曾文水庫由於長年淤積影響水庫正常供水，本研究探討其集水區土壤沖蝕和土砂來源，以供後續管理應用參考。研究係以土壤沖蝕經驗式建置為主，考量水文及地文等影響因子，建構特定雨場或連續降雨下之沖蝕深度經驗式；另假設坡面尺度並使用經驗式及物理模式(WEPP)推估坡面土砂變化，並配合經驗模式與年超越機率，建構機率性土壤沖蝕經驗模式；同時採用GeoWEPP程式模擬氣候環境變化，結合土地利用、數值地形及土壤類型，由降雨逕流、土砂生產、土砂運移、土砂堆積等輸送過程，由坡面尺度推演至集水區尺度，以模擬整個集水區土砂運移量。研究主要成果如下： 1. 土壤沖蝕量：建置適用於曾文水庫集水區的土壤沖蝕估算式，用於推估單一降雨事件或多年的降雨的土壤沖蝕量，並與物理模式進行比較，結果顯示： (1). 經驗模式比物理模式 (WEPP) 相對保守，但預測結果差異不大；惟應用時經驗模式需加入標準偏差，物理模式需將坡度放大，以利保守推估。 (2). 機率性土壤沖蝕經驗模式可掌握該預測該區降雨所引致土壤沖蝕深度和年超越機率，以推估水庫入流點土砂產量。研究觀察重現期及對應沖蝕深度關係為2年0.34cm、5年0.37cm、10年0.39cm、20年0.41cm和100年0.44cm。 2. 水道輸砂量：利用該區降雨特性，針對單一降雨事件及逐年降雨推估整個區域土砂產量，以掌握河道輸砂量變化，結果顯示： (1). 以土砂預測程式(GeoWEPP)預測2011年南瑪督颱風事件及2012年天秤颱風事件所產生之泥砂產量，透過流動路徑模式預測各子集水區、溪流、邊坡，乃至邊坡植生及土壤之土砂總流失量，在流域面積與最小流域長配置下，可達到與實際值相對誤差6.7%之最佳模擬。 (2). 利用土砂收支系統，由坡面土砂流失量和土砂遞移率乘積，可計算水庫集水區蓄水範圍以上子集水區出流點的土砂流出總量。 The long year’s siltation of Tsengwen Reservoir affects the normal water supply severely. This study is aimed to evaluate soil erosions and sediments at corresponding watersheds, in which the management can be easily accessed. Regression analysis was conducted to predict the soil erosions where the hydrological and physiographic factors were considered for a specific rainfall site and/or the cases of continuous raining. Additionally with the presumed slope dimensions and the application of empirical formula and physical mode (WEPP) to predict soil changes along the slope, the probabilistic model of soil erosion can be established. Furthermore, GeoWEPP program was conducted to simulate climate change. With land reclamation, digital topographic map and soil files, the amount of sand transports at the watersheds can be predicted based on the information of rainfall and transports of soil formations. Both slope and watershed levels’ assessment can be suggested. 1. Soil erosions: (1) Empirical formulas are conservative compared to the physical mode WEPP. There difference is not very significant. When using the regression formulas, one standard deviations of plus and/or minus are suggested. When using physical model, the dimensions of slope is better to be large in order to provide suitable predictions.. (2) The probabilistic soil erosion model can be used to provide soil erosion in correspondence with the annual exceednace of rainfall. It can be used to predict the sediment yield at the reservoir inflow station. It is suggested that the relationships of the return period and the soil erosion depth is two years-0.34cm、five years 0.37cm、ten years-0.39cm、twenty years-0.41cm and one hundred years-0.44cm. 2. Sand transports at watercourse: (1) Using the sand prediction program GeoWEPP to predict the sediment yields by typhoon Nanmadol in 2011 and Typhoon Tembin in 2012, with the flowpath model which predicts each watershed, river, natural and protected slopes and total amount of soil lost, as well as proper source area and channel length, an error of 6.7% can be found in the prediction. (2) Use sand sediment balance system, the soil loss of the reservoir watershed at the inflow stations can be computed by multiplying the soil loss of the slope with the sediment delivery ratio.
