Runoff and Water Quality Characteristics of Small Streams in Mountainous Drainage Basins in a Water Shortage Period

降雨流出機構における基岩層の寄与を定量的に明らかにするために，山地源流域の渓流において，基岩流出の寄与が大きいと考えられる渇水期の流量と水質についての調査を行った．調査地点は，粕屋演習林新建川上流部の11地点（流域面積6.23～80.6ha）で，本地域は古生層新建系角閃岩を基盤とする急峻な山岳地で，地質，地形，植生，降水量等において，調査された流域間の差はほとんど見られない．観測された流量を流域面積で除して比流量を求めたところ，最も大きい流域は1.15mm／day，最も小さい流域は0．32mm／dayで，11地点の変動係数は29％と大きなばらつきがみられた．この原因として，基岩中の地下水が流域界を越えて流れていることが考えられた．11地点の区間流域間の地下水収支計算から，このような基岩中の地下水移動量と，移動のパターンが流域毎に示された．各調査地点毎の水温および溶存成分濃度の平均値が示された．溶存成分の起源として，土壌や基岩中における鉱物の風化反応の他に，大気汚染物質や海水が風送されたもの等が考えられた．溶存成分濃度の特徴として，比流量の大きな流域ではSiO_2濃度が，比流量の小さな流域では陽イオン，陰イオンの濃度が比較的高い傾向が見られた．このことは，流域間の地下水移動によって，渓流水中に占める基岩からの流出成分と土壌からの流出成分の割合が，流域によって異なることを裏付けるものである．During a water shortage period, the discharge and the contents of dissolved components for 11 neighboring small streams were measured in the area of mountainous drainage basins (6.23-80.6 ha), upstream of the Tatara River in Northern Kyushu. This area, underlaid by Paleozoic Era formations, is covered with artificial forest (Cryptomeria japonica and Chamaecyparis obtusa) and evergreen broad-leaved forest. Despite the similarity of watershed characteristics, there were remarkable differences in the specific discharges of the 11 drainage basins, because groundwater in the bedrock flows across the watershed, as shown in Fig. 4. The flow rates of groundwater movement across the watershed could be evaluated by the groundwater balance among the 11 drainage basins. The dissolved components are supplied from two sources : chemically decomposed minerals both in the soil and the bedrock which are dissolved into water, and wet and dry deposits containing sea salts and air pollutants. There were two tendencies in the relationship between the specific discharge and the contents of dissolved components : among the 11 stream waters, a higher specific discharge stream water contains a larger amount of SiO_2, and smaller amounts of Na^+, K^+, Mg^{2+}, Ca^{2+}, Cl^-, {NO_3}^-, and {SO_4}^{2-}. This suggests that there are two runoff components with different contents of solutes : soil water with short residerlce time and groundwater in the bedrock with long residence time, and the mixing ratios of these two components are different for their respective drainage basins due to the groundwater movement across the watershed