人工林の針広混交林化による森林の洪水低減機能向上に関する研究

大流域における人工林の針広混交林化により期待される遮断蒸発率と地表面粗度の増強が森林の洪水低減機能にどれほど影響を与えるかを検討した．針広混交林を形成する一級河川那賀川流域の橋本林業地において水文観測を行うことで得られた遮断蒸発率と地表面粗度を長安口ダム流域に適用し，洪水シミュレーションを行った．ピーク流量やその発生時間等で洪水低減機能の向上具合を評価した．その結果，現地の将来予測100年確率雨量に対する洪水ピーク流量は，地表面粗度の増強により6.1％，遮断蒸発の増強によって3.6％の減少が推定された．

In this study, we examined the effects of the enhanced interception evaporation rate and surface roughness expected from changing needleleaf forest to needleleaf-broadleaf on flood mitigation functions. By conducting hydrological observations in the Hashimoto forestry area, which is a mixed forest of needle and broadleaf, the obtained interception evaporation rate and surface roughness were adapted to Nagayasuguchi dam and runoff simulation was conducted. We used a tank model for the spill simulation, which was evaluated in terms of peak flow rate and generation time. As a result, increasing the surface roughness reduced the peak flow rate by 6.1% relative to the 100 year Probabilistic rainfall, and increasing the interception evaporation rate reduced it by 3.6%. Therefore It was confirmed that this is an important function in flood reduction.

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