乾旱是自然且會重複發生的現象，其會對人類社會、經濟產生深遠的影響。評估乾旱風險除乾旱事件本身的發生頻率與嚴重性外，與各地區的社經因子與抵抗乾旱的能力亦息息相關，因此乾旱風險(risk)可視為危險度(hazard)、暴露度(exposure)及易損度(vulnerability)的函數。
本研究以供水與需水間的關係探討台灣地區的乾旱風險，首先以連續三個月的降雨資料為基礎並利用標準降雨指數(standardized precipitation index，SPI)定義乾旱的發生，而以各地區所發生乾旱的等級與其發生頻率來定義乾旱危險度指數(drought hazard index)；乾旱暴露度指數(drought exposure index)則依據各地區生活、農業及工業用水量的多寡來評估，乾旱易損度指數(drought vulnerability index)則以水庫供應各地區各標的用水的比例來評估各區域抵抗乾旱之能力。三項乾旱指數數值皆介於0與1之間，本文以此三乾旱指數的乘積定義乾旱風險指數(drought risk index)，其代表一地區之用水狀況可能遭受乾旱所導致缺水的影響程度。
本文應用此法於評估台灣地區15個區域之乾旱風險，研究結果顯示在現況(2008年)用水時，彰化區及屏東區為乾旱風險較大的區域，而新竹區、南投區及花蓮區則屬於乾旱風險較小的地區；未來(2021年)用水狀況之乾旱風險分析，與2008年結果類似，但台北及台東區之乾旱風險在未來的用水情境均略為降低，而桃園、新竹及宜蘭三區之乾旱風險則均增加。

Drought is a natural and recurrent phenomenon, which has far-reaching impacts on human society and economics. Evaluation of drought risk depends not only on the frequency and severity of droughts, but also on the socio-economic factors and resistance to droughts. Therefore, drought risk can be considered as a function of hazard, exposure, and vulnerability for a specific region.
In this study, water supply and demand based drought risk analysis is implemented in Taiwan. First, three-month rainfall data are transformed to the standardized precipitation index, which is used to define the incidence of drought. Drought hazard index is defined as the weighted sum of drought classes, which is a function of occurrence probability for various drought severity. Drought exposure index is used to measure what would be exposed to droughts. Water requirements for various purposes of each region is defined as drought exposure index in this study. Drought vulnerability index measures water consumption resistance to droughts. The ratio of water-supply from reservoirs to water requirements of each region are defined as the drought vulnerability index in this study. Drought risk index combines these three indices and defined as the product of these three indices, which is employed to evaluate impacts of drought on water supply.
The proposed methodology has been applied to assess drought risk of 15 regions in Taiwan. The results show that Changhua and Pingtung regions have higher drought risk, while the Hsinchu, Nantou and Hualian regions are belonging to low-risk regions in current (2008) water-use state. Drought risks in future scenarios (2021) are similar to the risk in 2008. However, Taipei and Taitung regions has decreasing trend, while the Taoyuan, Hsinchu, and Yilan show increasing risk trend.

1.經濟部水利署(1999)，「台灣地區南部區域水資源綜合計畫」
2.經濟部水利署(2002)，「台灣地區水資源開發綱領計畫」
3.經濟部水利署(2008)，「2008年-2012年積極推動節約用水計畫」
4.經濟部水利署(2008)，「經濟部水利署北區水資源局年報」
5.經濟部水利署(2008)，「經濟部水利署中區水資源局年報」
6.經濟部水利署(2009)，「旱災潛勢定義及其分析方法之建立」
7.經濟部水利署(2009)，「台灣地區水資源需求潛勢評估及經理策略檢討」
8.經濟部水利署(2010)，「脆弱度及風險地圖分析方法之研究」
9.經濟部水利署各項用水統計資料庫，2011年4月。http://wuss.wra.gov.tw/farmwater.aspx
10.經濟部水利署，水庫或壩堰營運概況年報，2011年4月。http://www.wra.gov.tw/ct.asp?xItem=20064&ctNode=5292&comefrom=lp#5292
11.經濟部水利署(2008)，現有水源與供水區域年報，2011年4月。http://www.wra.gov.tw/ct.asp?xItem=20063&ctNode=5292&comefrom=lp#5292
12.行政院農委會，農田水利入口網站，2011年4月。
http://doie.coa.gov.tw/index.asp
13.楊道昌、郭俊超、呂季蓉、游保杉，「標準化降雨指標應用於農業乾旱監測之研究」，農業工程學報，第五十一卷，第二期，第11-25頁(2005)。
14.蕭政宗、楊志傑，「台灣地區之區域乾旱頻率分析」，農業工程學報，第五十二卷，第二期，第83-101頁(2006)。
15.Downing, T.E., Bakker, K., (2000), Drought discourse and vulnerability. In: Wilhite DA (ed) Drought: A Global Assessment, vol. 2. Routledge, London
16.Kates, R.W., (1985) The interaction of climate and society. In: Kates RW, Ausubel JH, Berberian M (eds) Climate Impact Assessment: Studies of the Interaction of Climate and Society. Wiley, Chichister
17.Kron, W., (2005), Flood Risk =Hazard˙Values˙Vulnerability, Water International, 30(1), 58-68.
18.Kumar, M.N., Murthy, C.S., Sesha Sai, M.V.R., Roy, P.S., (2009), On the use of Standardized Precipitation Index (SPI) for drought intensity assessment, Royal Meteorological Society, 16(3), 381-389.
19.Loukas, A., Vasiliades, L., (2004), Probabilistic analysis of drought spatiotemporal characteristics in Thessaly region, Greece, Natural Hazards and Earth System Sciences, 4, 719-731,
20.McKee, T.B., Doesken, N.J., Kleist, J., (1993), The relationship of drought frequency and duration to time scales. In: Proceedings of eighth conference on applied climatology, American Meteorological Society, Jan17–23, 1993, Anaheim CA
21.Odeh, D.J., (2002), Natural Hazards Vulnerability Assessment for Statewide Mitigation Planning in Rhaode Island, Natural Hazard Review ASCE, 177(4), 1527-6988.
22.Peduzzi, P., Dao, H., Herold, C., Mouton, F.,(2009), Assessing global exposure and vulnerability towards natural hazards: the Disaster Risk Index, Natural Hazards, 9, 1149-1159.
23.Rouault, M., Richard, Y., (2003), Intensity and spatial extension of drought in South Africa at different time scales, Water SA, 29(4), 489-500.
24.Shahid, S., Behrawan, H., (2008), Drought risk assessment in the western part of Bangladesh, Natural Hazards, 46(3), 391–413.
25.Subash, N., Ram Mohan, H.S., (2011), Trend detection in rainfall and evaluation of standardized precipitation index as a drought assessment index for rice–wheat productivity over IGR in India, International Journal of Climatology, (in press).
26.Tingsanchali, T., and Karim, M.F., (2005), Flood hazard and risk analysis in the southwest region of Bangladesh, Hydrological Processes, 19(10), 2055-2069.
27.Tran, P., Shaw, R., Chantry, G., Norton, J.,(2009), GIS and local knowledge in disaster management: a case study of flood risk mapping in Viet Nam, Disaster, 33(1), 152-169.