Water demand and supply under future water development and climate change scenarios in the upper Blue Nile basin

Many of the world's major international river basins, which provide freshwater to billions of people and nature, may face increased stress in the coming decades (Zhang et al. 2018). This stress is frequently caused by changes in both water supply (as a result of climate change) and demand (Beck & Bernauer 2011).

Climate change is the supply-side driver, and it is expected to have a negative impact on the capacity of freshwater systems to meet water demands. It may indeed amplify water system variability by altering the water cycle, which determines how much water is available, when and where it is available, and ultimately how much we can use (Hublart et al. 2013). On the demand-side, population growth and higher per capita water consumption in the expanding urban, domestic, and industrial water sectors need sufficient water supply (Cosgrove & Loucks 2015). Indeed, global water resources are under threat due to incompatible relationships and the dynamic natures of water demand and supply.

To narrow the gap between water supply and demand due to the changing environment, a comprehensive water resources assessment and management tool is required because it considers the water supply and demand dynamics such as hydrological responses to climate change, allocation of water resources, agricultural economy, and other factors in designing water management options (Yimere & Assefa 2022). However, in many developing countries, the availability of hydrological data is a challenge, and as such, the water resource planning model may not be applied directly because it needs assumptions and input data from other models. This is the case with the Nile River basin, particularly in the upper Blue Nile basin, where most of the river sub-basins are ungauged (Touseef et al. 2021). The basin's water resources, on the other hand, are becoming increasingly stressed as a result of factors such as climate change, population growth, environmental degradation, and economic development needs, all of which pose challenges to water resource management and water security in the basin (Tibebe et al. 2022).

In the data-scarce basin, the integration of simulation models is required to estimate water supply and demand balance, evaluate scenarios, and develop effective water management strategies (Psomas et al. 2017; Touch et al. 2020; Izady et al. 2022), because hydrological models are reliable for simulating hydrological parameters at the basin level (Devia et al. 2015; Nyeko 2015; Lane et al. 2019). However, hydrological models, on the other hand, are not directly applied to water resource planning (Parra et al. 2018).

In terms of water resource allocation and management, with the exception of Ayele et al. (2017), Jeuland (2010), and McCartney & Menker Girma (2012), who analyzed the impacts of climate changes on water allocation and demand of the upper Blue Nile basin, literature focuses on only the impact of climate variability on hydrologic processes (Fentaw et al. 2018; Mengistu et al. 2021; Tariku et al. 2021) without considering the impact of data scarcity on the water management of the basin. Consequently, water allocation is evaluated for only a few sub-basins of the basin; to the best of the authors' knowledge, basin water allocation has not been fully investigated using water development scenarios.

This study attempts to fill this gap by integrating the SWAT hydrological model outputs with a water management tool, the Water Evaluation and Planning (WEAP) model. Since the upper Blue Nile basin is known for the scarcity of hydrological data and some major sub-basins are ungauged, the hydrological data for supply–demand analysis was derived from the SWAT hydrological model. SWAT is a trustworthy model for evaluating streamflow and water budgets at the regional scale (Akoko et al. 2021). To model water resource planning and management, water allocation was simulated using the WEAP model (Yates et al. 2005). WEAP can evaluate a wide range of water development and management options, accounting for multiple and competing uses of water systems (Abbas et al. 2022).

The paper is novel in that it uses the hydrological output of both the gauged and ungauged sub-basins of the basin as water resource inputs to the WEAP model, as well as to develop water supply and socioeconomic development scenarios. Previous research suggests a coupled model application that can predict multiple outcomes at multiple spatial and temporal scales for water resource allocation and management for the basin (Dile et al. 2018). In this case, the following questions were investigated: (1) How does climate change affect the water supply for irrigation and hydropower generation in the upper Blue Nile basin? (2) Is the available water sufficient to meet the basin's water demand? The findings of this study would give evidence for the benefits of employing hydrological model outputs as input to a water resource planning model for long-term water sustainability issues in the upper Blue Nile basin.

The physical characteristics of the reservoir (storage capacity, initial storage, volume–elevation curve) and operation rules (top of conservation, top of buffer, top of inactive, and buffer coefficient) were obtained primarily from the Ministry of Water and Energy of Ethiopia (MoWE) and the Eastern Nile Technical Regional Office (ENTRO). In addition, the planned reservoir data was collected from pre-feasibility, feasibility, technical reports, and Abbay River Basin Integrated Development Master Plan.

Irrigation data, including area (ha), yearly water consumption rate (Mm³), and monthly water need (Mm³) of each irrigation scheme, were collected from various sources. Data on existing irrigation schemes was obtained from the Ministry of Water and Energy of Ethiopia and previous reports and studies. The Abbay River Basin Integrated Development Mater Plan, ENTRO toolkits, ENTRO spreadsheet, feasibility studies, and earlier studies were used to determine the area of planned irrigation development schemes and water requirements for irrigation schemes. These data were then utilized to describe an annual activity level (i.e., irrigation area), the annual water use rate (i.e., the annual water user rate per annual activity), and the monthly water requirement variation (i.e., the monthly share of annual demand in percent).

The hydropower data for existing, under construction, and planned hydropower plants, such as installed capacity (GWh), discharge capacity (Mm³), head (m), maximum turbine flow (Mm³), tail water elevation (m), plant factor (%), power generation efficiency (%), and energy demand (GWh) were obtained from the Ministry of Water and Energy of Ethiopia, the Abbay River Basin Integrated Development Master Plan, the ENTRO Power Toolkit, ENTRO reports, and feasibility studies.

Proper modeling of the water supply and demand of the river basin is important and a prerequisite for decision-making. The present application of the WEAP model examines the interactions between current and future water supply and demand under the current and future climate and water development conditions of the upper Blue Nile basin. The Stockholm Environment Institute (SEI) developed the WEAP system (Sieber & Purkey 2015).

WEAP is a generic, deterministic water planning simulation modeling platform for managing water demand and supply, flows, storage, and discharge, as well as a number of other hydrological activities (Maliehe & Mulungu 2017). Among different IWRM models, the WEAP system is an exemplary application for connecting demand and supply-side requirements (Agarwal et al. 2019), by analyzing and simulating various water systems (Li et al. 2015). It uses an integrated approach to simulate water supplies, water demands, and environmental requirements, as well as the effects of policies on water quantity, water quality, and the ecosystem (Agarwal et al. 2019). This integration approach is used for the evaluation of specific water challenges within a larger framework. Integration encompasses supply and demand, water volume and quality, economic growth objectives, and environmental constraints (Yates et al. 2005). As a result, WEAP is well-suited for long-term water planning, confirming its role as a decision-support tool for selecting the best-performing future scenarios (Zerkaoui et al. 2018).

Natural hydrologic processes as well as anthropogenic activities are simulated in the WEAP model by balancing demand-side and supply-side dynamics. Correspondingly, all potential water uses, including rain-fed and irrigated agriculture, domestic, industrial, energy generation, and in streamflow requirements, are addressed on the demand side. It also employs a scenario-based strategy to predict water supply and demand, as well as their priorities for current and future periods (Fernández-Alberti et al. 2021).

Within WEAP, the spatial scope of the analysis must be defined by defining the catchment boundaries. Smaller rivers and streams flow into the main river of interest within these boundaries. Because these tributaries influence the distribution of water throughout the basin, it is necessary to divide the study area into sub-basins in order to characterize the spatial variability of river flows.

The basin was schematized based on current and future water supply resources and water development data. The current configuration includes five reservoirs, five hydropower schemes, and two large-scale irrigation schemes; 15 additional reservoirs, two hydropower schemes, and 19 irrigation schemes were added for the first planning horizon (the second configuration); and four additional reservoirs, three hydropower schemes, and three additional irrigation projects were added for the second planning horizon (the third configuration).

Model input parameters added after schematization include main river and tributary headflow, irrigation data such as irrigation area, monthly irrigation demand, water requirements per hectare, hydropower data such as maximum turbine flow, tailwater elevation, plant factor, power generation efficiency, hydropower priority, energy demand, reservoir data such as storage capacity, initial storage, reservoir zoning, and elevation–volume curvature.

The WEAP model optimizes water use in the basin by employing a linear optimization algorithm to allocate water to the various demand sites (DS) based on demand priorities ranging from 1 to 99, with 1 being the greatest priority (Abbas et al. 2022). Once there are insufficient water supplies for several demand points, the supply is prioritized to guarantee that the demand point is met since it is of greater importance and higher priority (Kou et al. 2018). The water supply priority for the upper Blue Nile basin was determined using the national water management policy introduced in 2006 (MoWR 2006). The primary national water policy objectives state that ‘priority must be given to providing basic human requirements and protecting ecosystems in the development and use of water resources’. However, taking into account the current state of agriculture productivity, this study emphasizes the irrigation sector. With these assumptions in mind, the upper Blue Nile basin's irrigation water use is prioritized first, followed by energy generation.

WEAP scenarios include assumptions about changes in the water resources system, such as changes in water use, hydrology, and water storage. In this study, the following assumptions are developed to model the water supply and demand for water management.

All reservoir developments are assumed to be online and operational during the specified scenario period; the transient stage (filling) and its short-term impacts are not taken into account. In the initial simulation condition, the water levels in all reservoirs in the system are assumed to be full. Irrigation land, water demand, and hydropower generation are expected to grow from 2021 to 2040 in the first term and from 2041 to 2060 in the second term. Because the study relied on future climate data, the basin's water supply fluctuated over the course of the study's time horizon. Taking these assumptions into account, future water demand scenarios based on the baseline situation were developed to investigate potential changes in the water supply–demand relationship.

The current account represents the present state of water resource development in 2020. Irrigation and hydropower schemes were already operational in this scenario. Water for domestic purposes, on the other hand, was not included in the current account period due to the absence of reservoirs needed to deliver water to the domestic sector. The hydrological data used to simulate the existing schemes was derived from SWAT output.

This scenario considered the large-scale irrigation schemes proposed for implementation in the first and second phases of the Abbay River Basin Integrated Development Master Plan period. In addition, one under construction project (GERD) and one proposed hydropower scheme (Karadobi). The simulation processes were carried out using the water supply generated under future climate conditions (2021–2040) as well as the ensemble Regional Climate Models (RCMs) under Repetitive Concentration Pathway (RCP) RCP4.5 and RCP8.5 climate change scenarios.

Based on an estimate of the maximum irrigation area, this scenario encompasses all large-scale potential future irrigation developments as well as all big hydropower projects in the main stream (including Mendaya and Beko Abo) and its tributaries (Lower Dedissa). This scenario considered all large-scale proposed irrigation schemes in the Abay River Basin Integrated Development Master Plan intended to implemented during the first, second, and third planning periods. Furthermore, this scenario assumes that all planned water resource development projects will be completed between 2041 and 2060. As a result, the simulations used the water supply generated under future climate conditions (2041–2060) and the ensemble RCMs under RCP4.5 and RCP8.5 climate change scenarios.

In many developing countries, the availability of hydrological data is a challenge, and as such, the WEAP model may not be applied directly because it needs assumptions and input data from other models. This is the case with the upper Blue Nile basin, where most of the river sub-basins are ungauged. Due to the capabilities of the hydrological and water resource planning models, researchers and decision-makers currently use a model integration approach.

In this study, previously simulated (Takele et al. 2022a, 2022b) hydrological outputs of the SWAT model were used as input for the WEAP system. The SWAT model provides the basin and sub-basin boundaries, as well as the river network, for WEAP configuration and location-specific hydrologic information across a river basin, which water resource planning models might employ for planning and water distribution (Psomas et al. 2016; Maliehe & Mulungu 2017; Touch et al. 2020; Dutta et al. 2021; Touseef et al. 2021; Abbas et al. 2022). The WEAP model is known for its performance in water resource management and planning, but a limitation of the model is that it is not best suited for detailed water balance component simulation in the watershed (Fernández-Alberti et al. 2021). To overcome this gap, it is preferable to use the SWAT results as input to the WEAP system since the hydrological output of SWAT is important in the WEAP system for examining the implications of an uncertain future on the basin's water demand due to irrigation and hydropower water requirements.

The integration of these models addresses the water supply data scarcity in unmeasured watersheds, and the water supply results from the SWAT model was utilized to drive the WEAP model, predicting water demand and supply under anticipated future climate change scenarios. Afterward, the WEAP model was used to assess water supply and demand relationships under the current and future climate conditions for water development and environmental flow scenarios.

Considering the upper Blue Nile basin encompassed both gauged and ungauged sub-basins, the calibrated and direct SWAT outputs were used as the main river and its tributaries' headflow. The river reach output, as the value FLOW_OUT extracted from SWAT output, was used as the headflow for the sub-basins (reaches). The SWAT outputs were subsequently linked to the WEAP model using the ReadFromFile wizard in the expression builder.

Once the system is described for the current account and the scenarios are defined over specified time horizons, water balance and allocation are calculated for each system component, such as river reach, reservoir, and DS. The results allow for an evaluation of the scenarios in terms of the current and future water demand for irrigation and hydropower generation. For each irrigation area per hectare, irrigation unit demands were analyzed using a disaggregated end-use approach, and the water demand for each demand node was computed.

Annual demand in the WEAP algorithm is:

The following indicators were used to assess the water supply–demand relationships in the study:

Water demand: The amount of water required at each demand location, excluding losses, reuse, and demand-side management savings.

Supply requirement: The amount required at each demand site after accounting for demand site losses, reuse, and demand-side management savings.

Supply delivered: The amount of water supplied to DS, specified either by source (supply) or by destination (DS). When listed by destination, the amounts reported are the actual amounts reaching the DS after any transmission losses have been deducted.

Unmet demand: The portion of each demand site's need that has not been reached. This report is valuable in determining the magnitude of the shortage when some demand places do not receive complete coverage.

Hydropower generation: The power generated by reservoirs and hydropower nodes.

The analysis was carried out at the basin level for irrigation and domestic and non-domestic supply and demand interactions, and at both the individual and basin levels for hydropower and environmental flow analyses. Furthermore, the unit of measurement for domestic and non-domestic, irrigation environmental flow is m³, and the unit of measurement for energy is GWh.

The upper Blue Nile basin's hydrological system was modeled using baseline water use (2020) and hydrological conditions. Reservoirs such as Lake Tana and Koga reservoirs in the Tana sub-basin; Finchaa, Amerti, and Neshe reservoirs in the Finchaa sub-basin; and two irrigation schemes that receive water from the Finchaa and Koga reservoirs were all included in the current account period. The Finchaa and Koga irrigation schemes have an existing irrigation land area of 26,412 ha, with annual water requirements of 178 and 55 Mm³, respectively. In these schemes, the total volume of water required for irrigation is 233 Mm³.

Table 1 shows the annual energy production of the basin during the baseline period. Tis Abbay I, Tis Abbay II, Finchaa, Neshe, and Tana Beles are the five hydroelectric plants under consideration in the current account period. According to the simulation, total annual energy production is estimated to be 3,159 GWh. Tana Beles has a maximum potential of 1,825 GWh, Finchaa has a capacity of 843 GWh, and Neshe has a capacity of 206 GWh. Tis Abbay I has a capacity of 62 GWh, and Tis Abbay II has a capacity of 223 GWh. Therefore, nearly all hydropower plants are expected to operate at full capacity.

The total irrigated area will increase significantly in this scenario due to the implementation of the first and second phases of the Abbay Basin Integrated Development Master Plan projects. The irrigation area is expected to increase from 26,412 to 259,746 ha between 2021 and 2040, with the annual irrigation demand increasing from 233 to 2,184 Mm³. The average annual water demand during this time period is 7,613 m³/ha, with the Lower Beles irrigation scheme having the highest average demand of 10,000 m³/ha.

Under the RCP4.5 climate change scenario, 1,917 Mm³ of water would be delivered for irrigation activity (supply delivery) from the total water requirement for irrigation (2,184 Mm³). This corresponds to meeting 88% of the irrigation water requirement. Under RCP8.5, the total amount of water delivered to the DS is estimated to be 1,971 Mm³. This is corresponding to 90% of the irrigation water requirement being met.

The maximum water requirement is expected in February, which is estimated to be 443 Mm³, while the delivered water for the required demand is 388 Mm³ under RCP4.5 and 395 Mm³ under RCP8.5. Although a large amount of water is required during the high-water demand season (November–May), the basin's water supply, however, did not meet the water requirements during the dry season.

The WEAP simulation for the near future development scenario revealed that annual unmet demand during this period is expected to be about 267 Mm³ under RCP4.5 and 213 Mm³ under RCP8.5, respectively. This equates to 12 and 10% of the basin's annual water demand, respectively. The increase in unmet demand is due to climate change's impact on the basin's water availability during the dry season as well as high irrigation water demand. This may result in increased pressure on the water resources of the basin and affect other water developments in the basin.

The energy production of the hydropower plants was simulated under the RCP4.5 and RCP8.5 climate change scenarios. Table 2 shows the annual energy production in the near future development scenario period under RCP4.5 and RCP8.5. The energy output of the hydropower plants is estimated to be 24,204 GWh under the RCP4.5 emission scenario and 25,303 GWh under the RCP8.5. The results correspond to 85 and 89% of their full potential, respectively (28,316 GWh). The two intended hydropower stations will contribute to a majority of energy production output. Under RCP4.5 and RCP8.5, the GERD station will produce 12,783 and 13,328 GWh, respectively, contributing more than 53% of the total production. The Karadobi stations also produce 8,731 and 9,168 GWh energy under RCP4.5 and RCP8.5, contributing approximately 36%.

This scenario takes into consideration the potential expansion of existing irrigation schemes between 2041 and 2060. According to the Abbay Basin Integrated Development Master Plan, the overall expansion in irrigation areas by the second phase is estimated to be 133,315 ha. Irrigation will thus cover around 393,061 ha of land until 2060. Annual irrigation water demand would rise by 50%, from 2,184 Mm³ in the near future scenario period to 3,276 Mm³ in the full development scenario period.

The total water requirement for irrigation was 3,276 Mm³ during the scenario period, and the total amount of supply delivered for irrigation demand was 2,682 Mm³ under RCP4.5. This corresponds to 82% of the irrigation water requirement being met. Under RCP8.5, the total amount of water delivered is estimated to be 2,665 Mm³. This is corresponding to 81% of the irrigation water requirement being met.

The amount of water that could not be physically delivered to demand nodes is expected to be approximately 594 and 611 Mm³ under RCP4.5 and RCP8.5 emission scenarios, respectively, in the years 2041–2060. The expected unmet demand is equivalent to 18 and 19% of the annual demand of the basin, respectively. This increase in unmet demand is attributable to climate change's influence on the basin's water resources during the dry season, as well as increased irrigation water demand. Furthermore, the main causes of the basin's highest unmet irrigation water demand are related to the extension of irrigation land in some schemes with on supply developments, they operated with the existing amount of water in their respective reservoirs. These would lead to severe water shortage problems.

Additional two mega and one medium-scale hydropower plants, namely Beko Abo, Mendaya, and Lower Didessa, were considered in this scenario. Table 3 depicts the annual energy production of the hydropower plants in the full development scenario period. Total annual energy production is predicted to rise from 24,204 to 44,921 GWh under RCP4.5 and from 25,303 to 44,539 GWh under RCP8.5. Under the RCP4.5, the energy output of the simulated hydropower stations in the upper Blue Nile basin will increase by 85% relative to that in the near future development scenario and increase by 76% under the RCP8.5.

The three intended hydropower stations would be expected to add 22,414 and 21,268 GWh of hydropower energy under RCP4.5 and RCP8.5, respectively. Under the RCP4.5 and RCP8.5 scenarios, the Beko Abo station will produce 10,951 and 10,767 GWh, respectively, contributing 24% of the energy production under RCP4.5 and RCP8.5. The Mandaya stations also produce 9,522 and 8,524 GWh of energy in RCP4.5 and RCP8.5, contributing 21 and 19% of total energy production, respectively. Lower Didessa stations additionally generate 1,941 and 1,977 GWh of energy under RCP4.5 and RCP8.5, respectively, contributing 4% of total energy production.

The overall analysis of water demand for and irrigation uses would change in the current and planning periods, with the water requirement increasing. During the current account period, the amount of water delivered for irrigation water requirements was balanced. However, the pattern of water demand and supply for irrigation purposes will alter beyond the current account period. The results of the simulations for the corresponding scenario periods reveal that the amount of water delivered for irrigation water demand varies based on the climate change scenarios (RCPs). Water delivered for irrigation water requirements under RCP8.5 is slightly better than water delivered under RCP4.5 in both scenario periods. This is because the basin's inflow is marginally increasing under the RCP8.5 climate change scenario.

The findings also demonstrate seasonal fluctuations in water demand and delivery. Water demand and water delivered to water users, particularly for irrigation, are higher in the dry season than in the wet. This seasonal variation is caused by irrigation intensity as well as the effects of climate change in the basin (especially for water supply). As a result, an immense quantity of water would be required throughout the dry season (December–May).

According to the total analysis for irrigation demand, irrigation water unmet demand will reach 267 and 213 Mm³ in the near future development scenario, and 594 and 611 Mm³ in the full development scenarios under RCP4.5 and RCP8.5, respectively. There are two reasons for the increase in unmet demand in the scenario periods. The first is increased water demand as a result of increased irrigation expansion, while the second is decreasing inflow to the basin as a result of climate change, particularly during the dry season. Various studies have confirmed a decrease in streamflow during the dry seasons (Malede et al. 2022). This is due to the basin's reputation for significant changes in streamflow related to variations in rainfall (Tariku et al. 2021; Worku et al. 2021). During the dry season, the combination of high irrigation intensity and low streamflow greatly contributed to the basin's unmet irrigation water demand.

Irrigation activities are likely to have a higher level of unmet demand. Thus, the results show that both irrigation expansion and climate change significantly contributed to the unmet demand for irrigation water in all scenario periods. The findings of this study are consistent with the findings of other studies, which demonstrate that projected losses in annual streamflow owing to climate change and rising water demand will result in water shortages for irrigation and scarcity for other uses (Orkodjo et al. 2022). According to Ougougdal et al. (2020), as water demand and unmet water demand increase in the future, so will the pressure on water resources, leading to water scarcity. Wijngaard et al. (2018) also noted that the combination of climatic change and socioeconomic growth is predicted to result in increasing water gaps, but socioeconomic development is the major driver in the evolution of the water gap. Furthermore, Adgolign et al. (2016) concluded that increased irrigation area contributed to unmet water demand for DS.

Water scarcity is most prevalent during the dry season, which lasts from January to May for all scenario periods and climate change scenarios. The greatest unmet demand is seen in April, the driest month of the year, with a slight recovery in May. This means that, except from the peak rainy season, water needs are not entirely supplied. Other research found a larger unmet water demand in their findings. According to Ahmed et al. (2021), unmet demand is higher during the dry season because water supply decreases while crop intensity increases. Similarly, according to Wang et al. (2019), seasonal scarcity, which occurs every year from April to June, has resulted in higher water pressure. Further, a decrease in precipitation and streamflow during the hot and dry seasons will almost certainly exacerbate future water shortages (Orkodjo et al. 2022). The water scarcity occurred primarily during three dry-seasonal months (January–March), with the peak of unmet demand occurring in February. These decreases in water shortage are the result of increased dry-seasonal river discharge. Water scarcity under RCP4.5 and RCP8.5 may still occur in the three dry-seasonal months of January–March (Khoi et al. 2021).

In general, the temporal fluctuation in unmet demand reflects changes in water availability as a result of climate change and water use as a result of irrigation intensity in the basin. Therefore, the overall results directed to the requirements of water resource management in order to increase water supply, minimize water demand, and reduce unmet demand, because the results of future unmet demands are important for formulating sustainable water management strategies in the river basin (Agarwal et al. 2019).

The basin's hydropower production from existing and proposed large-scale plants will be increased. In the current scenario, the basin can produce all of its hydropower capability. Despite the fact that planned large-scale hydropower schemes show an increasing trend in the future scenario period, the hydropower plants will not produce at full capacity, due to increased water demand in the irrigation sector, as well as variability in inflow into the basin due to climate change.

Climate change, in particular, will have an impact on the basin's energy generation. The availability of water through runoff determines energy generation, and variations in runoff affect energy production. When the basin's river inflow decreases in the future, hydropower generation within the basin will reduce, as seen in the near future and full development scenarios. As a result, hydropower is vulnerable to climate change (Wei et al. 2020). The study undertaken by Mtilatila et al. (2020) indicates the impacts of climate change on annual energy production; they conclude that due to climate change and reduced streamflow, hydropower productivity and dependability will decrease. According to their findings (Jakimavičius et al. 2020), changing climate and the resulting runoff patterns add unpredictability to energy generation.

This study's findings also reveal that there is temporal variation in energy generation as a result of discharge variance. The wet season would produce maximum energy (in terms of amount), whereas winter would produce the least. These findings are consistent with the findings of Fant et al. (2015), who identified that hydropower production decreases due primarily to predicted decreases in runoff and upstream irrigation demands. Hence, reservoir inflow and energy generation potential are projected to diverge over time, with dry scenarios increasing drier and wet scenarios growing wetter, resulting in significant basin climate sensitivity and uncertainty with energy generation potential (Kim et al. 2022). Furthermore, seasonal variations in streamflow caused by climate change will shorten the peak time of energy generation and decrease the peak value (Wei et al. 2020). In general, changes in energy output can be attributed to changes in stream pattern and rainfall-runoff regime, which are driven by changes in precipitation pattern and temperature (Beheshti et al. 2019).

The output of the SWAT hydrological model for upper Blue Nile basin sub-basins is used as input to the WEAP model for this study to investigate the interaction between water demand and supply under climate change, and water resource development. The current account period WEAP simulation depicts the balance of water supply and demand for existing irrigation and hydropower systems. As a result, there is no unmet irrigation demand, and hydropower units are operating at full capacity. On the other hand, future water demand for irrigation, and hydropower would be deficient.

The near future scenario of 259,746 ha of irrigation development may result in:

• Irrigation water requirements reach 2,184 Mm³ at the basin level.

The near future scenario water supply under RCP4.5 and RCP8.5 climate change scenarios may result in:

• 1,917 and 1,971 Mm³ of water were delivered for irrigation, respectively, under RCP4.5 and RCP8.5.

• Irrigation water unmet demand up to 267 and 213 Mm³.

• Hydropower generation increased to 24,204 GWh under RCP4.5 and 25,303 GWh under RCP8.5.

The full development scenario of 393,061 ha of irrigation development may result in:

• Irrigation water requirements reach 3,276 Mm³ at the basin level.

The full development scenario water supply under RCP4.5 and RCP8.5 climate change scenarios may result in:

• 2,682 and 2,665 Mm³ of water were delivered for irrigation under RCP4.5 and RCP8.5, respectively.

• Irrigation water unmet demand up to 594 and 611 Mm³.

• Hydropower generation increased to 44,921 GWh under RCP4.5 and 44,539 GWh under RCP8.5.

Therefore, the water resource deficit for socioeconomic and environmental needs is caused by two factors: the first is a high-water requirement for irrigation due to irrigation expansion, and the second is climate change, which affects annual and seasonal streamflow fluctuations. Due to climate change and irrigation water requirements, the proposed hydroelectric schemes will not operate at full capacity. Climate change, in particular, will have an effect on the availability and fluctuations of water through runoff, influencing energy generation. As a result, the wet season would generate the most energy (in terms of amount), while winter would generate the least. All of these findings show that the system's behavior is highly uncertain and sensitive to assumptions about future climate change scenarios; thus, there may be more opportunities for decreasing water scarcity and increasing water security through water management measures in the basin.

We thank the Ethiopian Ministry of Water, Irrigation, and Energy and Ethiopia's National Meteorological Service Agency (NMSA) for providing hydrometeorological data for the study region.

The study's inception and design were collaborative efforts by all authors. G.S.T., G.S.G., and A.N.E. prepared the materials, collected the data, and analyzed it. G.S.T. wrote the first draft of the manuscript, and all contributors provided feedback on prior drafts. All authors read and approved the final version of the manuscript.

Open access funding is provided by Addis Ababa University, the office of the vice president for research and technology transfer and Debre Birhan University.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.