近二十年來台灣地區進行水資源規劃採用缺水指數做為設計準則，缺水指數為將年缺水率以平方倍放大後再加總取平均值，目的在凸顯缺水的嚴重性。以缺水指數 1 而言，若缺水率每年均相同，則在模擬期間內的年平均缺水率為 10 %。由於各地區的水文隨機過程不盡相同，各年缺水事件之缺水量與延時亦不相同，宜以不同的缺水指數做為各水系水庫容量的設計標準。
本研究目的在探討面對水文的不確定過程下，家用及公共給水水庫設計容量之於用水需求，所面對之缺水率、發生機率及缺水延時，提出以用水者之「缺水可忍受限度」做為選定水庫容量設計標準之標準，並探討其與年缺水率、缺水指數、缺水百分率日數等評鑑指標之相關性，期能客觀地量化缺水衝擊，以供水資源規劃決策參考之用。
本研究選取台灣地區北中南三條溪流之水文歷程進行模擬分析，先依比例放大或縮小各水文歷程之歷史流量序列，使新流量序列的年平均流量均調整為相同。再探討供應相同需求水量且達到相同可接受的缺水程度下所需的水庫庫容，或相同庫容下的缺水特性。
在不同水文過程下，欲達到相同可接受的缺水程度，則各區所設計之水庫容量及其反應的缺水指數值亦不相同，應依各區之水文條件、社會需求及缺水可忍受程度之高低決定其缺水指數值。由於缺水指數、年缺水率、缺水百分率日數等指標均具相同一致之增減趨勢，故目前採用之缺水指數其內涵亦可以缺水百分率日數、年缺水率及發生機率等缺水特性來加以闡釋。本研究利用期望缺水百分率日數與期望缺水率，藉由迴歸分析明確找出缺水指數、年缺水率與缺水百分率日數等指標彼此之關係，進一步可由用水者之缺水可忍受限度下之期望缺水百分率日數或期望缺水率，建議適切的缺水指數值做為水庫規劃設計之標準。

The shortage index, which proposed by US Army Corps of Engineers, was adopted as a major criteria for designing the capacity of a reservoir in the last two decades in Taiwan. Shortage index was designed to demonstrate the severity of deficit by averaging the squares of annual shortage rates of all years. If the annual shortage rate is 10% in every year, the shortage index will equal to 1. However, the amount and duration of shortages are not uniform in different years due to stochastic hydrologic process. Therefore, by assigning a specific value of shortage index as design criteria for all reservoirs in different water resources systems would result different shortages.
This study investigated the characteristics of shortages, which included rates, durations and occurrence probabilities, from different design scenarios of required reservoir capacities and project demands. Three streams located in the northern, central and southern Taiwan respectively were chosen as case study to demonstrate the influences of different hydrologic processes. The historical flow series of the three streams were proportionally standardized first to make the annual average flow to be the same. This study also suggested the concept of “tolerable shortages” as design criteria of reservoir capacity. Required reservoir capacity of each stream was computed to supply the project demand by satisfying the tolerable shortages.
Appropriate tolerable shortage for designing the capacity of a reservoir can be determined based on the shortage impact to all purposes. Each stream should adopt its own shortage index to reach the corresponded tolerable shortage. This study proved that the shortage index, expected annual shortage rate and expected deficit percent-day had the same tendency to increase. The relationship is nearly one to one. This study developed the relationships between the shortage index, expected annual shortage rate and expected deficit percent-day with regression analysis. Therefore, one may choose either the expected deficit percent-day or expected annual shortage rate to demonstrate the shortage index.
Further research may be focused on developing the relationship between tolerable shortage and expected annual shortage rate or expected deficit percent-day. An appropriate shortage index can be determined for designing reservoir capacity from the expected annual shortage rate or expected deficit percent-day.

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