Hydrol. Earth Syst. Sci., 23, 2279–2303, 2019 https://doi.org/10.5194/hess-23-2279-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Multi-model approach to quantify groundwater-level prediction uncertainty using an ensemble of global climate models and multiple abstraction scenarios Syed M. Touhidul Mustafa 1 , M. Moudud Hasan 1 , Ajoy Kumar Saha 1 , Rahena Parvin Rannu 1 , Els Van Uytven 2 , Patrick Willems 1,2 , and Marijke Huysmans 1 1 Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium 2 Department of Civil Engineering – Hydraulics Section, KU Leuven, Kasteelpark 40 box 2448, 3001 Leuven, Belgium Correspondence: Syed M. Touhidul Mustafa (syed.mustafa@vub.be) Received: 20 November 2018 – Discussion started: 10 December 2018 Revised: 5 April 2019 – Accepted: 24 April 2019 – Published: 13 May 2019 Abstract. Worldwide, groundwater resources are under a constant threat of overexploitation and pollution due to an- thropogenic and climatic pressures. For sustainable manage- ment and policy making a reliable prediction of groundwater levels for different future scenarios is necessary. Uncertain- ties are present in these groundwater-level predictions and originate from greenhouse gas scenarios, climate models, conceptual hydro(geo)logical models (CHMs) and ground- water abstraction scenarios. The aim of this study is to quan- tify the individual uncertainty contributions using an ensem- ble of 2 greenhouse gas scenarios (representative concen- tration pathways 4.5 and 8.5), 22 global climate models, 15 alternative CHMs and 5 groundwater abstraction scenar- ios. This multi-model ensemble approach was applied to a drought-prone study area in Bangladesh. Findings of this study, firstly, point to the strong dependence of the ground- water levels on the CHMs considered. All groundwater ab- straction scenarios showed a significant decrease in ground- water levels. If the current groundwater abstraction trend continues, the groundwater level is predicted to decline about 5 to 6 times faster for the future period 2026–2047 compared to the baseline period (1985–2006). Even with a 30 % lower groundwater abstraction rate, the mean monthly groundwater level would decrease by up to 14 m in the southwestern part of the study area. The groundwater abstraction in the north- western part of Bangladesh has to decrease by 60 % of the current abstraction to ensure sustainable use of groundwater. Finally, the difference in abstraction scenarios was identified as the dominant uncertainty source. CHM uncertainty con- tributed about 23 % of total uncertainty. The alternative CHM uncertainty contribution is higher than the recharge scenario uncertainty contribution, including the greenhouse gas sce- nario and climate model uncertainty contributions. It is rec- ommended that future groundwater-level prediction studies should use multi-model and multiple climate and abstraction scenarios. 1 Introduction Groundwater is one of the major sources of high-quality freshwater across the world and one of the most important but scarce natural resources in many arid and semi-arid re- gions. However, these resources are under a constant threat of overexploitation and pollution all over the world due to anthropogenic and climatic pressure. Globally, groundwater provides 45 %–70 % of irrigation water (Döll et al., 2012; Shamsudduha et al., 2011; Taylor et al., 2013; Wada et al., 2013, 2014; Wisser et al., 2008), and the use of groundwa- ter is continuously increasing. Overexploitation of ground- water for irrigation is worldwide one of the main causes of groundwater-level depletion (Mustafa et al., 2017b; Rodell et al., 2009; Scanlon et al., 2012; Wada et al., 2014). Climate change will probably also have an impact on the future avail- ability of the groundwater resources (Brouyère et al., 2004; Chen et al., 2004; Goderniaux et al., 2009, 2011; van Roos- malen et al., 2009; Scibek et al., 2007; Taylor et al., 2013; Woldeamlak et al., 2007). Published by Copernicus Publications on behalf of the European Geosciences Union.