Emergence of wet-season hydrological droughts and the development of a “brittle” river system in the Vietnamese Mekong Delta

Abstract

River regulation works (RRWs) are a globally implemented strategy to enhance water security, primarily by increasing dry-season flows. However, this intervention can trigger unintended and often paradoxical consequences, fundamentally altering the nature of hydrological risk. This study investigates the profound impact of RRW development on drought characteristics in the Vietnamese Mekong Delta, a region of immense agricultural importance and ecological sensitivity. Building upon the established understanding of the Mekong’s altered flow regimen, we analyze how this engineered shift has transformed drought dynamics. Using the standardized precipitation index-3 and standardized streamflow index (SSI-3) from 2000 to 2024, we uncover a critical paradigm shift. While the frequency of traditional dry-season hydrological droughts has significantly decreased as intended, the system’s vulnerability to severe, prolonged wet-season droughts has increased dramatically. Correlation analysis reveals a significant “decoupling” between regional rainfall and streamflow in the current mega-dam era (2010-2024). The most alarming finding is the emergence of wet-season hydrological droughts, with the 2019-2020 event being the most extreme in the 25-year record, during which SSI values dropped to below −2.0 despite only a moderate meteorological drought. We introduce and define the concept of a “brittle” river system: By dampening the natural flood pulse, intensive regulation has removed the river’s inherent buffering capacity against monsoon rainfall deficits, making the delta extremely vulnerable to climatic shocks. These findings challenge the conventional understanding of drought in the Vietnamese Mekong Delta and have urgent implications for water management, emphasizing the need for strategies that enhance wet-season resilience while addressing growing risks.

Keywords

Drought paradigm / Hydrological drought / River regulation / Standardized streamflow index / Brittle river system

Funding

This study is supported by the Vietnam National University Ho Chi Minh City (VNU–HCM) through the project entitled “Identifying and quantifying drivers causing water level fluctuations in the Vietnamese Mekong Delta” (grant number: B2024-18-01).

Conflict of interest

The authors declare that they have no competing interests.

References

[1] Grill, G, Lehner, B, Thieme, M, et al. An index-based framework for assessing patterns and trends in river fragmentation and connectivity worldwide. Sci Data. 2019; 6(1):1-12. doi: 10.1038/s41597-019-0040-y

[2] Zarfl, C, Lumsdon, AE, Berlekamp, J, Tydecks, L, Tockner, K. A global boom in hydropower dam construction. Aquat Sci. 2015; 77(1):161-170. doi: 10.1007/s00027-014-0377-0

[3] Kondolf, GM, Rubin, ZK, Minear, JT. Dams on the Mekong: Cumulative sediment starvation. Water Resour Res. 2014; 50(6):5158-5169. doi: 10.1002/2013WR014651

[4] Poff, NL, Olden, JD. Can we stop assuming that fiscal flows trump ecological ones in freshwater conservation? Front Ecol Environ. 2017; 15(9):490-496. doi: 10.1002/fee.1633

[5] Oweis, T, Hachum, A. Water harvesting and supplemental irrigation for improved water productivity of dry farming systems in West Asia and North Africa. Agric Water Manag. 2006; 80(1-3):57-73. doi: 10.1016/j.agwat.2005.07.004

[6] Vörösmarty, CJ, McIntyre, PB, Gessner, MO, et al. Global threats to human water security and river biodiversity. Nature. 2010; 467(7315):555-561. doi: 10.1038/nature09440

[7] Grumbine, RE, Xu, J.Mekong hydropower development. Science. 2011; 332(6026):178-179. doi: 10.1126/science.1200990

[8] Eslami, S, Hakimpour, F, El-Shafie, A, Parnian, A, Ghaderi, K. The role of reservoir operation in aggravating or mitigating water scarcity downstream of multi-purpose reservoirs. J Hydrol. 2021; 603:126986. doi: 10.1016/j.jhydrol.2021.126986

[9] Hecht, JS, Lacombe, G, Arias, ME, Dang, TD, Piman, T. Hydropower dams of the Mekong River basin: A review of their hydrological impacts. J Hydrol. 2019; 568:285-300. doi: 10.1016/j.jhydrol.2018.10.063

[10] Manh, NV, Dung, NV, Hung, NN, Merz, B, Apel, H. Large-scale drivers of floods in the Vietnamese Mekong Delta. Hydrol Earth Syst Sci. 2015; 19(11):4707-4720. doi: 10.5194/hess-19-4707-2015

[11] Binh, DV, Kantoush, S, Sumi, T, Mai, NTP. Long-term alterations of flow regimes of the Mekong River and adaptation strategies for the Vietnamese Mekong Delta. J Hydrol Reg Stud. 2020; 32:100742. doi: 10.1016/j.ejrh.2020.100742

[12] Wild, TB, Loucks, DP. Managing flow, sediment, and hydropower regimes in the Sre Pok, Se San, and Se Kong Rivers of the Mekong basin. Water Resour Res. 2014; 50(6):5141-5157. doi: 10.1002/2013WR014917

[13] Sabo, JL, Ruhi, A, Holtgrieve, GW, et al. Designing river flows to improve food security futures in the Lower Mekong Basin. Science. 2017; 358(6368):eaao1053. doi: 10.1126/science.aao1053

[14] Li, D, Long, D, Zhao, J, Lu, H, Hong, Y. Observed changes in flow regimes in the Mekong River basin. J Hydrol. 2017; 551:217-232. doi: 10.1016/j.jhydrol.2017.05.061

[15] Ziv, G, Baran, E, Nam, S, Rodríguez-Iturbe, I, Levin, SA. Trading-off fish biodiversity, food security, and hydropower in the Mekong River Basin. Proc Natl Acad Sci U S A. 2012; 109(15):5609-5614. doi: 10.1073/pnas.1201423109

[16] Räsänen, TA, Someth, P, Lauri, H, Koponen, J, Sarkkula, J, Kummu, M. Observed river discharge changes due to hydropower operations in the Upper Mekong Basin. J Hydrol. 2017; 545:28-41. doi: 10.1016/j.jhydrol.2016.12.023

[17] Mekong River Commission. Hydrological Conditions in the Lower Mekong River Basin in January-July 2020. Laos: Mekong River Commission Secretariat; 2020.

[18] Yun, X, Tang, Q, Li, J, et al. Impacts of climate change and reservoir operation on streamflow and flood characteristics in the Lancang-Mekong River Basin. J Hydrol. 2020; 590:125472. doi: 10.1016/j.jhydrol.2020.125472

[19] Wang, D, Hejazi, M, Cai, X, Valocchi, AJ. Climate change impact on meteorological, agricultural, and hydrological drought in central Illinois. Water Resour Res. 2011; 47(9):W09527. doi: 10.1029/2010WR009845

[20] Dang, TD, Cochrane, TA, Arias, ME, Van, PTT. The Mekong River’s flow alterations and their effects on the Vietnamese Mekong Delta. Water. 2018; 10(7):843. doi: 10.3390/w10070843

[21] Räsänen, TA, Kummu, M. Spatiotemporal influences of ENSO on precipitation and flood pulse in the Mekong River Basin. J Hydrol. 2013; 476:154-168. doi: 10.1016/j.jhydrol.2012.10.029

[22] Hoang, LP, Lauri, H, Kummu, M, et al. The Mekong’s future flows under multiple drivers: How climate change, hydropower development and irrigation expansion drive streamflow changes. Sci Total Environ. 2019; 649:601-609. doi: 10.1016/j.scitotenv.2018.08.337

[23] Thanh, NC, An, DT. Assessing water level variability in the Mekong Delta under the impacts of anthropogenic and climatic factors. J Environ Earth Sci. 2024; 7(1):123-131.

[24] Anthony, EJ, Brunier, G, Besset, M, Goichot, M, Dussouillez, P, Nguyen, VL. Linking rapid erosion of the Mekong River delta to human activities. Sci Rep. 2015; 5(1):14745. doi: 10.1038/srep14745

[25] Schmitt, RJP, Rubin, Z, Kondolf, GM. Losing ground-scenarios for subsidence and sea-level rise in the Mekong Delta. Sci Total Environ. 2017; 596:70-79. doi: 10.1016/j.scitotenv.2017.04.043

[26] Jordan, C, Tuan, LH, Binh, TTT, Ruessink, G. A remote sensing-based assessment of long-term mangrove change in the Mekong Delta, Vietnam. Estuar Coast Shelf Sci. 2022; 265:107722. doi: 10.1016/j.ecss.2021.107722

[27] van Vliet, MTH, Franssen, WHP, Yearsley, JR, et al. Global river discharge and water temperature under climate change. Glob Environ Change. 2013; 23(2):450-464. doi: 10.1016/j.gloenvcha.2012.11.002

[28] Lee, SK, Dang, TA. Extreme rainfall trends over the Mekong Delta under the impacts of climate change. Int J Clim Chang Strateg Manag. 2020; 12(5):639-652. doi: 10.1108/IJCCSM-11-2019-0063

[29] Sridhar, V, Thilakarathne, M. Characterization of future drought conditions in the Lower Mekong River Basin. Weather Clim Extremes. 2019; 25:100213. doi: 10.1016/j.wace.2019.100213

[30] McKee, TB, Doesken, NJ, Kleist, J. The relationship of drought frequency and duration to time scales. In: Presented at: 8th Conference on Applied Climatology, Anaheim, CA; 1993.

[31] Telesca, L, Lovallo, M, Lopez, I, Kanevski, M. Investigation of scaling properties in monthly streamflow and Standardized Streamflow Index time series in the Apulia region (Southern Italy). Physica A. 2012; 391(4):1662-1678. doi: 10.1016/j.physa.2011.09.028

[32] Gudmundsson, L, Seneviratne, SI. Anthropogenic climate change affects meteorological drought risk in Europe. Environ Res Lett. 2016; 11(4):044005. Correction in: Environ Res Lett. 2017; 12(5):059501. doi: 10.1088/1748-9326/11/4/044005

[33] Pokhrel, YN, Felfelani, F, Satoh, Y, et al. Global terrestrial water storage and its drivers: A review. WIREs Water. 2021; 8(5):e1528. doi: 10.1002/wat2.1528

[34] Vicente-Serrano, SM, Beguería, S, López-Moreno, JI. A multiscalar drought index sensitive to global warming: The standardized precipitation evapotranspiration index. J Climate. 2010; 23(7):1696-1718. doi: 10.1175/2009JCLI2909.1

[35] Guttman, NB. Accepting the standardized precipitation index: A calculation algorithm. J Am Water Resour Assoc. 1999; 35(2):311-322. doi: 10.1111/j.1752-1688.1999.tb03592.x

[36] Stanley, JD, Warne, AG. The engineered Nile River delta: A losing battle against nature. Chrono. 2006; 12(1):1-13.

[37] Ho, HL, Umetsu, C, Quang, NH, Du Bui, D, Van, L, Dung, T. Assessing the impact of sea-level rise and upstream development on saltwater intrusion in the Mekong Delta, Vietnam. Hydrol Process. 2022; 36(1):e14457. doi: 10.1002/hyp.14457

[38] Holling CS. Resilience and stability of ecological systems. Annu Rev Ecol Syst. 1973; 4(1):1-23. doi: 10.1146/annurev.es.04.110173.000245

[39] Sadoff, CW, Grey, D. Beyond the river: The benefits of cooperation on international rivers. Water Policy. 2002; 4(5):389-403. doi: 10.1016/S1366-7017(02)00031-4

[40] Tran, DD, van Halsema, G, Hellegers, PJGJ, Ludwig, F, Wyatt, A. A stakeholder-based assessment of the impacts of and adaptation to water-level changes in the Vietnamese Mekong Delta. Water Int. 2019; 44(5):517-536. doi: 10.1080/02508060.2019.1627993