琵琶湖西北山地における融雪量観測

Abstract

琵琶湖の西北山地は, 標高はせいぜい900 - 1100mと低いが, 毎年1m以上の積雪がみられる。降雪は12月から3月にかけてであるが, 日平均気温が氷点下となるのはわずか2月ごろの1ヶ月程度で, 全般的には暖かく, 暖候性積雪地域である。本地域における積雪から融雪までの変換過程について, 1981年12月から1982年3月まで, 京大芦生演習林の事務所地点と滋賀県朽木村小川の民家の庭に設置した1×1mの集水板と地中に設置した一転倒100ccの転倒マス型流量計の観測結果を用いて, 日平均気温と日融雪量の関係を調べた。結果として日融雪量S (mm/日) は S=0. 005 (T+5)_3. 6 〔T : 日平均気温 (℃)〕 で推定できることが判明した。なお, この関係は, 同時期に小川で観測された日射量, 風速, 気温の結果からみれば, 融雪が短波放射熱と乱流伝達熱などの総合的な熱授受のもとに発生していると解釈できる。

The north-eastern mountain range of Lake Biwa is snowpacked more than 1m every year, though its altitude is only 900-1100m. Snow falls from the middle of December to the begining of March. However, it is only a month of February that the average daily temperature falls below 0℃. So, it is called as snowpack region under warmer climate. For investigating the process of transformation from snowfall to snowmelt, the observations were carried out at two points from December 1981 to March 1982. The first is Asyu where the office of experimental forest of Kyoto University is standing. The second is Kogawa, Shiga Prefecture. A principal apparatus for observation is consisted of a shallow box tray of 1m×1m gathering snowmelt water and a tipping bucket of 100cc per a turn, with heater preventing freeze, under the ground. Furthermore, temperature was observed at both points and a solar radiometer, thermometer with dry and an anemometer are attached to a steel pole near snowmelt apparatus at Kogawa. Consequently, it is clarified that daily snowmelt, S (mm/day) in this region can be calculated by the following equation using average daily temperature, T(℃). S=0.005(T+5)3.6 This relation is able to explain physically by the heat balance, or the short wave radiation and the sensible heat transferred by turbulence.