芦生演習林内の高原部における由良川最源流部の2流域の流出特性

Abstract

京都大学芦生演習林の気象・水文を把握するために, 演習林全体を含むおよそ4, 700haの由良川源流域において調査用施設を配置し, 観測を開始した。このうち, 由良川最源流に設定された上谷・下谷試験流域は芦生演習林の中では高原部に位置し, 標高は625 - 935mで南北に隣接するほぼ東向きの2流域である。流域面積はそれぞれ490ha, 303ha, 地質は丹波帯の中生層で頁岩を主体として砂岩, チャートを含んでいる。植生は上谷がスギと広葉樹 (ブナ, ミズナラ) が混交した天然林であるのに対して下谷は皆伐後の二次林とスギの人工林であり, 芦生の比較的標高の高い地域を代表する植生の面から言えば好対照の2流域である。今回はこの流域において60年前に行われた水文観測の結果をレビューし, 近年の観測結果と比較して流出の傾向が上谷と下谷で逆の傾向を示していることを指摘した。また, 上谷量水堰で観測された無雪期のハイドログラフにおいて水循環モデルの流域定数 (パラメーター) を決定し, 他の流域との比較を試みた。

Hydrological observations have been made in Kamitani Experimental Watershed and Shimotani Experimental Watershed situated on the source area of Yura River in Kyoto Prefecture, for understanding the meteorological and hydrological conditions of the University Forest in Ashiu. Kamitani and Shimotani Watershed is underlain by Mesozoic and Paleozoic Era formations. Kamitani is covered with natural forests of Cryptomeria japonica, Fagus crenata and Quercus mongolica var, grosseserrata. Shimotani is covered with artificial forests of Cryptomeria japonica and secondary forests dominated Cryptomeria japonica, Quercus mongolica var. grosseserrata and Betula grossa and Acer L.. The basin area of Kamitani is 490 (ha) and that of Shimotani is 303 (ha). The altitude of these watersheds range 625m-935m. These forests represent the upper parts of Ashiu. The results are as follows, 1) The observation data in these watersheds of 1932 are reviewed. Comparing old data with new data, it is clear that another characteristics appear in the discharges of Kamitani and Shimotani. Namely the discharge of Kamitani in normal water level was smaller than that of Shimotani in 1932, but there is the reversed tendency between Kamitani and Shimotani in 1989. 2) The hourly data of rainfall and discharge during non-snow seasons from May to October of 1992 and 1993 were used for determining the runoff constants on HYCYMODEL which is a synthetic runoff model. Comparing with the parameters already determined by HYCYMODEL in other regions, the characteristics of streamflow in Kamitani catchment are examined.