山地流域を対象とした水循環モデルの提示と桐生流域の10年連続日・時間記録への適用

Abstract

山地流域における水循環と水収支を一般的かつ定量的に評価するために, 水循環モデル (HYCYMODEL) を提示する。本モデルでは流域は流路系と林地斜面系に区分される。流路系においては, 直接流出と流路面蒸発が, また, 林地斜面系においては, 遮断蒸発と蒸散, および直接流出と基底流出が発生する。蒸散抑制機構と乾燥時における林地斜面系の直接流出量減少機構も組み込まれている。遮断蒸発モデルのパラメーターと蒸散量の季節分布および蒸散抑制の生ずる限界流量が定められれば, 9個のパラメーターによって, 降雨記録から時々刻々の流出量が求められる。本モデルは, 滋賀県南部に位置し, 風化花崗岩より成り, ヒノキとアカマツを主林木とする, 5. 99haの桐生流域の10年連続日・時間記録に適用され, その結果, たいへん良好な推定ハイドログラフが得られた。同時に, 水収支の各成分についても年間の総量が求められた。桐生流域の年平均流出量と年平均蒸発散量は年平均降水量の, それぞれ56. 0％, 44. 3％となる。また, 流路系の直接流出量と林地斜面系の直接流出量, 基底流出量は, 全流出量のそれぞれ6. 2％, 31. 3％, 62. 5％であり, 流路面蒸発量と蒸散量, 遮断蒸発量は全蒸発散量のそれぞれ, 3. 9％, 51. 9％, 44. 2％となる。

The hydrologic cycle model (HYCYMODEL) is presented for evaluating effects of forest on hydrologic cycle and water budget generally and quantitatively. A watershed is divided into a channel system and a forested hillslope system in the HYCYMODEL. A storm flow and evaporation occur on channels, and evaporation occurs on canopy. Transpiration, storm flow and base flow occur on forested slopes. The hydrograph of stream flow can be calculated from rainfall records with only nine parameters in the HYCYMODEL. The parameters relating to canopy interception, monthly transpiration rate, critical stream flow for constrain of transpiration, are decided previously by another methods. The HYCYMODEL is applied to continuous 10 years records of the Kiryu Watershed at intervals of both a day and an hour. This watershed is located in the south of Shiga Prefecture and has 5. 99 ha in basin area, and is underlain by weathered granite and covered with Chamaecyparis obtusa and Pinus desiflora. Calculated hydrographs are in good agreement with observed values. The annual amounts of each component which consist of the elements of water budget were estimated. Stream flow Q and evaporation E are 56.5% and 44.3% of precipitation in the Kiryu Watershed, respectively. Stream flow from channel Qc*, stream flow from forested slopes Qh*, and the base flow Qb* are 6.2%, 31.3% and 62.5% of Q, respectively. Evaporation from channel Ec, transpiration Et* and evaporation from intercepted water Ei* and 3.9%, 51.9% and 44.2% of E, respectively.