Hydrology and Droughts in the Nile: A Review of Key Findings and Implications

Abstract

The Nile Basin has long been the subject of extensive research, reflecting its importance, which spans from its historical role in the development of ancient civilizations to its current significance in supporting rapidly changing socioeconomic conditions of the basin countries. This review synthesizes studies focusing on the past and future climate, hydrologic, and drought outlooks of the basin, and explores the roles played by large-scale atmospheric phenomena and water infrastructure on the basin’s climate and hydrology. Overall, the studies underscore the complexity of the Nile hydrological system and the necessity for improved modeling and data integration. This review serves as a guide to areas warranting further research by highlighting the uncertainties and inconsistencies among the different studies. It underscores the interconnectedness of climatic and hydrological processes in the basin and encourages the use of diverse data sources to address the data scarcity issue and ensemble models to reduce modeling uncertainty in future research. By summarizing the data and modeling resources employed in these studies, this review also provides a valuable resource for future modeling efforts to understand and explore of the basin’s complex climatic and hydrological dynamics.

1. Introduction

The Nile River, being one of the longest rivers in the world, traverses diverse ecosystems that range from tropical rainforests to arid deserts across eleven African countries [1]. The basin has diverse physical, hydrological, and climatological conditions, coupled with significant variations in human interference (Figure 1). The population of these countries has tripled since 1980 [2,3] (Table S1) and their gross domestic product has doubled since 2000 [4] (Figure S1), underscoring the river’s growing significance for the socioeconomic development and livelihoods of the millions of people who depend on it. The Nile Basin, particularly the Upper Blue Nile (UBN) River Basin in Ethiopia, is also one of the most climate-sensitive basins in the world [5,6]. Understanding this sensitiveness based on past and future climate and hydrological patterns is crucial for effective water resource management and adaptation strategies in the basin, especially amid ongoing climate change and uncertainty [7]. Consequently, the basin has been the subject of several hydroclimatology studies with varying scopes and spatial and temporal coverages.

However, there remains a need for a comprehensive literature review on Nile hydrology and climate, as most existing reviews focus predominantly on specific parts of the basin, particularly, the UBN Basin. Such a review would serve as a crucial resource, offering a centralized overview of the entire Nile Basin. It would provide a one-stop reference for researchers, integrating findings from various sub-regions and addressing gaps in the current understanding of the basin’s hydrological and climatic dynamics. This comprehensive review could also highlight the interconnectedness of different parts of the basin, offering insights into the broader implications and interconnectedness of regional changes.

We conducted this review by first performing a scoping review of the literature on the climate and hydrology of the Nile River Basin. We used databases like PubMed, Web of Science, Google Scholar, and other relevant academic databases to identify relevant studies, articles, and papers on Nile River hydrology and climatology. The search used specific keywords and phrases, including Nile hydrology, climatology, and drought, and also explored references found in relevant literature. This led to the emergence of several key themes which we narrowed down to organize and synthesize the papers as reflected in the structuring of sections in our paper. We extracted and categorized the relevant data and information, such as methods, spatial and temporal coverages, hydrological or climatological variables, and main findings, from the selected studies. The collected data were synthesized to identify themes, patterns, and trends in the literature. We compare findings across studies, highlighting areas of agreement, disagreement, and gaps in the literature for our area of interest. Finally, we provide a concise summary of the main conclusions drawn from the literature and offer recommendations for future research based on identified gaps.

Most of the papers underscore the importance of understanding the intricate interactions between the river’s hydrological processes and the region’s diverse climatic conditions, climate change dynamics, and large-scale climate phenomena in shaping the hydrological processes of the Nile Basin. These interactions are complex and multifaceted, as the basin’s hydrology is influenced by varying rainfall patterns, evaporation rates, and seasonal shifts, all of which are subject to change under different climate scenarios. The studies also predominantly employ data analysis techniques and modeling approaches to explore the hydrology and climate of the Nile Basin. These methods are essential for simulating the basin’s complex hydrological processes and predicting future water availability under various climate change scenarios. However, the uncertainty inherent in such data analyses and models is not commonly or thoroughly addressed. Also, the majority of the studies highlighted the significant challenge of obtaining quality long-term data for the Nile Basin and recommended the use of different data sources and models to offset the insufficient data available for a detailed hydrological study of the basin.

Specifically, the studies highlight the importance of improving data collection and management [9]; addressing uncertainties in climate change projections and their impacts on water resources [10,11,12]; understanding the hydrological sources and sinks in the basin [13]; considering the influence of various factors such as the El Niño Southern Oscillation (ENSO) and land and water development [14,15]; improving the hydrological models to capture the underlying processes [16]; considering the impact of large dam constructions on the basin’s hydrology [17]; using regional climate models to capture the complex hydroclimatology of the Nile Basin [18,19]; and establishing basin-specific drought modeling and monitoring systems in the region [20]. By synthesizing these studies on the Nile, this paper aims to contribute to a more nuanced understanding of the Nile’s climate and hydrology and identify research gaps to guide future efforts to improve the understanding and ultimately aid the water management practices in the basin.

2. Past Trends of Rainfall, Temperature, Evaporation, Runoff, and Soil Moisture

The studies on past and future climate and hydrological trends in the Nile Basin do not always have consistent results, which might be due to the methods, models, basin (sub-basin) extent, data length, and data sources (observed and remote) utilized [7,21]. However, most studies agree on the presence of significant multi-year and seasonal rainfall and hydrological variability in the basin. This variability, coupled with the heavy reliance on the Nile water that goes back to ancient civilizations, has resulted in studies of the Nile River by renowned hydrologists like Harold Edwin Hurst [22].

Several studies on the Nile utilize historical paleoenvironmental data to offer insights into its long-term hydrological and climatic dynamics. These studies have revealed the impact of past climate shifts and human activities on the Nile’s hydrology and sedimentation patterns [23,24,25,26]. For instance, Williams et al. [25], using isotope data from sediments, showed the influence of summer monsoon on extreme flood and erosion in the basin during the early to middle Holocene. Kim and Kaluarachchi [27] and Nkwasa et al. [28] detected evidence of a long-term historical increase in river flows and sediment loads in the upper part of the Nile Basin, largely attributed to changes in climate. Williams [29] indicated that the widespread agriculture and deforestation in the upland headwaters of the Blue and White Nile accelerated land degradation and caused severe droughts during the late Holocene. Others emphasize the basin’s ongoing vulnerability to climate fluctuation by especially highlighting the increase in average temperature [30], changes in precipitation patterns [11] and land-cover dynamics [31], and increase in water use for agriculture and electricity [32,33].

The spatial and temporal trends of the Nile water are affected by an interplay of natural and anthropogenic factors. Nkwasa [28] studied the effect of climate change on river flow and sediment load for the years 1951–2019. The effects vary across the basin, with the White Nile showing an increase in river flow and sediment load that can be attributed to climate change, while no significant changes were observed in the Blue Nile and Main Nile Basins’ flow and sediment load. Taye et al. [21] also noted no significant change in the Blue Nile annual flows for the past five decades. Hasan and Tarhule [34] assessed long-term terrestrial water storage anomaly (TWSA) for the years 2002–2016 and showed that TWSA is mainly affected by precipitation in the tropical and upstream sections of the basin and by evapotranspiration in the semiarid and arid central parts of the basin where the Sudd wetland is located. Their findings also showed a significant spatial correlation between TWSA and population distribution in the basin.

Soil moisture in the basin is also unevenly distributed both temporally and spatially. Higher soil moisture levels are typically observed in the upstream sub-basins around the lakes area and the Blue Nile following their respective rainy seasons. In contrast, the lowest soil moisture values are found in the Main Nile and Tekeze-Atbara sub-basins [35]. Nigatu et al. [36], in their study of soil moisture changes in the Basin over the past 40 years, found varying trends across the basin. They observed an increase in soil moisture in forest covered areas of the Basin and when hydraulic structures like impoundment dams were constructed as observed in the Lave Victoria Region. They also found decreasing soil moisture in regions experiencing a higher rate of temperature increase, even when there is an increase in precipitation. An overall ecosystem decline [37], with a decrease of 18.1% and 2.9% in forest and wetland area, respectively, between 2005 and 2009, coupled with an increase of cultivated area by 5.9% in the basin across the same period [35] is also a reason for the decline in soil moisture. On the contrary, Ayehu et al. [38], explored autumn (September–November: the season that follows the main rainy season in the area) soil moisture in the UBN sub-basin and found an increasing trend for the years 1992–2017, which they attributed to an increase in precipitation that affects the soil moisture in the area more than evapotranspiration (temperature). Compared to other hydrometeorological variables, soil moisture studies are limited in the basin due to the technical difficulty of soil moisture measurement and the required dense network which is hard to have due to financial constraints [39].

Studies have also shown similar variability and uncertainty in the Nile temperature and evaporation patterns in the past. Mengistu et al. [40] observed an increase in mean annual minimum and maximum temperatures from 1981 to 2010 in the UBN River Basin, Ethiopia. Alemu et al. [41], using satellite data from 2002 to 2011, showed that most of the Nile Basin areas have positive correlations between monthly evapotranspiration (ET) and rainfall. In a similar study using satellite data from 1980 to 2014, Nooni et al. [42] found a declining trend in the long-term ET in humid regions caused by rainfall deficits but an increasing trend in arid–semiarid regions due to water availability from crop irrigation fields in the region. Nashwan and Shahid [43] used the Mann–Kendall (MK) tests, considering the effects of long-term persistence (LTP), to distinguish the unidirectional climate trends from natural variability for the period 1948–2010. Their results, not directly congruent with other studies, showed a decrease in spatial coverage of regions with a significant increase in minimum and maximum temperature. They also found a similar result for extreme rainfall, albeit in varying degrees, across the basin. Samy [44] observed high spatial and temporal variability in rainfall trends in the Blue Nile Basin despite an overall increase in average annual rainfall in eastern Africa.

In general, the historical trends and variability of Nile hydrology and climate are complex phenomena influenced by various factors such as climate change, human activities, and natural climatic variability. Studies have highlighted the contrasting certainty of increased water demand in the Nile Basin [27] with the uncertainty surrounding climate and human-induced changes in Nile River flow. This uncertainty and lack of consistent and verified hydroclimatological data hindered comprehensive evaluations and analyses of the change in the Nile waters.

3. Future Trends of Rainfall, Temperature, Evaporation, and Runoff

There are several studies evaluating the potential impacts of future climate change on the Nile Basin. Among the earliest works reviewed on the future climate outlook of the Nile is one conducted by Conway and Hulme [32]. In their work, they assess the impact of climate change on the White Nile and the Blue Nile, which collectively make up about 80% of the Nile’s annual flow. The uncertainty of precipitation outlook by GCMs, while a more consistent result of temperature, was evident in this early work, like many others to follow on the topic. Their follow-up hydrological modeling also revealed that Lake Victoria in the White Nile Basin is more sensitive to temperature changes than the Blue Nile, which is more sensitive to precipitation. They assert that temperature and the consequent effect on potential evapotranspiration might be more critical factors than precipitation for water availability in the large swamp and open water areas of upper White Nile and the semi-arid downstream countries like Sudan and Egypt, where intensive irrigation is practiced. They also evaluated Egypt’s ambitious Toshka project, which builds a system of canals to carry water from the Nile River to irrigate a vast desert land, and found out that such water-intensive projects could add additional pressure in an area where the available water is already limited.

Kim and Kaluarachchi [27] assessed precipitation and temperature change and the consequent effect on the UBN flow. While the trends in precipitation and streamflow are inconclusive, with river flow exhibiting both an increase and a decrease depending on the GCMs and emission scenarios considered, they show a 1.0 to 6.0 °C increase in temperature for the basin. Beyene et al. [6] also noted maximum and minimum temperature increases in all months and seasons in the UBN Basin under different emission scenarios. Coffel et al. [45] stress the effect of increased temperature by predicting that future hot and dry years will exacerbate water scarcity issues in the Upper Nile Basin, particularly affecting agriculture and population growth. Tariku and Gan [46], also predict the Nile Basin will be afflicted with high heat and flood due to future increased temperatures and extreme precipitation, the latter being more intense in the Lake Victoria area. However, their results were inconclusive for mean precipitation.

Gamal et al. [47] assessed the past and future precipitation patterns of the 11 capital cities of the Nile Basin countries and found varying trends in precipitation across cities and periods (note: one of the cities, Addis Ababa, is not in the Nile Basin). Although not with a discernible pattern, they found both an increase and decrease in extreme precipitation for some cities and recommended both enhanced flood and drought period management. Taye et al. [48] studied future climate impact on two upstream catchments of the Nile, Nyando and Tana, at the White Nile and Blue Nile, respectively. They concluded there would be an increase in mean and extreme runoff for the Nyando catchment, while nothing conclusive could be drawn for the Tana catchment. Booji et al. [49] assessed the present and future hydroclimatic response of the Nile Basin but did not reach conclusive characteristics of the basin due to large differences in climate and resulting discharge. Ezzat et al. [50] studied the future climate in the Blue Nile Basin, which contributes about 60% of the Nile’s annual flow. They found no consistent change in the total annual precipitation, increase in temperature by 2–5 °C, and potential evapotranspiration by 2–14%, leading to an overall reduction in soil moisture and annual runoff by about 3.5%. These show that, unlike the future rainfall and streamflow conditions, most of the studies reviewed agreed on a potential increase in the future temperature and evapotranspiration in the basin.

However, it should be noted that some of the studies are more conclusive regarding precipitation and flow. For instance, Siam and Eltahir [51] highlight an increase in river flow and interannual variability of the Nile River flow (by about 50%), leading to more frequent droughts and floods. Roth et al. [5] studied the hydrological response of the UBN River Basin to climate change, highlighting the potential alterations in streamflow and hydrology due to changing climatic conditions. Gebre and Ludwig [52] and Fenta and Disse [53] extended these studies by assessing the combined effects of land-use changes and climate variability on the UBN River flow and found a reduction in the annual flow. The summarized future prediction is shown in

Table 1. Future climatic and hydrologic predictions.

Factors	Change Direction	Blue Nile	White Nile	Entire Nile
Precipitation (including extremes)	Increase	[38]		[46,47]
	Decrease	[52,53]		[47]
	Inconclusive	[6,50]		[49]
Temperature (including extremes)	Increase	[6,45,50,52]		[49]
	Decrease
	Inconclusive
Runoff	Increase		[48,51]	[46]
	Decrease	[50]	[48,51]
(including extremes)	Inconclusive	[48]		[32]

.

Ombadi et al. [11] discuss the inconsistency in future precipitation patterns in the UBN Basin, contributing to the overall uncertainty in climate change projections for the region. Furthermore, Langman [54] points out substantial uncertainty in future precipitation patterns across the Nile Basin. Siam and Eltahir [51] also discuss the uncertainty surrounding results from studies on the impacts of climate change on the Nile Basin. Nonetheless, even with all the uncertainty surrounding the future outlook, the studies resound the necessity of caution, as the basin is likely to experience changes in rainfall patterns, evaporation rates, and streamflow dynamics due to climate change.

4. Droughts in the Nile

Drought in the Nile Basin has been extensively studied using various data sources and drought indicators. Most of the studies focused on characterizing the past droughts in the UBN sub-basin, with limited studies available for future droughts and droughts in the other parts of the Nile Basin. Several drought indices that use different hydrometeorological variables as inputs—with drought periods identified when lower than average values of the respective variables are observed—are used to characterize the different drought types and their spatial and temporal progressions. For example, Elkollaly et al. [55] used the standard precipitation index (SPI), which is the most widely used drought index in the basin, to characterize drought from 1965 to 2000. Both papers identified 1984 as a year of intense drought, which was consistent with the low flow recorded at the Aswan Dam in Egypt in the same year. Using the hidden Markov model based on the SPI for the years 1960–2008, Khadr [56] identified 1961, 1965, and 1980–1987 as drought periods in the UBN. He also asserted that short return period droughts only have small area coverage, while long return periods droughts might cover a wider area. Amare et al. [57] also utilized SPI and temperature changes to identify droughts during 1979–2014 in the UBN and identified drought years different from other papers reviewed. They identified 2002 and 2013 as severe drought years, while 1983, 1986, 1987, 1996, 1998, and 2010 were moderately wet years. They also found that the average temperature in the basin increased by about 1.66 °C between 1979 and 2014, with rainfall becoming highly variable under a warming climate. Bayissa et al. [58] used Z-score (similar to SPI) to assess droughts in the basin and found 2014–2015, 2009–2010, 1994–1995, and 1983–1984 to be drought years.

Additional and new indices were also considered by some studies to better characterize the different types of droughts in the basin. One such study was by Mabrouk et al. [59], who used the SPI and streamflow drought index (SDI) for the period 1973–2017. They used the SPI to identify metrological droughts in the UBN in Ethiopia. The SDI was used to identify hydrological drought using flow data obtained from Dongola station at the entrance of Lake Nasser. They utilized these two indices to see the translation of meteorological drought in the UBN basin in Ethiopia to hydrological drought in Egypt. Their results showed the correlation between the two indices. Bayissa et al. [20] used data from 1970–2010 in the UBN Basin to assess the performances of six drought indices compared to actual drought events as recorded in previous studies and the Emergency Events Database (EM-DAT), an international disaster database. They asserted that no single index on its own tells all the recorded historical drought periods, with the indices having different onset periods, some catching drought events earlier than others. They also found that drought indices have overlaps and correlations, indicating that the indices for meteorological droughts can also be utilized for agricultural and hydrological droughts. Finally, they concluded that all indices can be used in tandem to give a full picture of historical drought periods and that the years 2003/2004 and 1983/1984 were the worst drought periods in the study period.

Dilnesa [60] utilized SPI, SDI, and the reconnaissance drought index (RDI) based on precipitation and flow data from 1984–2014 and found different drought periods in portions of the UBN Basin. The study identified different drought periods for each index, except for 1984/1985, 1989–1992, and 1995/1996, in which all indices indicated drought periods. Kebede et al. [61] assessed drought during 1980–2016 using the SPI, standardized soil moisture index (SMI), and multi-standardized drought index (MSDI), which combines the SPI and SMI. Their results showed that the SPI and SMI do not always show drought periods simultaneously. Meteorological droughts based on SPI were observed for the years 1981, 1983, 1984, 2003, 2009, and 2015, while agricultural droughts based on SMI were observed for 1999–2005. They concluded that the MSDI is a better indicator of different droughts than the other two indices. Kebede et al. [62] used SPI and the standardized reconnaissance drought index (RDIst) for different time scales to assess the historic droughts in the UBN basin during 1980–2015 based on temperature and precipitation data from five meteorological stations. Once again, the results found were not always congruent with the indices, with the years 1984, 2002, 2009, and 2015 found to be drought years by both indices. Bayissa et al. [63] characterized the different forms of drought in the UBN basin using SPI, SMDI, and SRI, reflecting meteorological, agricultural, and hydrological droughts, respectively, and analyzed the local- (UBN) and regional-scale (Sudan and Egypt) associations from 1982 to 2019. All their indices captured the severe historical drought events like the 1984, 2002, 2008, and 2015 drought periods. They also found that all types of droughts were prevalent simultaneously in the basin and pointed out the mismatch in severity and type of droughts between the UBN and the downstream countries (Egypt and Sudan) by citing how devastating the 1984 drought was in Ethiopia but had a minimal hydrological impact on Egypt and Sudan, which is a different conclusion from Elkollaly et al. [55]. They also pointed out the proverbial elephant in the Nile Basin, which is the inevitable competition among the countries for surface runoff, “the less drought-affected and storable water system in the hydrologic cycle”, and the argument that Ethiopia should rely less on the Nile River due to the fact that its higher available precipitation is going to be challenged.

New indices were developed recently to further improve the characterization of the different types and propagation of droughts in the Nile Basin. A pair of indices were developed by Kebede et al. [64] to address decision-making needs using an adaptable framework. This was carried out with and without considering sea surface temperature (SST) in the meteorological variables. They established that both indices show relatively long and more severe droughts than that of the SPI and found 1983, 1984, 1985, 1994, 2002, 2009, and 2015 as drought years in the UBN. Ali et al. [65] integrated the SRI, SPI, SMI, and a new standardized evapotranspiration index (sETI) into a novel composite drought index for the Blue Nile Basin and found 2002/2003, 2009/2010, and 2015/2016 to be drought years within the 2000–2019 period. Although the paper by Awadallah [66] was not about a new drought index, it utilized a methodology developed by the author and previously used for flood risk assessment to assess the likelihood of drought using 130 years of discharge data at Aswan. The study concluded that there is a cyclic pattern of the drought risk, “confirming the previously suggested non-stationarity of the Nile discharge as one of the possible interpretations of the Hurst phenomenon”. Nigatu et al. [67] developed the Gravity Recovery and Climate Experiment (GRACE)-based groundwater drought index (GGDI) to characterize the propagation of meteorological and agricultural droughts to groundwater droughts in the Nile Basin. Their results showed higher correlations among the different droughts. Bayissa et al. [68] developed a fine-resolution vegetation outlook model and utilized the model for the 2001–2016 period, showing 2009 and 2015 to be drought periods. Overall, despite some consistency in severe drought years, like 1984 and 2002, the different drought indices identified different drought years in the basin. Some of these differences could be explained by the delaying effect of the different types of droughts and the different data ranges and locations used.

The limited studies available on future droughts note the worsening of drought in the Nile Basin. Ezzat and Elshamy [50] suggested that under the expected moderate changes in rainfall and increased temperature, the UBN might face increased moisture constraints in the future. Additionally, Mahmoud et al. [69] projected increasing drought trends in the Nile Basin due to the combined impact of warming and stronger ENSO and Indian Ocean Dipole (IOD) events. Coffel et al. [45] predicted that climate change would bring about more hot and dry years, exacerbating water stress and agricultural challenges in the UBN. The agreement of studies on drought incidents is shown in Figure 2 for recorded droughts between 1983 and 2010 over an overlapping study period. Associated information discussed above, along with additional selected papers that have studied drought in the basin—highlighting specific drought periods and areas of interest—can be found in Appendix A.

5. The Roles of Large-Scale Climate Phenomena in the Nile Basin

There have been various studies on the Nile that relate its prehistoric and recent climate and hydrology to global climate phenomena. One such phenomenon is the intertropical convergence zone (ITCZ) and its movement, which shifts seasonally from south to north and back, governing the climate in the Nile Basin [70]. In a study that reviews historical climate-induced discharge variations in the Nile during the Holocene, Sun et al. [71] indicate how the wet Nile period during 9.7–5.0 cal ka BP (9700 to 5000 calibrated years before 1950) was due to “increasing monsoon rainfall in both the headwater and the northern regions of the Nile basin driven by the northward migration of the ITCZ in the Northern Hemisphere”, while the dry Nile period after 5.0 cal ka BP reflected the opposite. Jalali et al. [72] also studied the Nile during the Holocene and found the high and low runoff were related to the migration of the ITCZ. Tedla et al. [73] explained how the movement of ITCZ affects precipitation in the tropics, where most of the Nile is positioned. In a work focusing on the recent period of 1995–2000, Mohamed et al. [19] analyzed the interaction between climatic and hydrologic processes for the Nile. They found the moisture recycling ratio (locally generated/total precipitation) to be low for the basin, with 86–89% of the waters of the Nile being generated outside the basin by the ITCZ, which draws moisture from the Atlantic and Indian oceans. ITCZ also affects the spatial distribution [5] and the seasonal patterns of rainfall in the Nile Basin [74].

The Nile’s climate and flow are also influenced by SST and the ENSO [75]. Onyutha and Willems [76] show the relationship between extreme rainfall in the different parts of the basin and the SST of the Atlantic, Pacific, and Indian oceans. The ENSO affects the interannual climate and rainfall variability in the basin [14,75]. Wang and Eltahir [77] emphasize the utilization of ENSO information in medium- and long-range forecasting of the Nile floods, highlighting the significant regulatory role of the ENSO in the natural variability of the annual flow of the Nile. Conway [78] also found a significant correlation between the seasonal rainfall and river flows in the Eastern Nile Basin with the ENSO. Eltahir [79] estimated 25% of the natural variability of the Nile River’s annual flow is associated with the ENSO. Zaroug [80], studying the connection between the ENSO and the rainfall regime of East Africa and UBN for the period 1982–2009, stated that La Niña and El Niño years are associated with increased and decreased rainfall, respectively. Moon [81] studied the Nile discharge at Aswan from 1870 to 2002 and found similar results, with the flooding and drought of the Nile being related to the La Niña and El Niño phases, respectively.

By studying the effect of the ENSO and IOD over the Nile for the years 2002–2011, Awange et al. [82] found the ENSO to have dominant effects on most of the basin areas, except for the Lake Victoria Basin and Ethiopian highlands (part of the Blue Nile Basin), where IOD influence was noticeable. Mohamoud et al. [69] associated the worsening drought severity in the basin with increased strength of the ENSO and IOD. The study highlights the significant impact of El Niño and IOD on the inter-decadal hydroclimate variability of the basin, indicating that future droughts in the basin are expected to worsen under the combined influence of warming and stronger ENSO and IOD events. Gamal et al. [47], in their study of future extreme precipitation in the Nile Basin, also found that the decrease in consecutive wet days and the increase in consecutive dry days are correlated with the IOD.

Overall, the temporal and spatial variation in the Nile Basin’s hydrology is influenced by the complex interactions and teleconnections among global air circulation, the ENSO, and SST of the Pacific, Indian, and Atlantic Oceans. In addition to these, Sene et al. [83] cited earlier works that indicate the sunspot and lunar cycles recurring every 10–11 and 18 years, respectively, to also influence the Nile climatic characteristics manifest via consequent climatic phenomena like the ENSO and SST. Understanding these climatic phenomena and their complex interaction with the hydrological system is crucial for managing water resources and predicting hydrological events in the Nile River Basin. The significance of the large-scale phenomena discussed in this section are summarized in

Table 2. Large-scale climate phenomena influencing the Nile’s flow.

Large-Scale Climate Phenomenon	References
ITCZ	33%
ENSO	43%
IOD	10%
SST	10%
Sunspot and lunar cycle	5%

.

6. The Roles of Water Infrastructure in the Nile’s Hydrology

The Nile Basin, specifically Egypt, was one of the first areas, if not the first, where irrigation started in the World. This irrigation practice grew into modern irrigation in the basin, starting with the construction of a series of diversion dams downstream of Cairo in the middle of the 19th century [84]. When it was completed in 1902, the first Aswan Dam (Aswan Low Dam) in Egypt was also the biggest dam in the world [85]. Since then, there have been numerous water infrastructure developments in the basin. These infrastructures are crucial for harnessing the full potential of available water resources in the Basin and addressing the droughts and water scarcity challenges by regulating the flow and providing water storage for irrigation, hydropower generation [27], and flood control [86].

Water infrastructure developments for consumptive use and hydropower generation were initially concentrated in Sudan and Egypt [87]. In recent years, Ethiopia has ramped up its use of the Nile, especially for hydropower [86]. The construction of the Grand Ethiopian Renaissance Dam (GERD), the largest hydropower dam in Africa with a planned annual installed capacity of 5150 megawatts [88], exemplifies the ongoing developments in the basin [89]. The Dam is seen as significant infrastructure that has challenged, if not broken, the institutionalized status quo on the utilization of the river—the 1959 Nile Waters Agreement, which allocates the entirety of the Nile flow shared by 11 countries to Egypt, Sudan, and environmental flow/loss [90]. However, it may also be perceived as an outcome of hydropolitical dynamics in the decade prior to its construction and as a promoter of cooperation in the basin [91]. Whether it is a catalyst or an outcome of a change in the basin, the GERD has ramped up extensive hydrological, legal, and political discussions that are unlikely matched by any past infrastructure development in the basin [87,92]. Both opportunities and concerns regarding the dam are reflected in different studies. The concerns include effects on downstream flow [93,94], sediment concentration [95,96], groundwater [97,98], and flow during prolonged drought [99], necessitating effective and collaborative management strategies [33,96,100,101,102]. The dam could also provide opportunities such as increased hydropower generation and storage capacity in Ethiopia during high flow periods [100]. Additionally, the dam can have a positive impact on averting the effect of climate extremes [103] and reduce evaporation loss in the basin [104].

The effects of water infrastructures in the basin are assessed either explicitly, like the ones mentioned above, or implicitly in climatic and hydrologic studies that require modeling the basin and its sub-basins for individual dams or a collection of them. It is also inevitable that they will continue to be central to future studies. After all, the largest water consumption in the basin is due to irrigation and evaporation from reservoirs. And there continues to be a change in storage across the basin to utilize its annual flow of 84–91 BCM, especially in the Eastern Nile sub-basin, where storage has increased from a mere 16 to 106.9 BMC from 1999–2020 and in the Main Nile from 167 to 180 BMC for the same period, while there is not much change in the White and Equatorial Nile, at 3.4 and 200.8 BCB respectively [105].

With the limited access to water, food, and energy in the basin, it is imperative for the development of infrastructure to continue. It is also imperative that basin countries work together to coordinate this infrastructure developments and their operations to minimize impacts and improve usage of the river. The Declaration of Principles signed by Ethiopia, Sudan, and Egypt in 2015 and the Cooperative Framework Agreement (CFA) planned to be taken into effect in October of 2024 to govern the utilization of the Nile are steps in this direction. In addition, unattained projects like the Join Multipurpose Program aimed at developing hydraulic infrastructures at geographically advantageous locations together by Sudan, Egypt, and Ethiopia [91] should be revamped in the future if the Nile is to be utilized to avert the effect of climate extremes and increased water demand by growing population. Lastly, although unacceptable to the Nile riparian countries except for Sudan and Egypt, parts of the 1959 Nile waters agreement [106], that detail the development of the basin by Sudan and Egypt actually can be a good springboard if broadened to include the rest of the riparian countries both in sharing the burden of developing the water resources of the basin and also reaping its benefits fairly and equitably.

7. Data and Modeling Resources to Study the Nile’s Hydrology

Verified data and models are essential to quantifying and understanding hydroclimatological processes, particularly those influencing rainfall variability in the Nile’s source regions. The lack of vetted data and limited accessibility has impacted the modeling effort and our understanding of the Nile River Basin’s hydrology and its complex and dynamic relationships with climate and human factors. Barnes [7] emphasizes the importance of refining our understanding of climate change impacts in the Nile Basin through improved modeling techniques and incorporating advancements in climate science into hydrological models. Other studies [107,108] emphasize the importance of satellite and secondary data sources in overcoming the limitations of in situ meteorological and hydrological observations in the Nile River basin due to inadequate monitoring networks. These data sources are crucial for calibrating hydrological models, assessing trends in hydro-meteorological variables, and understanding the impacts of climate change on water resources in the region [44].

Due to a lack of data, the Nile River Basin benefits significantly from satellite-derived data for streamflow modeling and hydroclimatic analysis [109]. Remote sensing data from satellites such as TOPEX, Jason-1, and Jason-2, as well as missions like GRACE and TRMM, are utilized to analyze water level fluctuations and water storage changes in the Nile Basin [109]. Studies emphasize integrating satellite-based precipitation and evapotranspiration data to capture the spatial variability and improve water balance estimation for the Nile Basin [13,110,111]. Khan et al. [112] highlight the use of satellite data in combination with artificial intelligence techniques to enhance rainfall estimation accuracy in the UBN Basin. Almost all current studies on the Nile rely on satellite data for some of their inputs. Appendix B.1, Appendix B.2 and Appendix B.3 list satellite and observed data and models used, respectively, in selected studies of the basin.

Secondary data such as tracers and radiocarbon dating are also utilized in the basin. For example, Langman [54] and Stanley et al. [113] suggested using the isotope record in the Nile Basin to reconstruct past hydrological conditions and gain insights into how changing climates may affect streamflow patterns. Cockerton et al. [114] have shown how stable isotopes can act as tracers to understand the seasonal hydrological variations within the basin. A growing number of studies employ radiocarbon and optically stimulated luminescence (OSL) dating to reconstruct Holocene River histories (e.g., [24,115]). Castañeda [116] used the sedimentary sequence data to determine the changes in river runoff, vegetation, and erosion in the Nile River watershed during the Holocene, shedding light on the long-term climatic changes that have influenced the basin. The sediment data was also used by Blanchet et al. [115] to study runoff in the Nile Basin during the early Holocene. Although the benefits of such data are promising, their availability is sparse.

Various hydrological models have provided valuable insights into Nile water resources’ responses to changes in its basin. Griensven et al. [117] and Betrie et al. [118] apply the Soil and Water Assessment Tool (SWAT) to analyze water balance and sediment loading in the Nile River Basin. The SWAT model was also applied by others [5,28] to study the impact of climate on the Nile hydrology and water quality. Gebre and Ludwig [52] used the Variable Infiltration Capacity (VIC) hydrology model for streamflow ensemble projections at inflow points to major dams and reservoirs in the Nile River Basin. Basheer et al. [119] also used VIC and the Routing Application for Parallel Computation of Discharge (RAPID) to generate historical and future streamflow of the Nile Basin. They also developed a model using the python library called Python Water Resources (Pywr) to assess the performance of the river infrastructure under climate change and different adaptive management options. Gebremicael et al. [108] developed a hydrological model in Wflow-PCRaster/Python modeling framework to simulate runoff at various locations within the Nile basin using rain gauge and satellite rainfall products as driving forces.

Hydrological modes have also been utilized to assess the effect of climate change in the basin. For instance, Gebre and Ludwig [52] use the Hydrologic Engineering Center Hydrological Modelling System (HEC-HMS) model to simulate the effects of climate change on the hydrological regimes of the UBN catchment. Mostafa et al. [120] used the River Basin Simulation Model (RIBASIM) to simulate the water system of the Eastern Nile Basin to assess the impact of climate change on the Nile flow at the High Aswan Dam. Yimere and Assefa [121] used a conceptual hydrologic model called NAM (Nedbør–Afstrømnings Model—a lumped conceptual precipitation–runoff model) to simulate runoff and analyze the water–energy nexus under climate change in the Nile Basin. Tedla et al. [73] used the Water–Energy Budget Rainfall–Runoff–Inundation (WEB-RRI) model to simulate past and future GCM outputs for Blue Nile flow projections. Nawaz et al. [122] used the Nile Forecast System (NFS) to study the sensitivity of the Blue Nile to climate change. Basheer et al. [86] developed a dynamic recursive annual computable general equilibrium (CGE) model to assess the basin countries economy in light of climate uncertainties and their sustainable development goals. Booij et al. [49] integrated the Hydrologiska Byråns Vattenbalansavdelning model (HBV) with a water distribution and allocation model called River Basin Simulation (RIBASIM) for the Upper Nile to assess the present and future discharge of the Nile River upstream of Lake Nasser. Kahsay et al. [123] used the Global Trade Analysis Project (GTAP–W) model, which considers irrigated and rainfed agricultural land for transboundary trade analysis in light of hydrological constraints. Taye et al. [48] used VHM (a Dutch abbreviation for “generalized lumped conceptual and parsimonious model structure identification and calibration”) and NAM to assess the impact of climate change on two source regions—Lake Tana and Nyando River—of the Nile. NAM was also used by Yimere and Assefa [121] for utilization with Regional Integration and Planning Assessment (RIPA) in their water–energy nexus study of the Nile Basin. Mohamed et al. [19] used the Regional Atmospheric Climate Model (RACMO) to assess the hydroclimatology of the Nile. Lazin et al. [103] used the Coupled Routing and Excess Storage hydrological model (CREST) for their study of the Grand Ethiopian Renaissance Dam in reducing hydrological extremes in the UBN Basin. The Nile has been extensively studied over the past 125 years, drawing interest in various fields of research. For a more in-depth examination of modeling efforts in the basin, as well as the associated challenges and opportunities, refer to the review by Digna et al. [124].

8. Discussion

The Nile Basin is a primarily semiarid/ arid region, with limited water yield [27], inadequate governance [125], deteriorating ecosystem [37] and is sensitive to climate change [53]. It exhibits contrasting characteristics, with upstream areas generating most of the runoff [126] while downstream areas have intensive water utilization. Furthermore, the countries within the basin are experiencing rapid urbanization and burgeoning populations, which place additional pressure on the already scarce water resources [105]. The Basin has also faced significant challenges due to the absence of integrated water management and effective water-sharing agreements, potentially leading to inefficient use of water, increased competition among riparian countries, and heightened tensions over water allocation. The interplay of these factors results in a complex array of problems for the Nile Basin, prompting numerous studies aimed at understanding and addressing the challenges. As such, researchers have explored various aspects of the Nile Basin, including water availability and distribution, climate change and water infrastructure impacts, and the socioeconomic implications of water resource management. They employ techniques ranging from historical data analysis and paleoenvironmental studies to complex climate models. These methods help reconstruct past conditions and assess both current and future hydroclimatic conditions for the basin. Below, we discuss the key findings from our review of these studies and provide some recommendations.

(1)

Historical trends in Nile hydrology and climate: The historical trends and variability in Nile hydrology and climate are complex and marked by significant uncertainty. There is particularly a notable difference between studies regarding the spatial and temporal trends of precipitation and runoff in the basin that can be attributed to factors like using different data sources and methodologies, focusing on different parts of the basin, and the variation in the basin’s climate and hydrological characteristics. Several strategies can be implemented to address the discrepancies and challenges in understanding precipitation and runoff trends in the basin, including standardizing the data collection and sharing, enhancing data quality and coverage by integrating different data sources, fostering regional collaborations, and improving the modeling and analytical techniques. Also, establishing a baseline model and data for the basin is crucial for effectively understanding and managing its hydrology and trends.

(2)

Future trends in Nile hydrology and climate: While studies confirm that water demand in the Nile Basin is rising, there remains uncertainty and variability in the literature regarding how climate change and human actions will impact precipitation and river flow in the region. These uncertainty and differences arise from using various socioeconomic scenarios, climate and hydrological models, and downscaling techniques, resulting in diverse projections of future climate impacts. The ensemble approach, using multiple hydrological and climate models, helps to address the limitations of relying on a single model and better captures the full range of possible outcomes and uncertainty. Establishing a long-term monitoring network to continuously collect and update observational data helps to validate models and refine projections. In addition, considering land-use change, population growth, large-scale climate phenomena, and other criteria, in addition to emission scenarios, when assessing future changes to account for the myriad complexities faced by the basin is highly advisable.

(3)

Past and future drought trends: Drought in the Nile Basin has been extensively studied using various data sources and indices, with a primary focus on past droughts in the Blue Nile sub-basin. Most studies have successfully captured severe historical drought events, such as the 1983/84 drought, but they vary in identifying other drought periods, their severities, and durations. While many studies suggest a cyclic pattern in past droughts, there is no consensus on the overall trends. There is limited research on future droughts. The few studies on future droughts indicate a worsening trend in drought conditions across the Nile Basin. Hence, there is a need for more studies focusing on future drought projections, considering the impacts of climate change and also for research extending beyond the Blue Nile sub-basin to include other regions of the Nile Basin, such as the White Nile and the lower reaches of the basin, to provide a more comprehensive understanding of drought dynamics and progression across the entire basin. The continuous use of multi-variable indices is essential to capture different aspects of drought and its propagation.

(4)

The need for Basin-wide studies: Most of the reviewed studies have focused on sub-regions of the Nile, particularly the Blue Nile Basin. There are two main reasons for the limited research addressing the entire basin. First, there is a severe lack of observed data, coupled with difficulty accessing the limited data. Second, the fact that most of the Nile’s discharge is generated in the Blue Nile region in Ethiopia has led to concentrated efforts in this area. With the prospect of intensified competition for water resources in the basin, basin-wide studies are becoming increasingly crucial to understanding the basin hydrology, resource distribution, and potential impacts of climate change across the entire basin. Basin-wide studies can also facilitate collaboration among the countries by providing a common scientific foundation, fostering transparency, and enabling the development of strategies that balance the needs of all riparian countries. Also, whenever there is the desire but also the intimidation of looking at the Basin in its entirety, focus can be given to the net inflows to Lake Victoria and losses in the Sudd swamps in addition to the discharge of the Blue Nile, as recommended by Sene et al. [83], to obtain a better understanding of the basin.

(5)

Improved monitoring and data sharing: High-quality data are essential for conducting accurate and meaningful hydrological studies, especially in complex regions like the Nile Basin. In addition, successful transboundary water management and analysis often relies on the transparent sharing of hydrological data and joint monitoring of water resources. The Nile Basin suffers from limited availability of comprehensive and long-term observational data, making it difficult to establish accurate baselines or identify trends over time. Expanding monitoring stations, leveraging increasingly available remote sensing data, and openly sharing data with the public through open-access platforms can help address data limitations in the basin. Organizations like the Nile Basin Initiative (NBI) can play a crucial role in this aspect by making their data easily accessible to the public and fostering cooperation among the riparian states to address the challenges and ensure sustainable water resource management in the region.

(6)

Use of remote sensed data and machine learning methods: Remote-sensed data are highly utilized and should continue to be utilized in basin studies, but they come with their shortcomings. The use of different remote sensing, hydrological, and climate models has resulted in varying and sometimes opposing results in the studies reviewed, especially regarding precipitation and discharge outlook, underscoring the presence of a significant level of uncertainty in predicting hydrological changes in the region. This research area needs continuous effort to refine the methodology to ensure reproducibility and to quantify uncertainty. Assessing previous works that compared different data sources [58,127] and did sensitivity studies [128], should be enhanced to rationalize the selection of data sources, parameters, and models and consulted at the bare minimum. Ground-truthing is necessary to validate and ensure the accuracy of remote sensing measurements. In addition, integrating diverse data sources and models is essential to address the data scarcity issue in the basin. Finally, while machine learning methods have shown success in various hydrological applications globally, especially for data-scarce and ungauged watersheds [129], their limited application in the Nile Basin presents an opportunity for harnessing advanced modeling and efficient data utilization techniques to improve hydrological modeling and water resource management in the region.

(7)

Lessons from other transboundary basins and adaptation strategies: using other transboundary rivers to gain valuable insight into water management that can be tailored to the unique challenges of the Nile Basin is beneficial. Many transboundary water systems (e.g., the Mekong River and Rhine River) have benefited from the establishment of robust institutional frameworks and legally binding agreements that outline water-sharing arrangements, rights, and responsibilities [130]. Similar frameworks can be adapted to the Nile Basin to enhance cooperation and address the current water equity issues among the countries. Furthermore, cooperative adaptive management that combines future climatic conditions, socioeconomic changes, and infrastructure developments in the basin should be explored [86,131].