Runoff Analysis in the Central Hokkaido Considering the Snow Coverage and Melting

東京大学北海道演習林では，1981年の台風を契機に，立木の被害面積率を異にする4試験流域が開設され，冬期間（12月～3月）を除いて降水量・流量の観測が行われている。本研究では1985年，1986年の2年分の水文資料を対象に流出解析を行った結果を報告する。日単位流出解析では，熱収支に基づく理論より積雪・融雪過程のモデル化を行い，日降水量・日平均気温・全天日射量という，気象台からの入手の容易な気象要素を用いた簡単な融雪量式を提案した。この式と，菅原のタンクモデルと組み合わせて日単位ハイドログラフを求めると，融雪のピークの発生時をよく再現することができた。また，短期流出では2つの高水例が紹介され，直接流出と地下水流出のピークが分離して発生するという珍しい形状を持つことが認められた。これは熔結凝灰岩という地質のもつ特異性によるものと判断された。さらに本州の風化花崗岩地帯の流出との比較から，熔結凝灰岩に覆われ豊かな林相を持つ当試験流域は極めて高い流出一様性に恵まれていること，また4試験流域間での比較から，風害面積率の小さい流域ほど流出の平準化が進んでいるという結果を得た。In the University Forest in Hokkaido, four experimental basins were set up for the observation of rainfall and runoff. The observation was not conducted during the winter season. The basins were damaged by a typhoon in 1981, which resulted in the different areal ratio of damaged forests. In this investigation, the results of runoff analysis on the data in 1985 and 1986 are reported. In the daily runoff analysis, the model of snow coverage and melting was induced from the theory of heat balance and by applying the model, daily amounts of snow melting can be determined in terms of daily precipitation, mean daily atmospheric temperature and global solar radiation which was obtained from the record of weather station. This model worked well with tank model by Sugawara for runoff calculation, resulting in good fit of the time when peak discharge occurred due to snow melting. In short period analysis, two examples of hydrographs are introduced and the fact was found that the shape has two separate peaks of direct runoff and baseflow which were thought to be caused by the newly formed volcanic geology. In comparison with the runoff characteristics from that of the weathered granite in Honshu, it was proved that these experimental basins with the cover of welded tuff and the abundant forest are endowed with very high uniformity of runoff. Meanwhile, comparing the runoff characteristics of these four basins, it was found that the less the damage was, the higher the uniformity of runoff became