山腹斜面の流出機構について (2) : 土壌水の挙動

Abstract

山腹斜面の流出現象を支配する大きな要因である斜面内の土壌水分の動きを, 試料採取による実測値と電気抵抗式の水分計による測定値の変化から考察した。試験実施場所は琵琶湖東南部の花崗岩より成る山地小流域にある, 幅5m・斜面長10mで傾斜16°の自然ライシメーターである。土壌の区分は, humus層が地表下15cmまでで, その間隙率は58. 3％, また根圏層は55cmで, 55. 3％, 非根圏層は2m以下に続き, 51. 5％である。水分計による測定は斜面の上・中・下部で, それぞれ土壌下10・50・100・150・200cmの深さで行なった。結果は次のようになる。1. 無降雨日が続けば, 土壌水分の容積含水率は, 土壌層の相違すなわち間隙率の変化にもかかわらず, 鉛直方向で, ほぼ同程度の値になる傾向が認められる。2. 重力水として移動すると考えられる最低の容積含水率は, ほぼ8 - 10％であった。3. 本斜面ライシメーターにおいて, 地表下2mまでの土壌中に, 重力水として含まれ得る最大可能水量は892mmになる。4. しかし, 測定例からは根圏層の飽和度は最大でも50 - 60％程度であり, 実可能水量は3項で述べた値より大きく下回る。5. humus層内水分の降雨終了後の減少過程は直線型よりも, 指数関数型に近い。さらに, 斜面の下方に向うに従って, その減少割合が低下するのは, 斜面方向と同方向の水分移動が存在することを意味する。6. 特に直接流出に関係する水分変化は, せいぜい地表下50cm程度まで, すなわち根圏層までである。この原因としては, それより下部の土壌層の透水性が悪いと考えるよりも, それより上部で貯留し得る水量が多く, かつ飽和する以前に, 斜面方向への流下が始まるためと考えられる。

The movement of soil water must be analized in order to understand runoff phenomena on a slope. For this purpose, the author observed the variation of soil moisture directly by the measurement of extracted soil samples and indirectly by a method of electric resistance. The experimental site is situated at No. 4 Lysimeter of Kiryu Experimental Catchment, which is 5 meters in width, 10 meters in length and 16°in gradient. The catchment is located in Otsu city, Shiga Prefecture and consists of granite. The results are as follows: 1. On consecutive clear days, it was recognized that the moisture percentage took an almost definite value on a vertical direction, in spite of the difference of soil porosity. 2. The minimum moisture percentage of gravitational water were considered to be about 8-10%. 3. In this Lysimeter, the maximum potential storage of gravitational water which was included in the soil layer up to 2 meters in depth, was 892 mm. However the actual maximum water content in the layer of intensive root zone was only half of saturation capacity. 4. The reccession curves of soil water in humus layer were expressed more appropriatly as an exponential function than as a linear function. In addition, the reccession rates decreased going down the slope. And it was assumed that the soil water in the humus layer was moving laterally along the slope. 5. The movement of soil water that was involved in the direct runoff was limitted within 0.5 meters in depth at the most, namely, within the intensive root layer. The reason was guessed that the lower part of soil layer did not have lower percolation capacity, while the upper part was able to store more water. And the runoff along the slope started before the soil saturated.