Impact of climate change on the hydrology of Blue Nile basin, Ethiopia: a review

Climate change is a change of climate in its mean properties which persists for a long period of time (Abdo et al. 2009). According to the Intergovernmental Panel on Climate Change (IPCC), the increase of greenhouse gases results in climate change, which in turn leads to heavy rainfall, extreme drought, and sea level rise. Nowadays, the issue of climate change has become a global agenda (IPCC 2008). Greenhouse gas concentrations in the atmosphere have increased since 1750 and are the main cause of global warming (Ayele et al. 2016). The concentration of carbon dioxide (CO₂) rose to 400 ppm during 2015 and is projected to reach from 470 to 1,099 ppm by the year 2100 (Prentice et al. 2001). The average temperature from the ocean and land surface has increased by 0.85 °C from the end of the 19th century to 2012 (Ayele et al. 2016). This change may be because of both internal and external processes like volcanism and increased solar radiation which occurs naturally and could contribute to the natural variability of precipitation, temperature, and other extreme weather events. Anthropogenic activity is the main cause of these external effects which has resulted in changing atmospheric composition since the industrial revolution. It affects all infrastructures and natural ecosystems. In the present time, climate change has become the major threat to water resources. It negatively affects certain components of the water cycle, particularly precipitation, evapotranspiration, infiltration, and runoff (Sridhar et al. 2018, 2019). Thus, climate change alters the variability and availability of water over space and time (Melaku Melese 2016).

The life of all human and natural ecosystems depends on the availability and quality of water. Climate change will increase the number of the global population who are living in regions where there is acute water stress (Bates et al. 2008). The impact will be worse for countries like Ethiopia where 84% of the population depends on rain-fed agricultural production (Central Statistics Agency 2007). Much research has shown that the increment of greenhouse gas concentration in the atmosphere results in changing of global climate, increasing of temperature, and alteration of the amount and distribution of precipitation. The variability and the availability of fresh water are adversely affected by climate change. According to the IPCC (2012), the increase of global warming, extreme climate events like drought, heat waves and flooding occur more frequently. Climate change increases uncertainty in water availability which negatively affects agricultural production, threatens the environment, and results in a socioeconomic problem.

The Blue Nile River (BNR) originates in Lake Tana, Ethiopia and is shared among Ethiopia, Sudan, and Egypt. These three countries have more than 200 million people who are heavily dependent on water availability from the River Nile for their survival (Keith et al. 2014). It is the largest of the 12 basins in Ethiopia and accounts for 55% of the annual renewable surface water resources of the country (King 2013). It plays a vital role in the hydrology of the riparian countries. Although the Blue Nile basin covers only 10% of the entire Nile basin area (Conway 2005), about 85% of the total streamflow of the River Nile reaching Egypt and Sudan is generated from Ethiopia (Roth et al. 2018). Thus, the streamflow variability of Ethiopia has great importance to water development in Egypt and Sudan. This hydrological imbalance among the three countries has been the cause of dissatisfaction and controversy among them. The potential climate change impact on the Blue Nile basin has become a particular concern due to its socioeconomic and geopolitical implications (Niang et al. 2014). The availability and quality of water in the basin is adversely influenced by climate change (Gebre & Ludwig 2015), which in turn, is threatening the life of people and life-supporting systems. The effect will be even worse in the future. The flow of the basin is estimated to be reduced in the coming decades due to the increasing withdrawal of water for irrigation, evapotranspiration, and declining precipitation (due to climate change) (McCartney et al. 2013). Beyene & Meissner (2010) indicated that the streamflow of the Blue Nile basin will decline after 2050 because of declining rainfall and increased evaporation. The outflow from Lake Tana is expected to decline by 2080 (Abdo et al. 2009), which in turn reduces the streamflow from the main river as Lake Tana is the main source of water for the whole basin. In agreement with this, Taye et al. (2011) also reported that there is a possible change in the streamflow of the Blue Nile river basin.

The lack of review papers on the assessment of climate change impact on water resources, especially in transboundary river basins like the Blue Nile basin, led to the collective preparation of the current review paper, which is an updated evaluation of the climate change effect on the different components of the hydrology of the Blue Nile basin. There have been a few studies conducted on the hydrological response of climate change in the different basins of Ethiopia. To the authors' knowledge, there are two review papers, Taye et al. (2015) which only focuses on the effect on the hydrological extremes and Goulden et al. (2009) which concerns adaptation mechanisms for climate change on the water resources of Africa. There are no review papers on the Upper Blue Nile River that assess and cite literature on the impact of climate change on water quality, extreme events, historical trend, and future impact, and suggest adaptation measures. The advances in climate and hydrological modeling achieved through climate change models have largely outstripped conventional models in terms of performance and led to an increase in associated research and resulting publication numbers. The major contribution of the current review paper is to comprehensively analyze and cite the impact of climate change on the different components of the hydrology of the Blue Nile river basin. In turn, this assessment will provide fresh ideas with respect to future research in the area of climate change impact. Moreover, the analysis and information from the current review paper can contribute to the implementation and development of effective hydrologic forecasting (owing to climate change) and thus, applicable precautionary measures. Therefore, the objective of this review paper is to assess the climate change impact on water resources of the Blue Nile river basin.

The livelihood of hundreds of millions of people of Ethiopia, Sudan, and Egypt are dependent on water availability in the Blue Nile basin. However, frequent droughts and floods, due to climate change, have rendered millions vulnerable to their impacts as the country's main economy is dependent on rain-fed agriculture. Studies by Melesse (2011) and Soliman et al. (2009) revealed that climate change would occur in the Upper Blue Nile basin, Ethiopia which would, in turn, reshape and shift the seasonal and annual climate patterns and cause erratic rainfall and variation, and reduce reservoir yield. The Upper Blue Nile basin was chosen as the case study for the following reasons: (1) it covers a wide climatic zone and is highly affected by climate change; (2) it is highly used for hydroelectric generation and irrigation so the effect on its water resources in Ethiopia will have an adverse impact on these sectors; (3) several towns and hundreds of million of people depend on the water availability of the basin and industrial enterprises lie within the basin. It is also chosen due to the fact that the Blue Nile river basin is the place where most Ethiopian water resource developments and projects are undertaken (e.g., Grand Renaissance dam and other projects). Therefore, it is critical to determine the potential impacts of climate change for sustainability of the project impacts and look for possible mitigation measures otherwise all the costs incurred will be lost in failing to meet the objectives.

The water resources system and hydrology in the basin are significantly affected by climate change. Extreme hydrological events like flood and drought at local and basin-wide scale are common in the basin (Taye et al. 2015). The impacts of climate change on water resources are highly significant because of the fact that all natural and socioeconomic systems critically depend on water (Mekonnen & Disse 2018). Climate change can directly affect the hydrology by changing the pattern and variation of the hydrologic cycle and causing droughts and floods. It also indirectly affects energy, food and agricultural production. This impact may be worse on transboundary rivers like the Blue Nile River where there is increasing competition for water from riparian countries (Kim & Kaluarachchi 2009). In the basin, rainfall is highly seasonal and erratic because of climatic variability and atmospheric drivers (Berhane et al. 2014).

Kim et al. (2008) examined the influence of climate change on the hydrology of the Blue Nile River. This study used six scenarios in predicting the effect of climate change on hydrology. The results of this study revealed that the rainfall from the six models changed from −11% to 44%. This indicates that rainfall in the basin is highly erratic and seasonal. Annual temperature is estimated to change by 2.3 °C when the weighted average scenario is used. The average potential evapotranspiration increased by 16% due to increased temperature. All of this shows that flow and water availability in the basin is decreasing from time to time.

Wale Worqlul et al. (2018) investigated the effect of climate change on streamflow of two sub-catchments in the upper part of the basin. According to this study, the predicted minimum and maximum temperature are expected to increase by 3.6 °C and 2.4 °C, respectively, at the basin level. Evapotranspiration in the basin is predicted to increase by 17.8% in 2100. The seasonal streamflow in the basin is expected to increase by 64% during dry seasons and decrease by 19% during wet seasons. That means during the wet season (June–August) there will be more of a flooding hazard while there is drought in the dry season.

Drought and flooding are the two natural hazards which are occurring more frequently in different climatic regions of the world (Thilakarathne & Sridhar 2017). Extreme flood is occurring more frequently due to climate change each year and causes human suffering and huge economic damage in different parts of the world (Chau 2017). Flooding and drought are the two most common natural phenomena in the Blue Nile river basin and create food insecurity and other complex social problems (Enyew et al. 2014). Severe drought occurs in every ten years in northern Ethiopia. High temperature, floods, and droughts have a further adverse impact on water quality and quantity (Delpla et al. 2009). These droughts will happen more frequently because of climate change. Flooding processes are influenced by a variety of non-climatic and climatic factors. Non-climatic factors includes soil type, slope, and antecedent soil moisture. Flood is produced by intensive and long-lasting rainfall, dam collapse, landslide, and snow melt. Climate change is altering rainfall which significantly affects the socioeconomic life of the people in the basin. Excessively low rainfall causes drought, while intensive rainfall leads to flooding (Zaroug et al. 2014). Climate change impact on the intensity and severity of future drought can be assessed using simulated meteorological and hydrological variables, temperature and precipitation projections, different drought indices, and various hydrological models (Kang & Sridhar 2017a, 2017b). By increasing the occurrence and intensity of flood and drought, climate change has a significant impact on the economy. Climate change causes the temperature to rise, which in turn increases crop water consumption and higher rainfall alone may not be sufficient (Sridhar 2013). Therefore, irrigation water requirement (due to higher ET) is expected to increase in the future (Hoekema & Sridhar 2013). Rain-fed rice yield in the Songkhram River basin of Thailand may reduce by up to 14% in the 2080s (Boonwichai et al. 2018). Agricultural production in northern Virginia, USA is projected to decrease up to 22% due to drought (Kang et al. 2019).

Zaroug et al. (2014) analyzed the occurrence of flooding and drought in the Upper Blue Nile catchment on La Niña and El Niño events. They used the El Niño index for analyzing the occurrence of this extreme hydrological event. For this study, 47 yearly (1965–2012) discharge data were used as the input for modeling. The results of this study indicated that when La Niña follows after the El Niño event, the occurrence of extreme flood in the basin reaches up to 67%. They also evaluated the occurrence of drought in association with El Niño using global models. It was indicated that the likelihood of significant drought occurrence was 83% when an El Niño event starts from April to June. A study by Bisht et al. (2018) in India also evaluated the characteristics of drought in different timescales in comparison with a reference period over future climatic scenarios. They used a Standardized Precipitation Evapotranspiration Index (SPEI) over India. The results of the study indicated an increasing trend in drought average length, duration, severity, and occurrences under warming climate scenarios. Moreover, the area under ‘above moderate drought’ conditions was also found to be increasing in projected climate scenarios.

Enyew et al. (2014) also assessed climate change impact on hydrological drought in Lake Tana and another four sub-basins of the Blue Nile River. The study used Hydrologiska Byråns Vatten balansavdelning (HBV), a rainfall–runoff model, to investigate climate change impact on drought on the indicated basins. This study found that temperature and evapotranspiration are expected to increase in the future while there is a significant reduction in rainfall and hence there is a more frequent drought in the basin. In agreement with this, Kang & Sridhar (2017a) used Soil and Water Assessment Tool (SWAT) and a variable infiltration model in Chesapeake Bay watershed to simulate different drought indices for both future and historical periods. They found that there will be an increase in agricultural drought in the entire basin. The reason for the increment was evapotranspiration and ground and surface flow.

Bayissa et al. (2015) investigated the spatial and temporal variation of meteorological drought in the Upper Blue Nile basin (Ethiopia). The study used Standard Precipitation Index (SPI) for comparison of the effect of using a different length of historical data on drought category. It also used short historical data (1975–2009) from 23 stations and long recorded data (1953–2009) from 14 meteorological stations as impute for the model. The result revealed that in the years 1978/79, 1984/85, 1994/95, and 2003/04 there were extreme meteorological droughts in the basin.

Impacts of climate change like desertification, drought, coastal flooding, and other extreme weather events could adversely affect food supply in developing countries (Immerzeel et al. 2012; Wagena et al. 2016). Climate change affects the water budget components and climatic variables, which in turn have a negative impact on the sedimentation of the water body (Peizhen et al. 2001). It alters and causes extreme precipitation and results in high sediment yield (Cousino et al. 2015; Bussi et al. 2016). In addition, climate change accelerates the rate of soil erosion by increasing the erosive power of wind and rainfall (Adem et al. 2015). This, in turn, creates a sedimentation problem in a water body (Nearing et al. 2004).

According to IPCC (2012) projections, temperature, rainfall, and evapotranspiration will increase in East African countries in the coming decades. Sediment yield is also caused by the change in land use over the catchment. Climate change impact on hydrology and water resources has been extensively studied (Immerzeel et al. 2012; Sridhar & Jin 2013; Cousino et al. 2015; Ayele et al. 2016; Mekonnen & Disse 2018). Yet, comparatively little attention has been given to its likely impact on sediment yield. Soil erosion and sediment transportation are accelerated by climate change, land use type, and other human activities due to their close link with the hydrology of a given catchment (Walling 2009). However, in this study, only the sediment yielded due to climate change is considered. Adem et al. (2015) found that both precipitation and temperature in the Upper Blue Nile basin will increase in the future. This study used A2 and B2 scenarios for future prediction. Due to the increases in climate variables, the resulting mean annual sediment yield of the Upper Blue Nile River was predicted to increase by 11.3%, 16.3%, and 21.3% in the 2020s, 2050s, and 2080s, respectively. This increment in sediment yield of the basin is doubled due to the increase in the runoff because of climate change.Wagena et al. (2016) also studied the effects of both climate change and land use on streamflow and soil erosion in the Gilgel Abay catchment from 2011 to 2025. The study used a SWAT model for analysis. The results of this study indicated that the average sediment load in this catchment will increase by 29%.

Adem et al. (2015) also investigated sediment yield with the response to climate change in the Gilgel Abay catchment using A2 and B2 scenarios. The result showed that sediment yield is estimated to increase by 21.3% and for B2 and A2 scenario, respectively, by 2100. This study also predicted the impact of climate change on the monthly sediment yield for the 2020s, 2050s, and 2080s.

Wagena et al. (2016) also studied the impact of climate change on water resources and sediment transport in the Blue Nile basin. Their study showed that the mean annual sediment concentration and annual flow in Tana and Beles basins is expected to increase 16–19% and 22–27%, respectively. Adem et al. (2015) modeled the contribution of climate change to sediment concentration on the Blue Nile basin using HadCM3 and SWAT models. The results showed the increase in downscaled temperature and precipitation in all future times for both B2 and A2 emission scenarios. These increments are expected to result in an increase of sediment yield by 11.3, 16.3, and 21.3% for A2 scenario and by 11.0, 14.3, and 11.3% for B2 scenario for the 2020s, 2050s, and 2080s, respectively.

Sediment yield at the basin level is generally related to precipitation and streamflow. According to Rodríguez-Blanco et al. (2016), the suspended sediment response to climate change generally follows the patterns of simulated changes in streamflow. The study used the SWAT model to project CO₂ concentration, rainfall and temperature impact on medium- (2031–2060) and long-term (2069–2098) sediment yield in Spain.

Based on their findings, sediment yield is expected to decrease by 11% and 8% in the years 2031–2060 and 2069–2098, respectively, mainly due to a decrease in streamflow. Nevertheless, an 11–17% increase in sediment transport is projected in winter due to increased flow and soil erosion.

Studying the availability trends of temperature, precipitation, and streamflow are generally perceived as a statistical approach to detect non-stationarity in time arrangement (Taye et al. 2015). Many studies on the trends in streamflow, temperature, and precipitation have been conducted in the Blue Nile river basin (Gebremicael et al. 2013; Taye et al. 2015). For example, Gebrehiwot et al. (2010), Gebremicael et al. (2013), Kebede (2009), Rientjes et al. (2011), Legesse et al. (2003), and others studied and analyzed the long-term trend of streamflow in the Blue Nile river basin. Nevertheless, there was no consensus in the conclusions among those studies. Gebremicael et al. (2013), Legesse et al. (2003), and Rientjes et al. (2011) indicated that there was an increasing trend of annual flow in the Blue Nile River, whereas Gebrehiwot et al. (2010) and Kebede (2009) revealed that there was a decreasing annual and seasonal flow at the basin. It can be understood that most of these studies focused on total streamflow and precipitation on an annual and seasonal basis with limited consideration of extreme conditions such as extreme heat. Therefore, this paper tries to include what is missed by other studies.

Gebremicael et al. (2013) studied the trend of total annual precipitation of the Blue Nile river basin from 1970 to 2009. This study found out that there is little change in average annual rainfall of eight out of nine stations in the Upper Blue Nile basin. Only one station out of the nine showed there were significant changes in annual precipitation. Tekleab et al. (2013) also investigated the total monthly and annual scale precipitation in Kiremt (rainy) season between 1970 and 2010. It was found that there were no statistically significant trends in the average annual and seasonal precipitation over 13 stations within the basin, whereas the mean annual temperature over the basin varies 10 °C over Ethiopia.

Seleshi & Zanke (2004) analyzed change in the annual rainfall of 11 stations located in different Ethiopian climatic regions over 1965–2002. They used the progressive Mann–Kendall trend test for analyzing the trend. The result showed that there was no total seasonal and annual rainfall change trend over northwestern, northern, and central Ethiopia. Tesemma et al. (2010) also investigated the trend of average monthly runoff and rainfall of the Blue Nile river basin over the period 1964–2003. The result of this study showed that there was no statistically significant change trend in the annual and seasonal precipitation in the basin. However, there was a significant increasing trend in runoff during the rainy season (June–September).

Mengistu et al. (2014) analyzed the temporal and spatial variability and trend of seasonal and annual temperature and rainfall in the Blue Nile basin of Ethiopia from the year 1981 to 2010. Regression line slope with the least squares method was used to evaluate the trends. In this study, the statistical significance of the trend was determined using F-distribution. The result revealed that there was a significantly increased (33%) trend in the annual minimum and maximum temperature of the basin, but the minimum temperature was increasing at a higher rate as compared with the maximum temperature. There was no substantial increasing trend shown in the seasonal and annual rainfall except spring season. The rainfall during the spring season showed a not significant decline (11%) trend in the basin.

Table 1 summarizes the surveyed studies.

Currently, many researchers have tried to analyze the effect of anticipated climate change on the availability and quantity of water (Goulden et al. 2009). Many studies have been conducted to analyze the climate change effect on hydrology and water resources of the River Nile basin (Di Baldassarre et al. 2011). All of them concluded that in the future, the basin will be vulnerable to acute water stress due to reduced water availability and excessive withdrawal of water for different purposes (Mccartney et al. 2013). It was estimated that Nile River flows will first increase until the mid 21st century, and then begin declining under different scenarios due to the fact that evaporation is increasing while rainfall is declining (Beyene & Meissner 2010). Variation in streamflow due to climate change has a direct effect on sediment yield of the catchment. Therefore, it is imperative to model the river flow using the latest methods (Yaseen et al. 2019). According to Wale Worqlul et al. (2018), the streamflow of Gilgel Beles River will decrease by 19% in wet seasons and increase up to 64% in dry seasons at the end of the century. A study by Seong & Sridhar (2016) in Chesapeake Bay watershed also showed a decreasing streamflow at the end of the 21st century compared to the baseline period.

Ayele et al. (2016) studied the water resources availability of Gilgel Abay watershed under future climate change. According to this study, the projected temperature has increased on average by 1.6 °C by the years 2021 to 2040 and by nearly 4 °C during the period 2081–2100, as predicted using different global climate models. This study also predicted that the rainfall in Gilgel Abay watershed is expected to decrease during the dry season by 4–25% and increases 5–23% during the wet season, whereas evapotranspiration is expected to increase by 23% and 6–19% during the dry and wet season, respectively, in the years 2020 to 2080. Evapotranspiration is expected to increase more than this due to increased temperatures. In agreement with this, a study by Seong et al. (2017) in Susquehanna watershed showed a decrease of evapotranspiration up to 5.5% when projected by using the Hamon (HM) method and an increase of up to 3.6% with the Hargreaves (HG) method. Jaksa & Sridhar (2015) also studied the long-term evapotranspiration change in a human-dominated river basin system. The results of this study showed that evapotranspiration has increased by 0.78 mm each year for the last 30 years. They mentioned that rising temperature due to climate change is the possible cause for increasing evapotranspiration. Moazenzadeh et al. (2018) also studied the effect of increased solar radiation because of climate change on evapotranspiration in Iran, applying the firefly algorithm coupled with support vector regression using ten years of data. This study found that evapotranspiration had the highest correlation with net solar radiation. One of the effects of climate change in water resources is that it increases the evapotranspiration. Therefore, it is imperative to model the hydrologic process (e.g., runoff) using the different latest hybrid artificial intelligence models (Wu & Chau 2011). Ghorbani et al. (2018) used an integrated artificial neural network quantum-behaved particle swarm optimization model to forecast pan evapotranspiration. The results of this study confirmed that the hybrid MLP-QPSO outperforms MLP-PSO and the standalone mode in accurately forecasting pan evapotranspiration. Sehgal et al. (2016) applied multiresolution Volterra models and multi-scale wavelet entropy for climatic downscaling (GCM) variables for monthly precipitation in different stations in India. The study found that the proposed wavelet-based multi-resolution models perform significantly better as compared to the artificial neural network (ANN) and traditional multiple linear regression (MLR) models.

Abdo et al. (2009) investigated the hydrological response of climate change in Gilgel Abay catchment. In Gilgel Abay catchment (Lake Tana), the projected mean annual streamflow variation is smaller compared to rainfall seasonal variability. The average annual streamflow is expected to reduce by 2.6% to 2.9% in the 2080s. However, the monthly and seasonal variation shows a very significant variation. The runoff during the rainy season is projected to reduce by 10.1–11.6% in the 2080s. Regarding monthly runoff, it is estimated to reduce by 59–66% and 16–20% in June and July, respectively, during the 2080s, whereas the monthly runoff in August is expected to increase from the 2050s to 2080s. A 5% increase in stream streamflow due to climate change was also shown in the Mekong River basin (Sridhar et al. 2019). A similar study was done by Sridhar & Jin (2013) in Salmon River basin and also revealed that streamflow is expected to decrease by 3% in the next 90 years while the temperature in the basin rises by 3–5°.

Setegn et al. (2011) also revealed that the actual evapotranspiration in the Blue Nile river basin will increase by 11.55% and 12.5% by 2045–2065 and 2080–2100, respectively. According to McCartney et al. (2013), the temperature in Blue Nile river basin was 20.9 °C and is expected to rise to 24.9 °C by the years 2071–2100. According to this report, the increment in average annual temperature and evapotranspiration is increased by 19.1% and 12.6%, respectively, while the rainfall is expected to decrease by 15.27% in 2100, as shown in Figure 1.

Jin & Sridhar (2012) studied the impact of climate change on the water resources of two river basins (Spoknane and Boise river basins) using SWT model and global climate models. Based on their findings, a substantial variability in evapotranspiration, precipitation, and temperature is expected in both river basins. Temperature is expected to rise by 3.5 °C, whereas a change in precipitation due to climate change is between −3.8% and 36%.

The recommended adaptation measure might be applied to the future expected climate stimuli or to already observed climate stimuli (IPCC 2007). Before applying any adaptation and mitigation measures in a given catchment it is important to study its characteristics. The Blue Nile basin is known for its dissected and steep terrain, highly erodible soils, fragmented land use intermixed with very low forest cover. These conditions are the main cause of land degradation and poverty in the basin (Zaitchik et al. 2012). Due to naturally erosive rains and erodible soils, the Blue Nile basin is highly susceptible to erosion. This again results in local soil fertility loss, sediment yield, and a reduced water holding capacity in the Blue Nile, which reduces water quality. Therefore, the adaptation and mitigation methods should be applied by considering the above-listed characteristics of the basin. One of the adaptation measures is flood forecasting and early warning to reduce the impact of flooding on the people living in the basin. Using a highly efficient and conjugate (use of surface and groundwater) irrigation method is another adaptation method that can be applied in order to reduce a change impact on water availability and crop productivity.

In addition to applying adaptation measures, it is important to mitigate climate change impact. The best and most commonly recommended mitigation measure is to promote watershed management activities such as agroforestry, tree planting, stream bank rehabilitation, runoff, and drainage management and soil conservation. They offer protection of land and water resources, building increased resilience to climate-related events such as droughts, floods, and landslides. They also reduce deforestation, store carbon, and contribute to climate change mitigation. Watershed management also takes steps to improve the resilience of wetland ecosystems.

From this study, it can be concluded that climate change is becoming a hot issue in the global environment because it affects water resources which humans rely on for drinking, crop production, and manufacturing. Climate change affects water resources in many ways. It alters the spatial and temporal availability of water. It causes too much water in some areas while there is a drought in some other regions. Climate change increases the demand for water while diminishing the supply (availability). The Blue Nile river basin is one of the rivers which are severely affected by climate change. Even though it covers only 10% of the Nile basin, the Blue Nile generates about 85% of the water that reaches Egypt and Sudan. Therefore, any streamflow reduction in the Blue Nile river basin due to climate change affects water development in downstream countries. Climate change affects the water resources of the Blue Nile basin in many ways. The sediment yield in the basin is high and will increase by 21.3% in the 2080s due to high erosion. Evapotranspiration in the basin is also increasing by 19%, while rainfall is reducing by up to 25%.

The basin is characterized by highly erodible soil, dissected and steep terrain, with fragmented land use dominated by smallholder livestock and crop production. The landscape and land use characteristics of the basin combined with the erosive nature of the rainfall cause high soil erosion, soil fertility loss, and lower water holding capacity and, in general, land degradation and poverty. Therefore, the mitigation and adaptation strategies should be applied by considering all these characteristics and problems of the basin. Both proactive and reactive adaptation measures can be applied to this basin. The best strategy is watershed management such as agroforestry, afforestation, soil water conservation, and stream bank rehabilitation. In addition to reducing the impact of climate change, this strategy can reduce carbon dioxide emissions.

The limitation of this study is that it does not include the impact of climate change on some water parameters (e.g., water quality). The reason is that there is no research done on this aspect and it was hard to get information in this regard.

Based on the review of more than 73 papers about climate change effect on the water resource of the Blue Nile basin, the following recommendations are given:

• Most of the studies done on climate change impact focused on precipitation and streamflow and a lower number of papers researched sedimentation load. Therefore, it is suggested that additional research should be undertaken on climate change impact on sedimentation.

• Water quality impact due to climate change is overlooked in the Nile basin. Therefore, it is strongly suggested that more research should be done on this.