• Journal of Korea Water Resources Association
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• v.51 no.10
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• pp.875-886
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• 2018

This study evaluated drought severity by bivariate frequency analysis using drought magnitude and precipitation deficit. A drought event was defined by Standardized Precipitation Index (SPI) and the precipitation deficit was estimated using reference precipitation corresponding to the SPI -1. In previous studies, drought magnitude and duration were used for bivariate frequency analysis. However, since these two variables have a largely linear relationship, extensibility of drought information is not great compared to the univariate frequency analysis for each variable. In the case of drought in 2015, return periods of 'drought magnitude-precipitation deficit' in the Seoul, Yangpyeong, and Chungju indicated severe drought over 300 years. However, the result of 'drought magnitude-duration' showed a significant difference by evaluating the return period of about 10, 50, and 50 years. Although a drought including the rainy season was seriously lacking in precipitation, drought magnitude did not adequately represent the severity of the absolute lack of precipitation. This showed that there is a limit to expressing the actual severity of drought. The results of frequency analysis for 'drought magnitude-precipitation deficit' include the absolute deficit of precipitation information, so which could consider being a useful indicator to cope with drought.

• Proceedings of the Korea Water Resources Association Conference
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• 2023.05a
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• pp.272-272
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• 2023

기후변화로 인해 많은 경우 강수량은 증가할 것으로 전망되지만 시공간적 편차 또한 커짐으로써 가뭄 위험은 증가할 것으로 예상된다. 가뭄 위험도 평가는 강수량, 유출량 등 수문자료로부터 추출한 가뭄변량의 빈도해석을 통해 이루어질 수 있다. 빈도해석의 대상이 되는 수문변량의 통계적 속성이 일정하게 유지되는 정상성의 가정은 기존 빈도해석 방법의 핵심이 되지만, 최근 기후변화로 인한 수문변량의 통계적 특성 변화가 발생할 것으로 예상되기 때문에 이러한 비정상성의 특성을 빈도해석 시 고려할 필요가 있다. 자료의 비정상성을 평가하는데 짧은 기록을 갖는 자료로부터 변화 추세를 신뢰성 있게 평가하는 것은 어려움이 크다. 이러한 점에서 지점자료를 통합적으로 활용할 수 있는 지역빈도해석 절차 도입을 통해 해석 결과에 신뢰성을 확보하는 것이 합리적이다. 본 연구에서는 유역단위에서 가뭄의 지속기간과 심도 사이의 상호의존성을 고려하기 위해 이변량 Copula 함수 기반 가뭄 지역빈도해석을 도입했으며, 두 가뭄변량의 주변확률분포의 매개변수는 시간에 따른 함수로 가정하였다. 모형의 모든 매개변수는 계층적 Bayesian 모형을 통해 동시에 추정하였다. 최종적으로 주어진 가뭄빈도에 해당하는 시간에 따라 변화하는 가뭄 위험을 평가하였다.

• Proceedings of the Korea Water Resources Association Conference
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• 2016.05a
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• pp.35-35
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• 2016

수문학적 빈도해석은 일반적으로 단변량 형태에 해석이 주를 이루고 있으나, 최근 다변량 해석에 대한 이해와 더불어, 해석 기술 발달에 따라 빈도해석에서도 다변량 해석적 접근이 이루어지고 있다. 기존 다변량 해석 방법으로는 Copula방법 적용이 활발하게 이루어지고 있으며, 특히 가뭄해석에 있어 지속시간과 심도를 동시에 평가하는 2변량 가뭄빈도해석에 대한 연구가 다수 이루어지고 있다. 그러나 기존 해석 방법은 정상성 해석 모형으로서 기상변동성과 같은 시변동성을 고려하는데 한계가 있다. 이러한 점에서 본 연구에서는 Bayesian 기반 Copula 함수의 매개변수를 추정함과 동시에 매개변수의 불확실성을 평가할 수 있는 2변량 비정상성 빈도해석 모형을 개발하였다. 본 연구에서는 최근 우리나라와 미국에서 발생한 2013-15년 가뭄빈도에 대한 평가와 동시에 이에 따른 불확실성을 정량적으로 평가하는 연구를 진행하였다.

• Journal of Korea Water Resources Association
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• v.56 no.2
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• pp.151-163
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• 2023

This study explored the 2022 drought over the Nakdong River watershed. Here, we developed a bivariate regional frequency analysis method to evaluate the risk of hydrological drought. Currently, natural streamflow data are generally limited to accurately estimating the drought frequency. Under this circumstance, the existing at site frequency analysis can be problematic in estimating the drought risk. On the other hand, a regional frequency analysis could provide a more reliable estimation of the joint return periods of drought variables by pooling available streamflow data over the entire watershed. More specifically, the Copula-based regional frequency analysis model was proposed to effectively take into account the tail dependencies between drought variables. The results confirmed that the regional frequency analysis model showed better performance in model fit by comparing the goodness-of-fit measures with the at-site frequency analysis model. We find that the estimated joint return period of the 2022 drought in the Nakdong River basin is about eight years. In the case of the Nam river Dam, the joint return period was approximately 20 years, which can be regarded as a relatively severe drought over the last three decades.

• Proceedings of the Korea Water Resources Association Conference
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• 2017.05a
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• pp.351-351
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• 2017

2014-2015년 우리나라 강수량이 평년에 비해 절반수준에 미치지 못해 극심한 가뭄을 일으켰으며, 이는 댐 용량 부족, 지하수 고갈 등 다양한 피해를 발생시켰다. 특히 소양강댐의 경우 1978년 이루 두 번째로 낮은 수위를 기록한바 있다. 우리나라의 경우 가뭄은 약 2-3냔 주기로 발생하고 있으며, 특히 2015년에 겪었던 가뭄은 물 용수공금 측면에서 막대한 영향을 미친 것으로 평가되어 신뢰성 있는 가뭄 분석이 중요한 요소로 대두되고 있다. 또한 지구온난화로 인해 기후변화의 영향으로 강수량의 증가가 일반적으로 전망되지만, 상대적으로 증가된 강우변동성으로 인해 가뭄 발생 빈도 및 강도도 동시에 증가할 것으로 전망되고 있다. 이러한 이유로 본 연구에서는 현재 가뭄을 신뢰성있게 평가하기 위해 Trivariate Copula 함수를 활용하여 가뭄분석을 수행하였다. 기존연구에서는 가뭄 지속시간(drought duration), 가뭄 심도(drought severity)를 활용한 이변량 가뭄 빈도 해석을 수행하였지만, 이는 다소 과소 추정 될 개연성이 있다. 이러한 이유로 본 연구에서는 가뭄강도(drought intensity) 변량을 추가로 분석하여 Trivariate frequency analysis 기법을 개발하였으며, 서울 관측소를 대상으로 분석하였다. 분석 결과 현재 가뭄은 역대 발생했던 가뭄 중 가장 큰 빈도를 기록하여 이에 대한 효과적인 가뭄 관리체계를 마련하기 위한 기초자료를 제공할 수 있을 것으로 기대된다. 또한 기존 Bivariate 빈도해석의 경우 Trivariate 빈도해석 보다 가뭄위험도를 다소 과소추정하는 것으로 나타나 Trivariate 해석이 다소 현실적인 접근 방법이라 사료된다.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.37 no.1
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• pp.231-238
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• 2017

Water supply safety index plays an important role on assessing the water supply capacity of hydrologic system. Due to the absence of consistent guidance, however, practical problems have been brought up on data period used for dam design and performance evaluation. Therefore, this study employed bivariate drought frequency analysis which is able to consider drought severity and duration simultaneously, in order to evaluate water supply capacity of multi-purpose dams. Drought characteristics were analyzed based on the probabilistic approach, and water supply capacity of five multi-purpose dams in Korea (Soyang River, Chungju, Andong, Daecheong, Seomjin River) were evaluated under the specific drought conditions. As a result, it would be possible to have stable water supply with their own inflow during summer and fall, whereas water shortage would occur even under the 1-year return period drought event during spring and winter due to low rainfall.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.30 no.6B
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• pp.599-606
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• 2010

Due to the short period of precipitation data in Korea, the uncertainty of drought analysis is inevitable from a point frequency analysis. So it is desired to introduce a regional drought frequency analysis. This study first extracted drought characteristics from 3-month and 12-month moving average rainfalls which represent short and long-term droughts, respectively. Then, the homogeneous regions were distinguished by performing a principal component analysis and cluster analysis. The Korean peninsula was classified into five regions based on drought characteristics. Finally, this study applied the bivariate frequency analysis using a kernel density function to quantify the regionalized drought characteristics. Based on the bivariate drought frequency curves, the drought severities of five regions were evaluated for durations of 2, 5, 10, and 20 months, and return periods of 5, 10, 20, 50, and 100 years. As a result, the largest severity of drought was occurred in the Lower Geum River basin, in the Youngsan River basin, and over in the southern coast of Korea.

• Journal of Korea Water Resources Association
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• v.50 no.11
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• pp.745-758
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• 2017

The copula-based models have been successfully applied to hydrological modeling including drought frequency analysis and time series modeling. However, uncertainty estimation associated with the parameters of these model is not often properly addressed. In these context, the main purposes of this study are to develop the Bayesian inference scheme for bivariate copula functions. The main applications considered are two-fold: First, this study developed and tested an approach to copula model parameter estimation within a Bayesian framework for drought frequency analysis. The proposed modeling scheme was shown to correctly estimate model parameters and detect the underlying dependence structure of the assumed copula functions in the synthetic dataset. The model was then used to estimate the joint return period of the recent 2013~2015 drought events in the Han River watershed. The joint return period of the drought duration and drought severity was above 100 years for many of stations. The results obtained in the validation process showed that the proposed model could effectively reproduce the underlying distribution of observed extreme rainfalls as well as explicitly account for parameter uncertainty in the bivariate drought frequency analysis.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.39 no.5
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• pp.561-568
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• 2019

Recently, Korea has suffered from severe droughts due to climate change. Therefore, we need to pay attention to the change of drought risk to develop appropriate drought mitigation measures. In this study, we investigated the changes of hydrologic risk of extreme drought using the current observed data and the projected data according to the RCP 4.5 and 8.5 climate change scenarios. The bivariate frequency analysis was performed for the paired data of drought duration and severity extracted by the threshold level method and by eliminating pooling and minor droughts. Based on the hydrologic risk of extreme drought events Jeonbuk showed the highest risk and increased by 51 % than the past for the RCP 4.5 scenario, while Gangwon showed the highest risk and increased by 47 % than the past for the RCP 8.5 scenario.

• Journal of Korea Water Resources Association
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• v.49 no.4
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• pp.275-282
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• 2016

The objective of this study is to propose a new method to determine the drought event and the design drought severity. In order to define a drought event from precipitation data, theory of run was applied with the cumulative rainfall deficit. When we have a large amount of rainfall over the threshold level, in this study, we compare with the previous cumulative rainfall deficit to determine whether the drought is relieved or not. The recurrence characteristics of the drought severity on the specific duration was analyzed by the conditional bivariate copula function and confidence intervals were estimated to quantify uncertainties. The methodology was applied to Seoul station with the historical dataset (1909~2015). It was observed that the past droughts considered as extreme hydrological events had from 10 to 50 years of return period. On the other hand, the current on-going drought event started from 2013 showed the significantly higher return period. It is expected that the result of this study may be utilized as the reliable criteria based on the concept of return period for the drought contingency plan.