Separation of Soil Water Runoff and Bedrock Water Runoff from River Hydrographs using the Storage Type Model

本論文では,森林の水源涵養機能を定量的に評価するために,森林が関与する土壌と関与しない基岩のそれぞれが,雨水流出過程に及ぼす調節機能を分離することを目的として,貯留関数型モデルによる流出成分の分離がおこなわれた.ここで扱われた流出成分は,従来の概念的な区分ではなく,流出の生起場としての土壌および基岩から流出する成分と定義され,それぞれ土壌流出成分,基岩流出成分と名付けられた.分離方法として,土壌と基岩をそれぞれひとつのタンクとして,その流出機構および土壌から基岩への浸透機構の物理性を反映した貯留関数型モデルが開発された.モデル化にあたって,基岩流出成分の運動式は線形関数で与えられ,ハイドログラフの逓減率から運動定数が定められた.また,土壌流出成分および土壌から基岩への浸透成分の運動式の係数については未知パラメーターとされた.九州大学宮崎演習林理水試験地の雨量と流量データについてモデルを適用した結果,基岩流出成分は全流出量の10%弱であり,その時間変化はきわめて緩慢であるという結果が得られた.また,得られたパラメーターの物理的意義が検討され,土壌流出成分の運動係数P_1が土壌の孔隙組成を,また,土壌から基岩への浸透成分の運動係数P_2が地質･地形的要因を反映するパラメーターであることが示唆された.流出成分の分離結果の妥当性については,本モデルの改良も含めて,他の手法との関連等,多面的な検討が必要であることが示された.In this paper, in order to evaluate the function of forest water yield, two runoff components are separated from a river hydrograph using the new model. These two components are a new concept. One is soil water runoff, which passes through soil into rivers. The other is bedrock water runoff, which passes through bedrock. The new model is the soil-bedrock storage model, which is composed of three tanks, representing channels, soil and bedrock respectively. Runoff from each tank and infiltration from the soil tank to the bedrock tank are expressed as formulas by storage functions that reflecting the physical mechanism of runoff. For modelling, the following assumptions are made. a) Bedrock is an impermeable layer and bedrock water moves through fissures. b) Bedrock water is free water. c) The infiltration mechanism from soil to bedrock is expressed as the change from capillary water to free water. Formulas are as follows, 1) The equation of motion in the bedrock tank is: Q_R=K_R \times S_R 2) In the channel tank and the soil tank respectively, equations are: Q_C=K_C \times S_{C^{5/3}} Q_S=K_S \times S_{S^{P_2}} 3) The equation of infiltration from the soil tank to the bedrock tank is: I_{SR}=K_I \times S_{S^{P_2}} In the above equations, K_R is determined by the recession rate in a water shortage period. The 5 other coefficients are parameters of this model. The soil-bedrock storage model is applied to 1990 and 1991 annual data from the Oyabu Experimental Basin taken at both daily and hourly intervals. This experimental basin, which is a part of the Miyazaki Forests of Kyushu University, is located in the north of Miyazaki Prefecture and has a 38 ha basin area at an altitude of 1000-1300m. It is on a bed of sandstone and shale of the Shimanto Supergroup, and covered with deciduous broad-leaved forest. In the results of the application, calculated hydrographs agree well with observed values. The annual amount of bedrock runoff Q_R is under 10% of total runoff, and its hydrograph shows little annual variation. Physical significance of parameters P_1 and P_2 are discussed. It is suggested that P_1 reflects the composition of soil pore and P_2 reflects geological and geomorphological factors of the catchment