ヒノキ林における樹体貯熱量の推定

Abstract

滋賀県桐生水文試験地において, ヒノキ林の樹体貯熱量の推定を行った.まず, 樹幹温度の実測を行い, 熱伝導方程式に表面温度を境界条件として与えて, 内部温度を解析し, 熱拡散係数の最適化を行った.解析結果は, 樹幹内温度をほぼ良好に再現したので, 樹幹表面における貯熱量を計算し, これを群落全体に拡張して, ヒノキ林における植物体の貯熱量を求めた.貯熱量の日較差は最大で約25W/m[2], 純放射量に対する割合は約5%であった.また, 群落貯熱量(顕熱・潜熱・土壌)全体の純放射量に対する割合は約12%であった.さらに, 気温から植物体貯熱量を推定する2つのパラメータ化手法として, 樹体表面温度の代わりに測定しやすい林内気温を熱伝導方程式に与えて貯熱量を推定する方法, 樹冠上の気温の時間変動から簡易に推定する方法を提示した.

Biomass heat storage was estimated in a Chamaecyparis obtusa forest in Kiryu Experimental Watershed, Shiga Prefecture, Japan. An application of heat conduction equation to a cylinder agreed with the inner temperature distribution of a tree stem, and the thermal diffusivity was successfully optimized. The result was scaled from the measured tree stem up to biomass heat storage of the forest using tree dimension data. Daily fluctuation range of the heat storage was approximately 25W/m[2], corresponding to 5% of the net radiation, and the total storage including biomass, sensible, latent and soil heat storage terms was equivalent to its 12% portion. Two parameterizations for biomass heat storage were presented based on air temperature records : one method was to estimate the storage using in-forest air temperature instead of stem surface temperature as the surface boundary layer of heat conductance equation, and the other was to simply calculate it from the temporal gradient of air temperature at the reference height above canopy.