桐生試験地の緩斜面における土壌水分と地下水の変動

Abstract

比較的傾斜の緩やかな小流域における土壌水分, 地下水の挙動の実態を3次元的に把握することを目的とした観測を, 桐生試験地内において1987年5月より開始した。これまでに得られた観測結果を, 傾斜の急な斜面についての既存の研究結果と比較しながら, 緩傾斜斜面における地下水体の変化と土壌水分の経時変化およびその空間分布を検討する。研究対象としたマツ沢の面積は0. 68ha, 地質は風化花崗岩である。植生は40年生のアカマツとヒノキの混交林で, 樹冠は閉鎖している。マツ沢では傾斜が15度以下の部分が62. 8％を占め, また傾斜25度以上の急傾斜部分は11. 2％と少ない。平均土層厚は103. 2cmであり, 地表面の形状と基岩面のそれが必ずしも一致しておらず, 地表面の谷と基岩面の谷の位置が異なる。観測結果では, 斜面の谷部では緩勾配であるためほぼ恒常的に飽和地下水が存在し, その地下水体の形状が流出量に大きな影響を与えている。また谷部の厚い表層土は, 降雨にともなう鉛直方向の水移動に対して大きなバッファとして作用する。このため降雨時の谷部の地下水位上昇量は, 降雨初期の不飽和域の土壌水分量に強い影響を受ける。土壌水分の縦断方向での分布をみると, 流域の下流側から尾根部へと行くに従って土壌が乾燥している傾向がみられる。一方, 鉛直分布を見ると, 表層土が厚い谷部では降雨によってもたらされた水分が土層の下部まで伝わらず, 1m付近を境界として上部は湿潤であるのに下部は水分が伝達されず, 乾燥化が進行することがある。このようにマツ沢における土壌水の挙動には, 緩勾配, 厚い表層土といった地形特性が大きく影響している。

Distributions of top soil layer, and saturated and unsaturated soil water movement act important roles to hydrological process in hillslope. Some models that are applicable to steep slopes for the prediction of discharge and soil water movement have already been proposed. However, soil water movement in gentle slope with thick soil layer have not been fully studied up to now. This study was carried out to clarify the phenomena of variations of soil moisture and groundwater table at the source area having gentle slopes in the Kiryu experimental watershed. The study area, MATSU hollow, is 0.68 ha. The observations started from May 1987. In this paper, observed data until February 1988 are used. The items of observations were rainfall by tipping-bucket rain gage, discharge by 30 degree V-notch weir, groundwater level by water stage recorder, and soil moisture by tensiometer. The observed results are compared with those of two other hollows which are located closely within 5 km. The annual mean rainfall is 1692.8mm, and the annual rainfall in 1987 is 1333.0mm. The observed period was under comparatively dry condition. The gradients of slopes that occupy 62.8 percent to whole area are milder than 15 degree in MATSU hollow, which has more gentle hillslope than those of other hollows. 25 percent of the soil depth is deeper than 2m in MATSU hollow, and the thick soil layer is situated in the bottom of hollow. There are also some parts where are deeper than 8m. The mean soil depth is 1.03m, and it is deeper than those of the other hollows. The groundwater tables fluctuate sensitively to rainfall events in the two other hollows, but the groundwater table rises up and falls down slowly in MATSU hollow. The antecedent rainfall is strongly reflected to the rise and expansion of groundwater table. When the area was under dry condition, the rainfall event of 100mm resulted slightly in the rise of the groundwater table by the observation on July 4, 1987. After antecedent rainfall, the rainfall event of 64.5mm resulted more in the rise of it on July 20, 1987. The rainfall intensities is reflected in the shallow parts. With higher rainfall intensity, higher groundwater table rise was shown in the upper and shallow part of area on July 18, 1987. Distributions of three dimensions unsaturated zone in surface layers are identified. Usually, moisture of surface layers is quickly dry after rainfalls at the upper and shallow parts. The soil layers at the lower and deep parts tend to be wetter in accordance with the increase of total rainfall and rainfall intensities at the lower and deep parts. In the dry period after August, the soil layer below 1 m depth was not easily wetted. When the unsaturated zone of surface layer is wetter, areas having groundwater extend and the discharge increases. Thick soil layer in mild slope acts greatly as bufferzone to the soil water movement and the rise of groundwater table.