花崗岩山地の2つの小流域について : 地形・植生・水収支・流出減衰曲線からみた特性

Abstract

琵琶湖東南部の2つの山地小流域について地形・植生・水収支・減衰曲線の面から検討を加えて, 次の結果を得た。桐生・川向両試験流域は同じ粗粒花醐岩から成り, 距離もたかだか数km以内に近接し, 外部的な環境条件はほとんど変らないと思われるが, 地形的にはかなりの差異を示す。すなわち形態的には, 桐生は老年期, 川向は満壮年期の特徴を示している。また, この地形的な差異が植生や水循環に大きな影響を与えていると推定される。桐生流域では, 森林によってほぼ閉鎖されており, A_o層も全面にみられ, かつ地下水の貯留機能にすぐれているが, 川向では森林の生育が悪く, A_o層も発達していない, かつ地下水の貯留機能も劣っている。

We continue to study the interaction between hydrologic cycle and forest, in the two mountainous basins, namely, Kiryu and Kawamukai catchments, where located on the south-east of Lake Biwa. The former is 5.99 ha. in area and the latter is 2.66 ha.. Air temperature, precipitation and run-off are measured at both catchments. The observation was started in 1968 at the former and in 1961 at the latter. In this report, we intend to clarify the characteristics of atmospheric phenomena, vegetation, topography, water balance and run-off depletion curves, in compariosn between Kiryu and Kawamukai catchments. Here we will give an outline of results; 1. Kiryu and Kawamukai catchments are the tributary of the River Kusatsu and the River Kisshoji respectively, and both are consisted of weathered granite. The terrain at both catchments is about 180-260 meter in altitude. Moreover, the distance between both catchments is only several kilometers. 2. The annual precipitation and the monthly distribution of the precipitation (Apr. 1968-Mar. 1971) are nearly equal at both catchments. The air temperature at the former faced to the north side is slightly lower than at the latter faced to the south-west side. 3. Formerly, the soil conservation work was done to prevent erosion, and the plants such as Pinus and Alnus were afforested at both catchments. Now, at the former, the surface coverage of the plants such as Chamaecyparis obutsa and Pinus densiflora is comparatively satisfactory for the most part. At the latter, however, the denudation or the thinning tendency of the stand is observed as a whole. 4. In regards to topography, the former reveals a low slope angle on an average and a mild change in a longitudinal profile and is small in drainage density, as compared with the latter. 5. The tendency of the observed groundwater storage increaces from winter to spring and decreases from spring to autumm. Neverthless, it seems that the soil moisture in the summer season at the latter is less than at the former. 6. From the analysis of run-off depletion curves, it seems that the former is better in capacity of groundwater storage than the latter. The matter that will be conjectured from above results is as follows; The denudation or the thinning tendency of the stand at Kawamukai catchment seems to be mostly caused by shortage of soil moisture. As the outside condition of the circumstance is hardly different between both catchments, in disregard of exposure difference of two catchments, the remarkable difference of the catchment characteristics may be caused by topography. But the reason why that difference of topography occured is not clear.