逕流曲線法於森林集水區之改善研究

Abstract

台灣森林覆蓋率約60%，許多主要河川與水庫上游集水區都是森林覆蓋，隨著環境保育觀念抬頭，森林的保水能力也備受重視，加上近年來台灣頻受颱風侵襲，山坡地和河川上游集水區發生土石流及水患的情形屢見不鮮，人們開始關心森林對水災、土石流的影響，植樹造林的成效又是如何。目前工程上逕流估算常用的是美國水土保持局的逕流曲線法，它是一個簡單且方便使用的方法，但應用在森林集水區時並不能有效地將植被的影響反應出來，而其他適合模擬森林集水區的水文模式又過於複雜，不利於工程實務應用。 本文的研究目的是提出改良版的逕流曲線法，希望能反映出植被對水文的影響又不至於過度複雜。因此本研究利用現有的水文模式「降水徑流建模系統(PRMS)」中的數個模塊並參考其運算方法，以逕流曲線法為主體架構做修改，重新定義參數中的各個細項，取代以逕流曲線值推估參數的方式。選擇南勢溪與桶後溪集水區做PRMS的參數校正與模式驗證，再將改良版的逕流曲線法與原版、模式結果分別做長期與短期的比較與討論。根據研究的結果，改良版的逕流曲線法提高了土壤的入滲能力及初始損失，能改善高估逕流量的情形，但因為公式先天的架構使得改善效果有限。除此之外，結果也推論出逕流曲線法中的超滲降雨並非只有地表逕流，其中還包含了部分的淺層地下水。

The forest coverage rate in Taiwan is about 60%. Many major rivers and upstream catchment of reservoirs are covered by forests. With the rise of the concept of environmental protection, the ability of forests to conserve water and soil resources has also received great attention. In addition, Taiwan has been frequently attacked by typhoons in recent years. It is not uncommon for landslides and floods to occur in mountain and upstream of rivers. People have begun to care about the impact of forests on floods and landslides, and the effectiveness of afforestation. At present, the commonly used method for runoff estimation in engineering is the Curve Number Method of the U.S. Soil Conservation Service. It is a simple and easy-to-use method, but it cannot effectively reflect the influence of vegetation when applied to forest catchment. However, other hydrological models suitable for simulating forest catchments are too complex and are not conducive to engineering applications. The purpose of this study is to propose an improved version of the Curve Number Method, hoping to reflect the impact of vegetation on hydrology without being overly complicated. Therefore, this study uses several modules in the existing hydrological model Precipitation Runoff Modeling System (PRMS) and refers to its calculation method and uses the Curve Number Method as the main structure to modify, redefine each item in the parameters, and replace the method of characterize the runoff properties for a soil and ground cover by the Curve Number. The Nanshi creek and Tonghou creek catchments were selected for parameter calibration and model verification of PRMS, and then the long-term and short-term results of the modified Curve Number Method were compared and discussed with the original and model results. According to the results of the study, the modified Curve Number Method improves the soil infiltration capacity and initial loss, and can improve the situation of overestimating runoff, but the effect is limited due to the innate structure of the formula. In addition, the results also deduce that the excess rainfall in the Curve Number Method is not only the surface runoff, but also includes part of the shallow groundwater.