The Universal Soil Loss Equation (USLE) is presently one of the most widely used models for evaluating soil erosion. The rainfall amount is often used to estimate the R factor for Universal Soil Loss Equation (USLE) modeling. Daily, monthly, and annual rainfall data are commonly available. The purpose of the present study is to establish three models for estimating rainfall erosivity based on daily, monthly, and annual precipitation, respectively, and to identify the most suitable model in the Ailiao River watershed located in southern Taiwan. The data set consists of 2266 storm events, monitored by 6 rainfall stations there. The results show that the 6 stations indicate strong positive relationship (r2 > 0.75) between annual rainfall erosivity and annual rainfall with a 99% confidence level, which means the simplified estimation methods based on annual precipitation are useful for predicting long-term annual rainfall erosivity in most of locations in the Ailiao River watershed. The results also show that annual and monthly precipitation correlate with erosivity better than does daily precipitation based on the analysis results of root mean square error (RMSE) and mean absolute percentage error (MAPE) in the Ailiao River watershed.降雨沖蝕指數是通用土壤流失公式 (universal soil loss equation, USLE) 重要的參數之一，由於計算降雨沖蝕指數需要30 分鐘或更短量測間距的雨量資料，但大部分地區只有日、月或年雨量資料。因此利用何種降雨資料推估年平均降雨沖蝕指數較為接近30 分鐘雨量資料計算的年平均降雨沖蝕指數，是本研究的重點。本研究以隘寮溪集水區為研究區域，蒐集區域內6 個雨量站10 年 (2002-2011 年)10 分鐘等間隔的降雨資料，計算年平均降雨沖蝕指數 (Ryc)，另以日、月及年降雨資料，建立三種降雨量與降雨沖蝕指數關係式，利用RMSE、MAPE 及Bias 等誤差分析方法，評估三種降雨沖蝕指數推估關係式之適用性。研究結果顯示，利用年降雨資料推估年降雨沖蝕指數的適用性較好。本研究結果可提供缺乏短量測間距雨量資料地區年降雨沖蝕指數推估之參考依據
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