景美溪集水區流量及雨量頻率分析

Abstract

頻率分析常被應用於防災規劃、水工設計及災害風險評估中，透過適當的機率分佈可推估最大降雨量或洪峰流量，有助於人們瞭解集水區內的水文特性，提升集水區經營管理之效率。本研究以景美溪集水區作為案例，從1970年至2009年間，針對降雨與逕流，透過年最大日事件、豐、枯季節量與全年總量，比較極端值第一型分佈、對數常態分佈、皮爾森第三型分佈及對數皮爾森第三型分佈之適切性，並據此探討近期氣候變遷之趨勢。結果顯示，研究區內年最大日流量頻率分析以極端值第一型分佈最為適切，後期(1987-2009年)的年最大日流量推估值均比前期(1970-1987年)高出許多，變動率介於9.45%至54.59%間，後期的年最大日流量平均值也比前期高出17.77%，變異係數更高出32.45%。後期的年一日最大雨量推估值均比前期高，變動率介於12.87%至56.34%間，後期的年一日最大雨量平均值比前期高出16.74%，變異係數則高出65.19%。在季節變動部份，枯水期流量及雨量下降，導致豐、枯季間差距亦更趨明顯。豐水期流量推估值較前期增加，變動率介於9.12%至12.62%間，後期豐水期流量平均值上升8.34%，變異係數高出35.50%。枯水期流量推估下降，變動率介於8.55%至14.01%，平均值減少13.42%，變異係數增加10.52%。流量不僅在年最大事件中出現增強的趨勢，豐、枯水期差距亦有趨於極端之現象。整體而言，景美溪集水區的年最大日流量及年一日最大雨量均逐漸增高，變動率大，頻度亦增強，顯示出未來的極端水文事件將具有上升之趨勢。 景美溪為台北都會區外圍重要河川，隨著都市的擴張，都市聚落多沿著河谷地向上游擴張，多使用河道兩側土地與山坡地。在全球氣候變遷下，台灣氣候型態受到明顯轉變，地表逕流亦受到相當程度的影響，因此，防洪減災成為景美溪流域經營的重要目標才能因應氣候變遷下流量及雨量特性的轉變。The frequency analysis is often applied in disaster prevention planning, hydraulic works design, and hazard risk analysis. It is possible to estimate the maximum volume of intensive rainfall and peak discharge based on a suitable probability distribution. The frequency analysis is also helpful for people to understand the hydrological characteristics in a watershed, and to enhance the efficiency of watershed management. The Chingmei stream watershed is selected as a study area in this research. The extreme hydrological events are analyzed, including the annual maximum daily discharge and annual one-day maximum rainfall during the period from 1970 to 2009. Four probability distributions are compared, including extreme-value type I distribution, logarithmic normal distribution, Pearson type III distribution, and logarithmic Pearson type III distribution. The estimated annual maximum daily discharge in late stage (1987-2009) is higher than that in early stage (1970-1986). Their variation is from 9.45% to 54.59%. The average of annual maximum daily discharges in late stage is bigger surpassing up to 17.77%, and the standard deviation is also increasing up to 32.45%. The estimated one-day maximum rainfall in late stage is higher than that in early stage. Their variation is from 12.87% up to 56.34%. The average of one-day maximum rainfall in late stage is bigger surpassing up to 16.74%, and the standard deviation is also exceeding up to 65.19%.The other is that the amounts of runoff and rainfall become larger during the wet season and lower during the dry seasons. Discharge in the wet season variation is from 9.12% to 12.62%. The average of wet season discharge in late stage is bigger surpassing up to 8.34%, and the standard deviation is also increasing up to 35.50%. Besides, the discharge variation is from -8.55% to -14.01% in dry season. The average of discharge in the dry season in late stage decreases to 13.42%, and the standard deviation is also up to 10.52%.These results present that extreme events increase interannually, including floods and drought. In conclusion, there is an increasing trend for the annual maximum daily discharge and annual one-day maximum rainfall in the Chingmei stream watershed. The variation is enhancing and the frequency is strengthening. It reveals that the occurrence probability of extreme hydrological events is rising in the further. The Chingmei stream is an important river in the suburbs of Taipei city. As the settlement extended along the valley, flood plains were occupied and communities were close to channel. Weather pattern and surface runoff in Taiwan have been considerably affected by the change of global climate. Therefore, the integrated watershed management should focus on land-use planning in Changed steam watershed. This will help us response to the change of runoff and rainfall characteristics in change stream.