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www.nature.com/scientificreports OPEN Future hydrology and hydrological extremes under climate change in Asian river basins Sangam Shrestha1*, Deg‑Hyo Bae2, Panha Hok1, Suwas Ghimire1 & Yadu Pokhrel3 The diverse impacts of anthropogenic climate change in the spatiotemporal distribution of global freshwater are generally addressed through global scale studies, which sufer from uncertainties arising from coarse spatial resolution. Multi‑catchment, regional studies provide fne‑grained details of these impacts but remain less explored. Here, we present a comprehensive analysis of climate change impacts on the hydrology of 19 river basins from diferent geographical and climatic conditions in South and Southeast Asia. We fnd that these two regions will get warmer (1.5 to 7.8 °C) and wetter (− 3.4 to 46.2%) with the expected increment in river fow (− 18.5 to 109%) at the end of the twenty‑ frst century under climate change. An increase in seasonal hydro‑climatic extremes in South Asia and the rising intensity of hydro‑climatic extremes during only one season in Southeast Asia illustrates high spatiotemporal variability in the impact of climate change and augments the importance of similar studies on a larger scale for broader understanding. Te spatiotemporal variability of global freshwater has been increasing in the past few decades1–3. Among several associated reasons, anthropogenic climate change has been identifed as a principal driver of such hydrological alteration4,5. Previous studies show that streamfow in major rivers across the globe is expected to range from − 96 to 212% within 50 years accompanied by an increase in intensity of hydrological extremes in terms of both magnitude and frequency 1,2,6. In such a scenario, an understanding of river hydrology and its evolution within future climatic conditions is of immense signifcance for water resources management and sustainable devel- opment. Previous studies show that most climate variables, with the exception of temperature, are expected to experience varying trends in time and space, as does the fow regime7–11. However, it is crucial to note that most global scale studies are ofen conducted at a spatial resolution which is too coarse to capture local climatic phenomena. On the other hand, basin scale studies can capture the local phenomena, but are limited within the watershed scale, failing to provide a holistic picture over a large region. In this context, this study investigates the impact of climate change on the hydrology of 19 river basins (Supplementary Table 5 and Supplementary Fig. 6) in Asia with the aim of capturing trends in relation to diferent geo-climatic conditions. Te Southern and Southeast Asia region is recognised as being one of the fastest growing in the world, com- prising mostly developing countries and catalysing national growth with heavy investment in water infrastruc- tures. Consequently, a clear understanding concerning the variation of future water availability and its spatiotem- poral distribution could be a signifcant asset to the scientifc community and policymakers in investigating the associated risks while formulating any kind of river basin management plans and disaster risk reduction (DRR) policies. Tus, this study utilises locally available data in conjunction with a physics-based hydrological model and an ensemble of climate models to provide a reliable outlook at regional level. Among 19 General Circulation Models (GCMs), an ensemble of fve was selected from the Coupled Model Intercomparison Project (CMIP5)12 based on their performance evaluation. Tis was followed by statistical downscaling using quantile mapping and bilinear interpolation to address the methodological variation13,14. Finally, bias correction 15,16 was applied using distribution and empirical-based quantile mapping to obtain the future climate under two diferent emission scenarios: Representative Concentration Pathways (RCPs) 4.5 and 8.5 in three future periods: the 20 s (2011 to 2040), 40 s (2041 to 2070), and 80 s (2071 to 2100). Te results were then compared with those for the control period (1976 to 2005). Tus, climate data obtained was employed in the Soil and Water Assessment Tool (SWAT)17 model to execute future hydrological responses. Here, hydrological model performance was evaluated through graphical interpretation and statistical indicators18 whereas future 1Water Engineering and Management, School of Engineering and Technology, Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathum Thani 12120, Thailand. 2Department of Civil and Environmental Engineering, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul 143-747, Korea. 3Department of Civil and Environmental Engineering, Michigan State University, East Lansing, MI 48824, USA. *email: [email protected] Scientifc Reports | (2021) 11:17089 | https://doi.org/10.1038/s41598-021-96656-2 1 Vol.:(0123456789) www.nature.com/scientificreports/ hydrological responses were examined through the absolute or percentage change in annual and seasonal fows. Similarly, potential hydrological extremes were evaluated based on streamfow exceeding a given percentage (Q5 and Q95) of time over the analysis period19. Results Changes in future temperature. An overall increasing trend was found in annual and seasonal maxi- mum temperature over the region (see Fig. 1 and Supplementary Fig. 2). Figure 2 and Supplementary Figure 3 illustrate similar results for minimum temperature. In addition, most of the river basins possess a slightly higher increment in maximum temperature during the months of June, July, and August (JJA) under emission scenario RCP8.5, while no characteristic trend is found in projected temperature across seasons within the river basins. In the South Asian river basins, the annual change in maximum temperature is estimated to remain within the range 1 to 1.5 °C (1.2 to 2 °C), 1.8 to 3.1 °C (2.7 to 4.7 °C), and 2.3 to 4.1 °C (4.5 to 7.6 °C) under RCP4.5 (RCP8.5) in the 20 s, 50 s, and 80 s. Similarly, in the Southeast Asian river basins, the annual change in maximum tempera- ture is estimated to remain within the range 0.5 to 1.4 °C (0.8 to 1.8 °C), 1.1 to 2.6 °C (1.8 to 3.9 °C), and 1.5 to 3.2 °C (3.1 to 5.8 °C) under RCP4.5 (RCP8.5) in the 20 s, 50 s, and 80 s, respectively. At the end of the twenty-frst century, the estimated change in both maximum and minimum annual tem- perature in the South Asian river basins is expected to remain at around 1 °C higher than for Southeast Asian river basins under both emission scenarios. Although the results for individual basins may vary, Supplementary Figures 2 and 3 clearly show that the South Asian river basins are projected to become much hotter than those of Southeast Asia in future. Moreover, at the end of the century, it is estimated that the change in both maximum and minimum temperature under the higher greenhouse gas (GHG) concentration emission scenario (RCP8.5) is expected to be around 1 °C higher than for the lower GHG concentration emission scenario (RCP4.5) in both South and Southeast Asian river basins. Details of the seasonal variation in maximum and minimum temperature for all river basins are provided in Supplementary Figures 2 and 3. Changes in future precipitation. Figure 3 shows an overall increasing trend in annual precipitation over the region except for the Swat River Basin which exhibits a decreasing trend. Interestingly, the change in aver- age annual precipitation in the Southeast Asian river basins is projected to be around twice as much as those in South Asia throughout the century under both emission scenarios. In South Asian river basins, the percentage change in annual precipitation is estimated to remain within the range − 0.5 to 4.9% (− 0.3 to 7.5%), − 4.3 to 12.6% (− 3.2 to 10.7%), and − 3.4 to 16% (1.4 to 18.8%) in the 20 s, 50 s, and 80 s under emission scenario RCP4.5 (RCP8.5). Similarly, the percentage change in annual precipitation within the Southeast Asian river basins is estimated to remain within the range 2.2 to 14.1% (1.4 to 12.3%), 3.4 to 23.6% (6.7 to 31.8%), and 5.4 to 31.5% (7 to 46.2%) in the 20 s, 50 s, and 80 s under emission scenario RCP4.5 (RCP8.5). Considering the seasonal change in precipitation, most of the South Asian river basins show a decreasing trend in the dry period and vice versa except for Swat and Satpara which show contrasting trends (see Supplementary Fig. 4). However, Southeast Asian river basins show an increasing trend in future precipitation throughout the year except Chindwin and Bandung which exhibit similar characteristics to South Asian river basins. Details of the seasonal variation in precipitation for all river basins are provided in Supplementary Figure 3. Changes in future streamfow. Figure 4 illustrates an overall increasing trend in annual fows for most of the river basins with a few exceptions like Swat and Satpara which show a decreasing trend. Te projected annual river discharge in Southeast Asian river basins is also prominently higher, (around fve times) than in South Asian river basins. In South Asian river basins, the percentage change in annual discharge is estimated to range from − 2.8 to 6.6% (− 7.6 to 8.5%), − 12.3 to 15% (− 19 to 14.2%), and − 14 to 18.8% (− 18.5 to 26.3%) in the 20 s, 50 s, and 80 s under emission scenarios RCP4.5 (RCP8.5). Similarly, the annual percentage change in discharge within the Southeast Asian river basins is estimated to range from − 0.9 to 25.1% (− 0.7 to 27.6%), 2.1 to 53.2% (6.8 to 70.3%), and 4.9 to 71.9% (6.3 to 109%) in the 20 s, 50 s, and 80 s under emission scenario RCP4.5 (RCP8.5).

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