Contrasting dynamics of hydrological processes in the Volta River basin under global warming

1International Water Management Institute, CSIR Campus, No. 6 Agostino Neto Road, Accra, Ghana 2Institute of Earth Surface Dynamics (IDYST), Faculty of Geosciences and Environment, University of Lausanne, 1015 Lausanne, Switzerland 3Laboratoire des Sciences du Climat et de l'Environnement (LSCE-IPSL), CEA/CNRS/UVSQ, Université Paris-Saclay Centre d'Etudes de Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette, France 4School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK 5School of Geography and the Environment, University of Oxford, South Parks Road, Oxford OX1 3QY, UK 6UK Centre for Ecology and Hydrology, Wallingford, Oxfordshire OX10 8BB, UK 7Institute of Geography (GIUB) and Oeschger Centre for Climate Change Research (OCCR), University of Bern, 3012 Bern, Switzerland


Introduction
Climate warming is projected to occur at a faster rate in West Africa than the global average during the twenty-first century (Todzo et al., 2020). Anthropogenic greenhouse gas emissions have led to an unprecedented increase in surface air temperature, which has resulted in the intensification of the hydrological cycle (Sylla et al., 2016). Therefore, recurrent floods and droughts could persist in the future because rainfall is projected to decrease in frequency but increase in intensity (Aich et al., 2014;Dosio 40 et al., 2020). In the face of climate change and variability, West Africa is particularly vulnerable because of its high reliance on rain-fed agriculture and limited institutional capacities to cope with climate change and variability (Karambiri et al., 2011;Sultan and Gaetani, 2016;Yira et al., 2017). Climate change and anthropogenic pressures pose an increasing stress on water resources (Sood et al., 2013). Freshwater shortages that lead to a decline in basin-scale irrigation water availability could have dire consequences for sustainable agriculture (Sylla et al., 2018b). Consequently, global warming is a serious threat for 45 water and food security in West Africa. However, addressing this threat is currently difficult given the high variability in climate projections, which can impose very different hydrological implications for West Africa (Dosio et al., 2020), thus imposing greater urgency for further investigations on the impacts of climate change on hydrological processes.
In the transboundary Volta River Basin (VRB) located in West Africa, water resources are fundamental for agriculture, hydropower generation, fisheries and other ecosystem services (Williams et al., 2016). Most of the agriculture is rain-fed but 50 many regions rely on irrigated agriculture (Roudier et al., 2014). Hydropower is a major source of electricity production with the potential to grant more access to energy in the region Stanzel et al., 2018). Future water resource developments in the VRB focus primarily on hydroelectricity and irrigation (McCartney et al., 2012). However, severe impacts of climate change on water resources in the VRB will impede future socio-economic development (Sood et al., 2013).
Anticipation of the future evolution of the hydrological cycle in the VRB is essential to ensuring the adaptive capacities of the 55 riparian countries to the regional consequences of global warming (Jin et al., 2018). However, there is relatively little knowledge of the impacts of climate change on the future water resources in West Africa in general (e.g., Kasei, 2010;Oyerinde et al., 2016;Yira et al., 2017), and only a few studies targeted the VRB (Jung et al., 2012;Okafor et al., 2019;Roudier et al., 2014). These studies usually focused on climatic variables (i.e. precipitation and temperature), and when considering hydrological modelling, they focused usually on one variable (e.g. streamflow). The limitations of climate change impacts 60 studies on water resources in West Africa arise from the lack of hydrological and meteorological observations to drive models, in addition to uncertainties related to climate projection data as well as hydrological models (Dembélé et al., 2019;Oyerinde et al., 2016;Sidibe et al., 2020;Sylla et al., 2018a). Despite considerable progress in improving climate projections and the efforts in investigating climate change in West Africa (e.g., Diallo et al., 2016;Mahé et al., 2013;Nikiema et https://doi.org/10.5194/hess-2021-525 Preprint. Discussion started: 27 October 2021 c Author(s) 2021. CC BY 4.0 License. al., 2017), the need for understanding the future evolution of the hydrological cycle under varying scenarios still exists (Eyring 65 et al., 2019;Sidibe et al., 2020;Sylla et al., 2016).
The goal of this study is to analyse repercussions of projected changes of rainfall and temperature on the seasonal and annual trends of various components of the hydrological cycle (i.e., streamflow, total runoff, potential and actual evaporation, groundwater recharge, soil moisture and terrestrial water storage) and water availability in the twenty-first century, under multiple future scenarios. Therefore, this study investigates the dynamics of various hydrological processes in the VRB under 70 a changing climate and the implications for water availability and extreme events like floods and droughts. The results can provide knowledge to inform research and water planning to develop and implement adaptation and mitigation measures that alleviate the impacts of global warming in the VRB.

Study area
The Volta River Basin (VRB) is a major transboundary basin in West Africa (Figure 1), and the ninth largest drainage basin 75 in sub-Saharan Africa (UNEP-GEF, 2013). It is located between latitudes 5°40' N and 14°55' N and longitudes 5°25' W and 2°20' E, and covers approximately 415,600 km 2 shared among six countries (Benin, Burkina Faso, Côte d'Ivoire, Ghana, Mali, and Togo). The population of the VRB was estimated to 23.8 million people in 2010 and it is projected to reach 56.1 million in 2050 (Williams et al., 2016). The altitude ranges from zero to 940 m a.s.l., but the topography is predominantly flat with a mean altitude of about 255 m a.s.l., as most of the basin lies below 400 m a.s.l. (Dembélé et al., 2020c). The land cover of the 80 basin area is composed of savannah (75%), cropland (13%), forest (9%), water bodies (2%) and bare land and settlements (1%). The climate is driven by the latitudinal and seasonal oscillation of the Inter-Tropical Convergence Zone (ITCZ), which governs rainfall occurrence in the VRB. Rainfall depicts a south-north gradient of increasing aridity, with four eco-climatic zones (i.e. Sahelian, Sudano-Sahelian, Sudanian and Guinean), and it is characterized by interannual and multidecadal variabilities (Nicholson et al., 2018). The Volta River flows north-south over 1,850 km and drains into the Atlantic Ocean at 85 the Gulf of Guinea after transiting into the Lake Volta formed by the Akosombo dam. The drainage system is composed of four sub-basins known as Black Volta (152,800 km 2 ), White Volta (113,400 km 2 ), Oti (74,500 km 2 ) and Lower Volta (74,900 km 2 ) (Dembélé, 2020

Overview of the methodology
The methodology adopted for the assessment of climate change impacts on water resources in the VRB is summarized in Figure 2. The main steps consist of the bias correction of the RCMs forced by the GCMs, the modelling of hydrological 95 processes based on the climate projection datasets, and the analysis of the future changes in the modelled hydrological processes.
Uncertainties in the climate projections are addressed by employing a large ensemble of twelve General Circulation Models (GCMs) downscaled by five Regional Climate Models (RCMs) under three Representative Concentration Pathways (RCPs; Moss et al., 2010;Van Vuuren et al., 2011). The RCMs are obtained from the Coordinated Regional-climate Downscaling 100 Experiment (CORDEX) for Africa (Giorgi et al., 2009). Only considering the highest RCP8.5 scenario as the "business as usual" scenario in climate change studies is increasingly criticized because the assumption of the heavy use of coal in RCP8.5 is unrealistic (Hausfather and Peters, 2020;Ritchie and Dowlatabadi, 2017). However, the current emissions are found to be in line with the RCP8.5 scenario (Peters et al., 2013), and there are suggestions for giving RCP8.5 a high priority (O'Neill et al., 2016). 105 The RCM-GCM datasets are first evaluated by comparing them to the best performing satellite and reanalysis datasets of 110 rainfall and temperature for hydrological modelling in the VRB. Subsequently, a multivariate bias correction is applied to the climate projection datasets using the Rank Resampling for Distributions and Dependences (R2D2) method (Vrac and Thao, 2020). Finally, the bias corrected climate projection datasets are used as input in the fully distributed mesoscale Hydrologic Model (mHM) to assess the impact of climate change on multiple hydrological fluxes and state variables. Although the performance of a model during past and present conditions does not guarantee its reliability for future projections (Stanzel et 115 al., 2018), having a well performing model that simulates realistic hydrological processes is a prerequisite for any sound impact study (Krysanova et al., 2018).

Climate projection datasets
CORDEX generates high-resolution historical and future climate projections for regional domains, by downscaling GCMs participating in the fifth Coupled Model Intercomparison Project (CMIP5; Taylor et al., 2012). Based on data availability on 120 the Earth System Grid Federation (ESGF) platform (https://esgf-data.dkrz.de, last accessed on 22.03.2020), twelve GCMs from CMIP5 dynamically downscaled with five RCMs available from the CORDEX-Africa initiative are selected for this study ( Table 1). The spatial resolution of the RCMs is 0.44° (~ 50 km) in latitude and longitude. As not all GCMs are downscaled by the RCMs, there are 21 RCM-GCM combinations available for the global runs in the historical period , while 43 RCM-GCM combinations are available for the future projections (2006-2100) under various representative 125 concentration pathways (RCP2.6, RCP4.5 and RCP8.5). The RCPs correspond to different scenarios of future concentrations and emissions of greenhouse gases and air pollutants, and land-use change until 2100, relative to the pre-industrial times (Moss et al., 2010;Van Vuuren et al., 2011). The three RCPs used in this study are (i) RCP2.6 corresponding to a mitigation scenario with a very low radiative forcing level that peaks at ~3 W m -2 (~490 ppm CO2 equivalent) and declines to ~2.6 W m -2 by 2100; (ii) RCP4.5 representing a medium stabilization scenario without overshoot pathway to 4.5 W m -2 (~650 ppm CO2 equivalent) 130 at stabilization after 2100; and (iii) RCP8.5 corresponding to a very high emission scenario with rising radiative forcing pathway leading to 8.5 W m -2 (~1370 ppm CO2 equivalent) by 2100. Approximatively, RCP2.6 corresponds to a +2 °C global temperature stabilization by 2100, while RCP4.5 and RCP8.5 correspond to +3.5 °C and +5 °C increase in global temperature by 2100 (Konapala et al., 2020). The climate variables used in this study are daily data of rainfall and minimum, maximum and average air temperature, which are obtained for the climate model realization r1i1p1. 135 Table 1. List of the CORDEX-Africa General Circulation Models (GCMs) datasets downscaled by Regional Climate Models (RCMs) and their availability per Representative Concentration Pathways (RCP) marked with a cross (x).

Multivariate bias correction of climate data
Assessing the reliability of climate projections data in reproducing observations is a precondition to impact studies (Eyring et al., 2019). However, observations are also subject to uncertainties. To address these uncertainties, the climate projection 140 datasets are evaluated with respect to ten satellite-based and reanalysis-based rainfall datasets and six temperature datasets ( Table 2). The selected satellite-based and reanalysis-based rainfall datasets demonstrated the best performances for largescale hydrological modelling in the VRB as shown by Dembélé et al. (2020c). Hereafter, the datasets composed of both satellite and reanalysis products will be referred to as observations.

JRA-55
Japanese 55 year ReAnalysis (JRA-55); rainfall: fcst_phy2m125; temperature: anl_surf125 https://jra.kishou.go.jp/JRA-55/index_en.html T 1959-NP Kobayashi et al. (2015) As discrepancies are observed between the cumulative distribution functions of the observations and the climate projection datasets ( Figure 3), a bias correction is applied before using the climate datasets for hydrological modelling, as usually recommended (Hakala et al., 2018;Teutschbein and Seibert, 2012). The R2D2 method (Vrac and Thao, 2020) is adopted for 150 a multivariate bias correction of the climatic variables. R2D2 is a rank analogue-based method that adjusts not only the univariate distributions of climatic variables, but also the inter-variable and inter-site dependence structures (Vrac, 2018). Bias correction with the R2D2 approach is achieved in two steps. First, the marginal distributions of univariate time series are adjusted using any univariate bias correction method. Here, the "cumulative distribution function transform" (CDF-t) approach (e.g., Vrac et al., 2012) is used to adjust the marginal properties of the univariate time series. Second, R2D2 is used to adjust 155 the dependence structure between several variables, independently of their marginal distribution (i.e. empirical copula function). R2D2 has performed better compared to other bias correction methods as demonstrated by François et al. (2020).
In the current multivariate bias correction, rainfall and temperature datasets are corrected simultaneously to preserve the temporal and spatial dependences between the climatic variables. The bias correction is done using only one of the observational datasets as reference data, the WFDEI, because it has both rainfall and temperature data over a long period 160 . Furthermore, the WFDEI dataset previously demonstrated good performances in the VRB (Dembélé et al., 2020b). Therefore, it is chosen as reference data to limit uncertainties in the bias correction (Tarek et al., 2021). The period 1981-2005 is taken as the reference period for training the bias correction of the climate projection datasets, whose time series are divided into several 25-year periods over the period 1951-2100 to correspond to the length of the reference period. The multivariate bias correction is applied by grouping the data per calendar month in each sub-period of 25 years, which improves 165 seasonality in the corrected data.

Model set-up and modelling period
The fully distributed mesoscale Hydrologic Model (mHM) simulates dominant hydrological processes with seamless spatiotemporal patterns in the modelling domain (Kumar et al., 2013;Samaniego et al., 2010;Samaniego et al., 2017). Telteu The calibrated model is run over the entire data availability period (1951-2100) of the RCM-GCM datasets, with 1951-1960 as spin-up period. The baseline or historical period for climate change impact assessment is 1991-2020. Projections are assessed for the near-term future (2021-2050), the long-term future (2051-2080) and the late-century (2071-2100). In total, 21 RCM-GCM combinations are available for the historical runs, while for future projections, 9 RCM-GCM combinations are available for the RCP2.6, 16 for RCP4.5 and 18 for RCP8.5 (Table 1)

Model reliability 185
The realism of the hydrological simulations is verified with the Budyko framework (Budyko, 1974), which helps to estimate mean annual water availability as a function of aridity. The supply-demand framework, which is increasingly used in hydrological modelling (Greve et al., 2020;Wang et al., 2016), is valid for large catchments under steady state, considering long-term water balance and energy balance (Donohue et al., 2010;McVicar et al., 2012). The exercise consists of verifying if the ratio of the long-term mean annual potential evaporation to precipitation (aridity index) and the ratio of long-term mean 190 actual evaporation to precipitation (evaporative index) of the climate projections are coherent with the energy and water limits for a given climate (Sposito, 2017;Donohue et al., 2011). Here, the term evaporation involves all sources of evaporation including transpiration (Miralles et al., 2020;Savenije, 2004). The Budyko curve is formulated with equation 1, after Budyko (1974), as follows: where � is the long-term mean annual precipitation, a ��� is the long-term mean annual actual evaporation and is the aridity 195 index. Uncertainties in the model inputs and outputs are assessed in terms of variability between the simulations corresponding to different climate models using the second order coefficient of variation (V2), which addresses the limitations of the classical coefficient of variation (Kvålseth, 2017), and is defined as follows: where s is the standard deviation and ̅ is the mean of a sample data x = (x1,…, xn) ∈ R n . V2 represents the distance between x 200 and ̅ relative to the distance between x and the origin zero, and it varies from 0 to 1 or 0% to 100%. Finally, the percentage of RCM-GCM agreement is calculated as the number of models agreeing on the same direction of change (i.e. increase or decrease) relative to the total number of models, which shows the robustness of the ensemble of climate projections.

Multivariate bias correction
The raw RCM-GCM datasets are evaluated by comparing their cumulative distribution functions to those of the observations over the period 1983-2005 corresponding to the concomitant availability period of all the observation datasets ( Figure 3). The distribution of most of the raw RCM-GCM datasets presents discrepancies with the observations, with larger gaps for temperature than rainfall. The multivariate bias correction with the R2D2 method visually performs well by adjusting the 210 distributions of the RCM-GCM datasets to the WFDEI reference dataset for all the climatic variables. Therefore, the corrected RCM-GCM datasets are expected to provide reliable hydrological simulations in the VRB.

Plausibility of hydrological simulations
The general plausibility, here used to mean water and energy balance consistency within the Budyko framework, of the hydrological simulations using various RCM-GCM datasets as inputs to the mHM model under various RCPs and various 220 periods is illustrated in Figure 4. All the RCM-GCM datasets provide plausible hydrological simulations, at least in terms of water and energy balance, as they respect the water and energy limits imposed within the Budyko framework. On average, the evaporative index (Ea/P) is between 0.86 and 0.97, while the aridity index (Ep/P) is between 2.2 and 4.4, which corresponds to expected values for subhumid and semi-arid environments such as the VRB (Gunkel and Lange, 2017). It is noteworthy that future projections are 225 shifted towards a lower evaporative index and they large model dependent variability in aridity ranges, particularly under RCP4.5 and RCP8.5.

Seasonal changes in hydroclimatic variables
The annual cycles of climatic and hydrological variables are illustrated in Figure 5 for RCP 8.5 (see Figures S1-S10 for other RCPs). The hydroclimatic variables analysed are rainfall (P), average air temperature (Tavg), potential evaporation (Ep), actual evaporation (Ea), root-zone soil moisture (Su), terrestrial water storage (St), total runoff (Qrun) and groundwater recharge (Rr). 235 It is noteworthy that Tavg and Ep show a clear bimodal annual cycle in the VRB, with the first mode peaking between March and April, corresponding to the hottest period, while the second mode starts in September and peaks around October and November. For the hydroclimatic variables depicting a unimodal annual cycle, the peak of the cycle is observed around August and September, corresponding to the rainiest period.  Figures S11-S30 illustrate the distribution of changes in the seasonal cycle of the hydroclimatic variables across RCM-GCM combinations. Overall, monthly P is projected to increase between +2.9% (RCP2.6) and +136.7% (RCP8.5) on average in the dry months (November-March) and decrease between -14% (RCP8.5) and -0.6% (RCP2.6) in the wet months (May-September) over the twenty-first century. However, the reduction in rainfall over the wet months is more accentuated from the 250 beginning to the middle of the rainy season (May-July), whereas there is an increase in rainfall intensity over the late rainy season (August-September). Consequently, monthly rainfall is projected to be more concentrated at the end of the rainy season, i.e. in August and September, as compared to the baseline period. All RCM-GCM models agree for air temperature on the increasing trend across all RCPs over the twenty-first century. There is a clear increase for Tavg in all months with a similar pattern for each month, depicting an increase with time from 2021 to 2100 and with increasing radiative forcing level from 255 RCP2.6 to RCP8.5. On average, Tavg increases between +2.4% and 16.3% (i.e., +0.6 °C and +4.4 °C) across RCPs and over the twenty-first century. Similar to future projections of Tavg, monthly Ep also shows a consistent increase across RCPs and for https://doi.org/10.5194/hess-2021-525 Preprint. Discussion started: 27 October 2021 c Author(s) 2021. CC BY 4.0 License. different periods in the twenty-first century. The average changes in Ep are projected from +1.1% in 2021-2050 for RCP2.6 to +10.2% in 2071-2100 under RCP8.5 ( Figure 6). The changes in simulated monthly Ea follow a similar pattern to P, with an average increase between +0.2% under RCP2.6 and +8.1% under RCP8.5 for Ea in the dry months and a reduction of -0.2% 260 and -9.9%, respectively under RCP2.6 and RCP8.5, in the wet months (Figure 7).  Overall, the inter-model (i.e., RCM-GCMs) variability is lower for the historical period than for the future periods and increases with increasing radiative forcing level from RCP2.6 to RCP8.5 (Figure 8). Among the climatic variables, low inter-model 275 variabilities are simulated for monthly temperatures and potential evaporation, with an average V2 varying from 1% to 4% over the twenty-first century. However, P shows higher inter-model variabilities with V2 ranging between 30% and 44%, which have a direct repercussion on the projected hydrological variables. The highest inter-model variabilities are projected for groundwater recharge, exceeding V2 of 48%. The monthly inter-model variability for all the hydroclimatic variables and all the RCPs are illustrated in Figure S31. It is noteworthy that the inter-model variability for monthly rainfall, actual evaporation, 280 total runoff and groundwater recharge is lower during the wet months (May-September) than the dry months (November-March), while an almost constant trend across months is simulated for the other variables ( Figure S31).

Annual changes in hydroclimatic variables and water availability
Considering all RCPs over the historical period in the VRB, the multi-model ensemble mean of long-term annual estimates of climatic variables is as follows: Tavg = 28.4 °C, Ep = 2580 mm/year, P = 994 mm/year. The trends in annual estimates of projected hydroclimatic variables and water availability are illustrated in Figure S32. Water availability is assessed as the net 290 water flux into the land surface, using a proxy metric that is the difference between rainfall and actual evaporation (P-Ea) (Konapala et al., 2020;Greve and Seneviratne, 2015;Mishra et al., 2017). The largest uncertainty in annual climate projections over the historical period is observed for rainfall with a V2 of 3.2% for the inter-model variability.
The changes in the future projections of annual climatic and hydrological processes over the twenty-first century (2021-2100) as compared to the historical period  show contrasting trends between RCPs and between projection periods 295 https://doi.org/10.5194/hess-2021-525 Preprint. Discussion started: 27 October 2021 c Author(s) 2021. CC BY 4.0 License.
( Figure 9). All climatic variables (rainfall, temperature and potential evaporation) are projected to increase over the twentyfirst century for all RCPs and projection periods, at the exception of rainfall that is expected to decrease under RCP2.6 and RCP4.5, and increase under RCP8.5. Figure 9. Annual changes in future hydroclimatic variables and water availability (P-Ea) relative to the historical period for RCP2.6 300 (9 models), RCP4.5 (16 models) and RCP8.5 (18 models).
In contrast to the climatic variables, the hydrological variables (total runoff, actual evaporation, groundwater recharge, soil 310 moisture and terrestrial water storage) decrease under RCP2.6 and RCP4.5, and only increase under the RCP8.5 scenario during the period 2021-2100 (Figure 9), thereby following the patterns of the projected changes in rainfall. The multi-model ensemble mean of long-term annual estimates of hydrological variables for all RCPs over the historical period is as follows: The multi-model ensemble mean of water availability is 65 mm/year over the historical period. The changes in water 320 availability are driven by the variations in rainfall, with a projected median increase of +20.3% only under RCP8.5 over the twenty-first century, and a reduction of -11.3% under RCP2.6 and -0.6% under RCP4.5.

Spatial patterns of hydroclimatic variables across climatic zones
The spatial patterns of the inter-model median of annual climatic variables show a north-south gradient of increasing rainfall and decreasing air temperature and decreasing potential evaporation (Figure 10), in line with the aridity gradient of the eco-325 climatic zones in the VRB (Figure 1). The spatial average of the multi-model ensemble of hydroclimatic variables per climatic zones in the VRB considering all RCPs over the historical period is given in Table 3. Annual average of hydroclimatic variables per climatic zones corresponding to the median of all RCM-GCM combinations and all RCPs over the historical period. The changes in spatial patterns of climatic variables under RCP8.5 illustrate a clear increase in air temperature and potential evaporation over the twenty-first century, with higher increase rates in the Sahelian zone and the northern regions of the VRB (Figure 10, see other RCPs in Figures S33-S37). The future projections show a decrease in rainfall under RCP2.6 and RCP4.5 from the Sahelian to the Sudanian zones, which correspond to the northern and central parts of the VRB (Figures S33-S37). 335 The increase in rainfall is projected mainly under RCP8.5, with higher increase rates localized from the Guinean to the Sudano-Sahelian zones, which correspond to the south and central regions of the VRB (Figure 10). The spatial patterns of the hydrological variables under RCP8.5 are depicted in Figure 11 (see Figures S38-S43 for other RCPs). Changes in annual actual evaporation, surface runoff, groundwater recharge depict similar spatial patterns to those of rainfall, with the exception that surface runoff and groundwater recharge tend to increase in the Sahelian zone. Soil moisture and terrestrial water storage projections show a persistent decrease over most of the VRB area and particularly over the eastern regions. The analysis of projected future water availability per climatic zones in the VRB reveals an opposite pattern to the 345 "dry gets drier, wet gets wetter" paradigm (Byrne and O'Gorman, 2015;Greve et al., 2014). The dry regions of the VRB (Sahelian and Soudano-Sahelian zones) are projected to become wetter, while the wet (Sudanian and Guinean zones) are projected to become drier under RCP2.6 and RCP4.5. However, all climatic zones in the VRB are projected to become wetter under RCP8.5.
The spatial inter-model variability depicts different patterns across hydroclimatic variables and across RCPs and projection 350 periods ( Figures S44-S54). For RCP8.5, the highest differences among the simulations from climatic models are found generally in the northern dry regions (Sahelian zone) and in the forested southwestern wet region (Guinean zone) of the VRB

Change in high and low flows
The streamflow (Q) projections at the outlets of the sub-basins in the VRB are used for the analysis of high and low flows.
Here, high flows correspond to streamflow with a return period of 10 years, i.e. streamflow peaks that are equalled or exceeded for 10 percent of the time (Q10). Low flows represent streamflow that is equalled or exceeded for 90 percent of the time (Q90). 365 Therefore, Q90 and Q10 are the tenth and ninetieth percentiles of daily streamflow, corresponding to the 90% and 10% probability of exceedance, respectively. An increase in Q10 implies an increase in flood risk, whereas a decrease in Q90 represents a higher risk for river drought (Aich et al., 2014). The streamflow gauges are Bui-Amont, Daboya and Saboba at the outlets of the Black Volta, White Volta and Oti sub-basins (Figure 1), respectively. No gauge at the outlet of the Lower Volta sub-basin was available for the study. 370 The historical and future projections of high and low flows are illustrated in Figure S55. The projected future changes in high and low flows are summarized in Figure 13. Changes in low flows indicate an overall median increase in Q90 in all the subbasins under RCP8.5 as follows: Black Volta (+30%), White Volta (+16%) and Oti (+14%), with an RCM-GCM agreement on the direction of change of 69%, 63% and 57%, respectively. It is noteworthy that median Q90 is projected to decrease by -4% in the White Volta over the period 2051-2080 under RCP8.5. The trends in the median changes in Q90 are contrasted across 375 the future periods for RCP2.6 and RCP4.5. Table S1-S2 provide details on the direction of change between the RCM-GCM combinations, which in average varies between 50% and 78% for Q90, and between 56% and 75% for Q10, depending on RCPs and climate projection periods.
For high flows, Q10 is projected to increase in all the sub-basins under RCP8.5 on average by +20% in the Black Volta, +2% in the White Volta and + 6% in the Oti over the period 2021-2100, with an RCM-GCM agreement on the direction of change 380 of 65%, 63% and 59%, respectively. However, it is noteworthy that Q10 for the period 2051-2080 under RCP8.5 is projected to decrease by -14% in the White Volta and by -10% in the Oti. Under RCP2.6 and RCP4.5, a decrease in Q10 is projected from 2051 to 2100 in all the sub-basins. The highest median decrease in Q10 over the twenty-first century in the sub-basins is -6% in the Black Volta under RCP4.

Climate sensitivity
The sensitivity of selected hydrological variables to changes in climatic variables over the historical and future periods is illustrated in 395 Figure 14 (see Figure S56 for additional variables). The changes in hydroclimatic variables are estimated in percentage over the three future periods compared to the historical period. Larger changes in hydroclimatic variables are simulated with increasing radiative forcing levels over the future period.
The VRB illustrates highly nonlinear behavior. An increase in annual rainfall by 10% results in an increase in total runoff by ~50%, and an increase in actual evaporation by ~6%. A 10% decrease in annual rainfall leads to ~30% decrease in total runoff, 400 and ~7% decrease in actual evaporation. Annual actual evaporation represents 93% of annual rainfall, while total runoff represents 7% of annual rainfall. However, small changes in the rainfall regime might strongly affect the total runoff regime.
A 5% increase in annual average air temperature leads to an increase of ~3% in potential evaporation. However, heteroscedasticity is observed for changes in air temperature and potential evaporation as the variability increases with higher rates of change ( 405 Figure 14f). This analysis reveals a high sensitivity of major fluxes (Qrun and Ea) to even small changes in P in the VRB, a finding consistent with previous studies in the region (Roudier et al., 2014), and more generally in dryland regions globally (Berghuijs et al., 2017).

Discussion
A direct comparison of our results to previous studies in the region is restricted by the differences in the choice of RCM-GCM models, RCPs, future projections periods, and the baseline period, which alone might lead to different outcomes (Liersch et 415 al., 2020). However, our results generally corroborate with comparable previous studies in the region (Aich et al., 2016;Roudier et al., 2014;Sidibe et al., 2020). It is noteworthy that total runoff is projected to increase while rainfall decreases under the RCP2.6 and RCP4.5 during the period 2021-2050. This paradoxical phenomenon of rainfall-runoff negative correlation is commonly referred to as the "Sahelian paradox" (Mahé and Paturel, 2009). The percentage of agreement of 63% for hydrological projections between the climate models is similar to that of rainfall, which supports that rainfall is the key driver 420 of hydrological processes in the VRB, thereby confirming the findings of Roudier et al. (2014). Vetter et al. (2015) also found a high variability in rainfall projections among climate models in the Upper Niger basin. Therefore, the improvement of rainfall representation in climate models, which hardly represent the West African monsoon (Akinsanola et al., 2020;Philippon et al., 2010;Xue et al., 2010), would ultimately enhance the reliability of the assessment of climate change impacts on water resources (Dosio et al., 2021). The development of convection-permitting climate models to better capture complex climate features in 425 the region at higher resolutions would help Kendon et al., 2017;Kendon et al., 2019). The projected decrease of rainfall under RCP2.6 and RCP4.5, whereas an increase is projected under RCP8.5, can be explained by the higher warming level under RCP8.5. Higher warming would lead to increased evaporation, resulting in increased water vapour in the atmosphere, which would turn into increased precipitation (Donat et al., 2016;Trenberth, 2011).
The large ensemble of RCM and GCM datasets used in this study allows the quantification of model-related projection 430 uncertainties in terms of inter-model variabilities. In general, the selection of the best-performing climate models for hydrological modelling is not straightforward (Dosio et al., 2019;Hakala et al., 2019). Therefore, all the RCM-GCM combinations are used in this study without weighting or excluding models because the performance of models in the future is not necessarily related to their performance in the present (Dosio et al., 2020). More detailed analyses using methods for climate model selection (Kiesel et al., 2020;Ross and Najjar, 2019;Merrifield et al., 2020;Abramowitz et al., 2019;Ahmed et 435 al., 2019) with the updated CMIP6 models (Eyring et al., 2016) and the new Shared Socioeconomic Pathways (SSPs) (O'Neill et al., 2017;O'Neill et al., 2014;Riahi et al., 2017) are left for future work. For uncertainty dependence on ensemble composition, further attention should be given to single-model initial condition large ensembles (SMILEs) (Milinski et al., 2020;Maher et al., 2021).
A further potential limitation arises from the imposed stationarity of the dependences between variables related to the used 440 bias-correction method, although the used R2D2 method assumes some non-stationarity in the marginals (i.e., univariate distributions). However, predicting non-stationarity of dependence biases under climate change is not straightforward. With advances in multivariate bias correction methods (François et al., 2020), the added value of methods that consider nonstationarity in climate (e.g., dOTC; Robin et al., 2019) could be investigated in climate impact studies on water resources (Yang et al., 2021b). 445 Although multiple RCMs, GCMs and RCPs are used in this study, a single hydrological model is used for the hydrological projections. Therefore, the results are also subject to potential deficiencies of the mHM model, as hydrological models are known to be a source of uncertainty in climate change impact studies (Vetter et al., 2017;Hattermann et al., 2018;Mendoza et al., 2015;Giuntoli et al., 2015;Hagemann et al., 2013). For instance, mHM does not explicitly consider the effect of vegetation dynamics associated with climate change that would modify runoff processes, which can be a limitation that is also observed 450 in previous studies (Duethmann et al., 2020;Hanus et al., 2021;Wu et al., 2016). However, the mHM model used in this study has been thoroughly calibrated to provide realistic simulations of hydrological state variables and fluxes in the VRB (Dembélé et al., 2020a).
In addition to the hydrological model itself, the choice of the method for the calculation of potential evaporation can introduce uncertainties in the hydrological projections as reported in previous studies (Prudhomme and Williamson, 2013;Seiller and 455 Anctil, 2016). The lack of atmospheric coupling between purely temperature driven potential evaporation and the hydrological cycle in the hydrological model projections means some important feedbacks, such as surface resistance, may be missing (Milly and Dunne, 2016;Yang et al., 2019), and will not incorporate other mechanisms important in atmospheric demand.
However, it is worth noting that the importance of this missing feedback has thus far only been evaluated for conditions in which water availability is not a limiting factor, a condition that is rarely met in the VRB. Moreover, even a more suitable 460 method for estimating potential evaporation (e.g. Penman-Monteith), which can also be adjusted for surface resistance changes in future climates (Yang et al., 2019), is to the first order still expected to generate potential evaporation rates higher than precipitation. Therefore, the increase in soil moisture and actual evaporation will likely still be very large in response to the increase in precipitation. Nonetheless, the long-term CO2 -vegetation driven evaporation feedbacks not considered here could be very important for lower flow and drought conditions and need consideration in future work. The use of RCMs to downscale, 465 which generally do not have the appropriate CO2 -vegetation feedbacks in contrast to GCMs, is also an issue that needs to be addressed in future downscaling projects. Furthermore, the hydrological projections generated in this study focus on changes in climate and do not explicitly account for land use and land cover change or changes in water management practices in the VRB. Although, land use and land cover changes play an important role in the hydrological processes, the primary focus in this study is constraining the impact of 470 climate change alone. However, land use changes are assumed to be accounted for to some extent in the RCPs, as their development is based on assumptions regarding future evolution of land use and land cover (Van Vuuren et al., 2011).
Nevertheless, given the dominance of the evaporation response, future work accounting for potential land use and land cover changes in response to the climate scenarios provided here will be needed. Finally, large basins studies of climate change impacts on water resources need to consider human interactions with the hydrological cycle to better integrate the co-evolution 475 of the human-water systems across scales (Yang et al., 2021a). -Rainfall is projected to decrease under RCP2.6 and RCP4.5, while an increase is projected under RCP8.5, with a direct correlated and contrasting impact on water availability in the Volta River basin. Compared to temperature, there are more uncertainties in the trend of the changes in rainfall projections as there is only an agreement of 63% on the direction of change between the RCM-GCM models, which leads to more uncertainty in the prediction of hydrological variables. 490 -The seasonality of rainfall is projected to shift forward in the future, with the concentration period of the rainy season moving towards the months of August and September.
-Annual actual evaporation, total runoff, groundwater recharge, soil moisture and terrestrial water storage decline under RCP2.6 and RCP4.5, while they increase under RCP8.5 following the trends in rainfall. In fact, a strong sensitivity of hydrological processes to climate variability is found. 495 -The analysis of high and low flows suggests an increased risk for floods under RCP8.5 over the twenty-first century, while an increased risk for hydrological drought is projected under RCP2.6 and RCP4.5 from the mid-twenty-first century.
-The spatial projections of future water availability per climatic zones depict a "dry gets wet and wet gets dry" pattern under RCP2.6 and RCP4.5. 500 -Contrary to the other RCPs, under RCP 8.5, the projected climate changes lead to a clear intensification of the entire hydrological cycle, i.e. an increase in the magnitude of hydroclimatic variables.
The changes in the hydrological cycle have important implications for future floods and droughts in the Volta basin, thereby amplifying the vulnerability of the local population to climate change. These findings can contribute to the elaboration of regional adaptation and mitigation strategies of climate change. However, significant inter-model variabilities of the climate 505 models and low to moderate agreement between RCM-GCM combinations on the direction of changes highlight the complexity and uncertainties related to the assessment of climate change impacts on water resources. Therefore, more work is required to improve climate modelling in West Africa. A strong collaboration between climate and water resources scientists, practitioners and policymakers is key for advancing knowledge and development. Review statement. This paper was edited by Editor's name and reviewed by referees names(or anonymous). 535