water Article Changes in Extreme Precipitation across 30 Global River Basins Xin Feng 1, Zhaoli Wang 1,2, Xushu Wu 1,2,* , Jiabo Yin 3, Shuni Qian 3 and Jie Zhan 4 1 School of Civil Engineering and Transportation, South China University of Technology, Guangzhou 510641, China; [email protected] (X.F.); [email protected] (Z.W.) 2 State Key Laboratory of Subtropical Building Science, South China University of Technology, Guangzhou 510641, China 3 State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China; [email protected] (J.Y.); [email protected] (S.Q.) 4 China Water Resources Pearl River Planning Surveying & Designing Co., LTD., Guangzhou 510640, China; [email protected] * Correspondence: [email protected] Received: 8 May 2020; Accepted: 25 May 2020; Published: 27 May 2020 Abstract: Extreme precipitation can cause disasters such as floods, landslides and crop destruction. A further study on extreme precipitation is essential for enabling reliable projections of future changes. In this study, the trends and frequency distribution changes in extreme precipitation across different major river basins around the world during 1960–2011 were examined based on two of the latest observational data sets respectively collected from 110,000 and 26,592 global meteorological stations. The results showed that approximately a quarter of basins have experienced statistically significant increase in maximum consecutive one-day, three-day and five-day precipitation (RX1day, RX3day and RX5day, respectively). In particular, dramatic increases were found in the recent decade for the Syr Darya River basin (SDR) and Amu Darya River basin (ADR) in the Middle East, while a decrease in RX3day and RX5day were seen over the Amur River basin in East Asia. One third of basins showed remarkable changes in frequency distributions of the three indices, and in most cases the distributions shifted toward larger amounts of extreme precipitation. Relative to the subperiod of 1960–1984, wider range of the three indices over SDR and ADR were detected for 1985–2011, indicating intensification along with larger fluctuations of extreme precipitation. However, some basins have frequency distributions shifting toward smaller amounts of RX3day and RX5day, such as the Columbia River basin and the Yellow River basin. The study has potential to provide the most up-to-date and comprehensive global picture of extreme precipitation, which help guide wiser public policies in future to mitigate the effects of these changes across global river basins. Keywords: extreme precipitation; trend; frequency distribution change; global river basins 1. Introduction Flood, landslide and soil erosion triggered by extreme precipitation are among the major hazards that pose threats to society and the environment [1–7]. The understanding of extreme precipitation features is beneficial for the forecasting and management of these hazards. However, extreme precipitation is becoming substantially more intense and unpredictable, with larger fluctuations than the past largely due to climate change, and it always displays high spatiotemporal heterogeneity [8–11]. Exploring extreme precipitation behaviors across various regions enables a comprehensive understanding of how it changes in space and time under the changing environment [12,13]. Water 2020, 12, 1527; doi:10.3390/w12061527 www.mdpi.com/journal/water Water 2020, 12, 1527 2 of 12 A variety of literatures have revealed extreme precipitation changes at different spatial scales. Alexander et al. (2006) examined global trends and probability distributions of extreme precipitation, showing that extreme precipitation has increased on the whole for the 20th century, but the probability distribution of maximum one-day precipitation (RX1day) does not exhibit remarkable changes [11]. Asadieh and Krakauer (2014) analyzed trends in global extreme precipitation and found that both observations and models show generally upward trends in extreme precipitation since the beginning of 1900s, and the changes for tropic regions are the largest [14]. Through using a global atmospheric model for forecasting of extreme precipitation, Akio et al. (2016) showed that RX1day and maximum five-day precipitation (RX5day) are increasing even though the mean precipitation is decreasing [15]. Shi and Durran (2016) held the viewpoint that, in mountainous regions, the sensitivity of extreme precipitation to global warming is lower than in oceanic regions or plains [16]. Zhang and Zhou (2019) stated that, for global monsoon regions, extreme precipitation is on the rise and its correlation with warming climate is distinctive [17]. Regionally, in North America, a study has revealed a strong relationship between extreme precipitation and hurricane activities based on a 25-year observational analysis [18]. Costa and Soares (2007) assessed the uncertainty of spatiotemporal interpolation of an extreme precipitation index using the southern region of continental Portugal as an example and made conclusions of a higher spatial continuity of extreme precipitation but a weaker relationship between altitude and the index in recent decades [19]. Vyshkvarkova and Voskresenskaya (2018) pointed out an abnormal phenomenon over the southern Russia where the wetting trend is negligible, not necessarily following the global overall upward trend of extreme precipitation [20]. A similar phenomenon is found for the Northwestern Highlands of Ethiopia, where evidence of increasing extreme precipitation is lacking [21]. Over eastern Asia, Gayoung et al. (2018) projected the average and extreme precipitation intensity in Korea in future and found an increased intensity for the future period 2021–2100 compared to the present [22]. For China, there are also many studies regarding extreme precipitation across different regions. For example, Wu et al. (2016) investigated extreme precipitation characteristics over 11 basins in China and revealed strengthened RX1day in the Liaohe River Basin (LB) together with strengthened RX5day in the Songhua River Basin [23]. Another example is the study of Wang et al. (2015) which explored seasonal extreme precipitation changes over the arid regions of northwest China [24]. In India, Gupta and Jain (2020) discovered that the annual total precipitation is decreasing, while extreme precipitation shows an opposite trend [25]. Although the aforementioned studies provide beneficial information on extreme precipitation features, most of them are regional and their conclusions may not completely agree because of the use of varying indices, data sets, methodologies and study periods. Compared to regional studies, global studies enable a direct comparison of extreme precipitation characteristics across different regions and can provide a large-scale picture of extreme precipitation characterization [26,27]. However, there are relatively few global-scale literatures at present and, more importantly, the existing studies seldom investigate extreme precipitation characteristics (e.g., frequency or probability distributions) from different subperiods, but rather, from the whole period of record, which hampers the formulation of integrated information regarding changing properties of extreme precipitation under the backdrop of climate change. Furthermore, there is no systematic study of extreme precipitation over various river basins worldwide. From the hydrological perspective, flood analyses rely on precipitation conditions within a basin rather than in a region, country, or continent [28]. The lack of sufficient knowledge on extreme precipitation over different basins across the globe has restricted governors or river managers from making flood adaptations and mitigation strategies appropriate to each basin. To overcome the shortage, we set out to reveal trends and frequency distribution changes in extreme precipitation across different major river basins globally. Our study aims to draw the most up-to-date and comprehensive global picture of extreme precipitation at the basin scale which would be beneficial for river flood managements over different basins around the world. Water 2020, 12, 1527 3 of 12 2. Materials and Methods 2.1. Sourced Data and Global Basins Daily precipitation data used in this study came from two sources; one is the data covering 1929–present generated from more than 110,000 meteorological stations around the world, which were obtained from the National Oceanic and Atmospheric Administration (https://www.ncei.noaa. gov/data/global-summary-of-the-day/archive/), and the other is the data for the period 1929–2017 obtained from the National Climate Data Center GSOD dataset produced by the National Centers for Environmental Information, covering 26,592 stations globally (https://www.ncdc.noaa.gov/). Most of the analyses in the current study were based on the first data set and the second data set was served as the complementary one; if there is no data from the first data set for a specific basin, or the data are less sufficient than the GSOD data set, then the GSOD data are used instead. Meanwhile, before the data were used, a quality control procedure was applied, including examination of internal consistency, and suspected and erroneous data. To reduce errors caused by insufficient sampling and record length, we chose 9181 stations with near-continuous (less than 10% of missing data each year) observations during 1960–2011 in which the missing data are minimum. There are a wide range of river basins around the world and