琵琶湖西北山地における積雪水量の高度分布

Abstract

琵琶湖西北山地に属する梁ヶ谷において, 1980 - 1981年, 1981 - 1982年に積雪期間を通して積雪水量調査が実施され, 積雪水量の高度分布の時間変化が示された。山地の積雪水量が最大になる時期は, 山麓よりも遅れ, 3月上旬であった。また, 山麓での積雪水量が減少し始める時期は, 融雪が盛んになる時期と一致し, 日平均気温が0℃以上になる時期であることがわかった。ある期間内における積雪水量の変化量を高度別に見ると, 測定地点, 測定期聞により, かなりのばらつきがあるものの, 全体的に見れば, 降雪量は高度に伴って増加し, 融雪量は逆に減少する傾向が認められた。地域内の積雪水量は, 高度に伴って増加し, その高度分布を直線で近似した時, 直線の勾配は時間の経過とともに増加した。琵琶湖西北, 北部山地, 石川県宝達川において, 山麓部の積雪水量が最大となる2月 - 3月上旬の積雪水量の高度分布直線の勾配は, 井上ら (1979) が示した, 海岸からの距離との関係に近い値をとることがわかった。梁ヶ谷と琵琶湖の間に南北に連なる比良山地の積雪深データより, 積雪の全層平均密度と経過日数の関係を用いて積雪水量を推定し, 積雪水量の高度分布直線を求めた結果, 直線の勾配は, 梁ヶ谷よりも大きく, 比良山地においても, 海岸からの距離が長くなるにつれて高度分布直線の勾配が小さくなることがわかった。

For the purpose of investigating the distribution of the water equivalent of snow cover by the altitude in nothwestern mountains of Lake Biwa, snow surveys were carried out in the three areas, Yanagatani eatchment, the area around Hacchodaira, Asiu Experimental Forest of Kyoto university. The depths and the water equivalents of snow cover were measured by snow sampler in the snow season from December 1980 to March 1981 and December 1981 to March 1982. The results on the changes in the water equivalents of snow cover, the additional water equivalent of snowmelt, and the daily mean temperature across time during the snow season are shown in Fig.3. As seen in this figure, the daily mean temperature on the foot of the mountain rose up to 0℃ on the latter half of February, and the amount of snowmelt became larger, so that the water equivalent of snow cover began to decrease. But the water equivalent of snow cover at higher altitude increased till the first half of March. The water equivalents of snow cover were larger at the higher altitudes in each area and such relationships between the water equivalent of snow cover (Hw:mm) and the altitude(H:m) can be represented by linear regression lines; Hw=aH+b throughout the accumulation and the ablation season. The value of 'a' increased across time. When the water equivalent at the foot of the mountain became maximum, the value of 'a' showed the relation by J.Inoue and C.Nakajima (1979) which the value of 'a' decreases with the distance from the coast. The water equivalents of snow cover in Hira mountainous region near the Yanagatani catchment were estimated by data of the snow depths using the relation of the mean density of snow cover (D:g/cm) and the number of days from January 1(N); D=0.07+0.004N, and the value of 'a' in Hira mountainous region were estimated. The results of these estimations show that the values of 'a' in Hira mountainous region were larger than the values in the Yanagatani catchment, and decreased with the distance from the coast.