度の経時変化を詳細に観測した。生成に地球化学的ソースの影響が大きいSiO_2濃度は流量ピーク時に必ず低下したが, 生物学的ソースの影響が大きいNO_3_-濃度は降雨イベントごとに上昇する場合と低下する場合が見られた。溶存物質負荷量の変動を考慮するとその違いは降雨イベント前の流域の水分条件, 飽和地下水帯の水位・ソースの空間分布が影響すると考えられる。地下水中で濃度が高いSiO_2は表層土壌からの流出水で希釈される。一方表層土壌内の中層から下層にかけて濃度が高いNO_3_-は流出水の通過した層の違いによって上昇と低下の2パターンが現れる。また, 降雨の規模が大きくなるに連れて表面流出成分・河道上への降雨成分の影響が大きくなることも濃度低下の原因となる。In order to clarify the mechanism of the influence of a rainfall event on the chemical changes of forest runoff water, we sampled intensively the forest runoff water and monitored in detail the temporal changes in the concentrations of both geochemically generated Si02 and biochemically generated NO3- in Kiryu Experimental Catchment. As a result, we found that Si02 concentration decreased at the peak of discharge in all three observed events, while NO3- concentration increased in one event, decreased in another event and didn't clearly change the other event. Taking the variations in the loads of dissolved substances into account, it was suggested that the differences in the variation result from the vertical distribution of elements, the soil moisture condition and groundwater level of the saturated zone in existence before the occurrences of the event. Si02 concentration, which is higher in groundwater than that in soil solution, decreases due to a dilution by the direct runoff water. On the other hand, NO3- concentration, which is higher over the range from the middle to the lower layers than that in the upper layer within the surface soil, increases after runoff water passed through the lower layer, and decreases after runoff water passed through the upper layer. And the surface flow which may occur in larger rainfall and the channel precipitation are another cause of the concentration decrease