Implications of climate change on hydrological extremes in the Blue Nile basin: A review

AbstractStudy regionThe Blue Nile river basin in East Africa.Study focusThis review paper presents the current understanding of hydrological extremes in the Blue Nile River basin under historic and future climate conditions, largely drawing on research outputs over the past decade. Characteristics of precipitation and streamflow extremes, including historic trends and future projections, are considered.New hydrological insightsThe review illustrates some discrepancy among research outputs. For the historical context, this is partially related to the period and length of data analyzed and the failure to consider the influence of multi-decadal oscillations. Consequently, we show that annual cycle of Blue Nile flow has not changed in the past five decades. For the future context, discrepancy is partially attributable to the various and differing climate and hydrological models included and the downscaling techniques applied. The need to prudently consider sources of uncertainty and potential causes of bias in historical trend and climate change impact research is highlighted

Evaluation of regional climate models performance in simulating rainfall climatology of Jemma sub-basin, Upper Blue Nile Basin, Ethiopia

This study examines the performance of 10 Regional Climate Model (RCM) outputs which are dynamically downscaled from the fifth phase of Coupled Model Inter-comparison Project (CMIP5) GCMs using different RCMs parameterization approaches. The RCMs are evaluated based on their ability to reproduce the magnitude and pattern of monthly and annual rainfall, characteristics of rainfall events and variability related to Sea Surface Temperature (SST) for the period 1981–2005. The outputs of all RCMs showed wet bias, particularly in the higher elevation areas of the sub-basin. Wet bias of annual rainfall ranges from 9.60% in CCLM4 (HadGEM2-ES) model to 110.9% in RCA4 (EC-EARTH) model. JJAS (June-September) rainfall is also characterized by wet bias ranges from 0.76% in REMO (MPI-ESM-LR) model to 100.7% in RCA4 (HadGEM2-ES) model. GCMs that were dynamically downscaled through REMO (Max Planck Institute) and CCLM4 (Climate Limited-Area Modeling) performed better in capturing the rainfall climatology and distribution of rainfall events. However, GCMs dynamically downscaled using RCA4 (SMHI Rossby Center Regional Atmospheric Model) were characterized by overestimation and there are more extreme rainfall events in the cumulative distribution. Most of the RCMs’ rainfall over the sub-basin showed a teleconnection with Sea Surface Temperature (SST) of CMIP5 GCMs in the Pacific and Indian Oceans, but weak. The ensemble mean of all 10 RCMs simulations was superior in capturing the seasonal pattern of the rainfall and had better correlation with observed annual (Correl = 0.6) and JJAS season rainfall (Correl = 0.5) than any single model (S-RCM). We recommend using GCMs downscaled using REMO and CCLM4 RCMs and stations based statistical bias correction to manage elevation based biases of RCMs in the Upper Blue Nile Basin, specifically in the Jemma sub-basin

July-September rainfall in the Greater Horn of Africa: the combined influence of the Mascarene and South Atlantic highs

July-September rainfall is a key component of Ethiopia’s annual rainfall and is a source of rainfall variability throughout inland Greater Horn of Africa. In this study we investigate the relative influences of the Mascarene (MH) and South Atlantic (AH) highs on July-September rainfall in a covarying region of the Greater Horn of Africa using CHIRPS observed rainfall and the ERA5 reanalysis. We show that a mixed metric using the circulation at 850 hPa of these two subtropical anticyclones (AH-MH), is better correlated with rainfall than individual high circulations. Variations in remote circulation are translated by changes in Central African westerlies and Turkana Jet wind speeds. We apply the AH-MH mixed metric to the CMIP5 and CMIP6 ensembles and show that it is a good indicator of mean July-September rainfall across both ensembles. Biases in circulation are shown to be related to the Hadley circulation in CMIP5 atmosphere-only simulations, while causes of biases in CMIP6 are more varied. Coupled model biases are related to southern ocean warm biases in CMIP5 and western Indian Ocean warm biases in CMIP6. CMIP6 shows an improved relationship between rainfall and Turkana Jet and Central African westerlies across the ensemble

The Grand Ethiopian Renaissance Dam: Source of Cooperation or Contention?

This paper discusses the challenges and benefits of the Grand Ethiopian Renaissance Dam (GERD), which is under construction and expected to be operational on the Blue Nile River in Ethiopia in a few years. Like many large-scale projects on transboundary rivers, the GERD has been criticized for potentially jeopardizing downstream water security and livelihoods through upstream unilateral decision making. In spite of the contentious nature of the project, the authors argue that this project can provide substantial benefits for regional development. The GERD, like any major river infrastructure project, will undeniably bring about social, environmental, and economic change, and in this unique case has, on balance, the potential to achieve success on all fronts. It must be stressed, however, that strong partnerships between riparian countries are essential. National success is contingent on regional cooperation

Potential predictability of the Ethiopian summer rains: understanding local variations and their implications for water management decisions

Understanding the influence of large-scale oceanic and atmospheric variability on rainfall over Ethiopia has huge potential to improve seasonal forecasting and inform crucial water management decisions at local levels, where data is available at appropriate scales for decision makers. In this study, drivers of Ethiopia's main rainy season, July–September (JAS), are investigated using correlation analysis with sea surface temperature (SST). The analy- sis showed local spatial variations in the drivers of JAS rainfall. Moreover, the analysis revealed strong correlation between March to May (MAM) SST and JAS rainfall in particular regions. In addition to the influence of SSTs, we highlighted one of the mechanisms explaining the regional pattern of SST influence on Ethiopian rainfall, the East African Low-Level Jet. Moreover, examining the occurrence of large-scale phenomena provided additional infor- mation, with very strong ENSO and positive IOD events associated with drier conditions in most part of Ethiopia. A sub-national analysis, focused at a scale relevant for water managers, on the Awash basin, highlighted two dis- tinct climate zones with different relationships to SSTs. June was not included as part of the rainy season as in some areas June is a hot, dry month between rainy seasons and in others it can be used to update sub-seasonal forecasts with lead time of one month for JAS rainfall. This highlights the importance of understanding locally rel- evant climate systems and ensuing sub-seasonal to seasonal forecasts are done at the appropriate scale for water management in the complex topography and climatology of Ethiopia

Integrated water availability modelling to assess sustainable agricultural intensification options in the Meki catchment, Central Rift Valley, Ethiopia

The Meki catchment in the Central Rift Valley basin of Ethiopia is currently experiencing irrigation expansion and water scarcity challenges. The objective of this study is to understand the basin’s current and future water availability for agricultural intensification. This was done by simulating scenarios through an integrated SWAT-MODFLOW model to assess the water balance. The scenarios were codeveloped with communities who expressed their aspirations for agricultural intensification in conjunction with projected climate change. The results show that with the present land use and climate, the catchment is already water stressed and communities cannot meet their irrigation water demand, particularly in the first irrigation season (October–January). However, in the second irrigation season (February–May) water resource availability is better and increasing irrigated area by 50% from the present extent is possible. With a climate change scenario that favours more rainfall and shallow groundwater use, agricultural intensification is feasible to some extent

Climate Change Impact on Water Resources in the Awash Basin, Ethiopia

Rapid growth of agriculture, industries and urbanization within the Awash basin, Ethiopia, as well as population growth is placing increasing demands on the basin’s water resources. In a basin known for high climate variability involving droughts and floods, climate change will likely intensify the existing challenges. To quantify the potential impact of climate change on water availability of the Awash basin in different seasons we have used three climate models from Coupled Models Inter-comparison Project phase 5 (CMIP5) and for three future periods (2006–2030, 2031–2055, and 2056–2080). The models were selected based on their performance in capturing historical precipitation characteristics. The baseline period used for comparison is 1981–2005. The future water availability was estimated as the difference between precipitation and potential evapotranspiration projections using the representative concentration pathway (RCP8.5) emission scenarios after the climate change signals from the climate models are transferred to the observed data. The projections for the future three periods show an increase in water deficiency in all seasons and for parts of the basin, due to a projected increase in temperature and decrease in precipitation. This decrease in water availability will increase water stress in the basin, further threatening water security for different sectors, which are currently increasing their investments in the basin such as irrigation. This calls for an enhanced water management strategy that is inclusive of all sectors that considers the equity for different users

Impact of Climate Variability and Change on Hydrological Extremes of the Upper Blue Nile Basin (Impact van klimaatvariabiliteit en -verandering op hydrologische extremen in het boven-rivierbekken van de Blauwe Nijl)

Over the recent decades, the world has witnessed frequent and intense hydro-climatic extremes in the form of floods, droughts, heat waves, cyclones and other phenomena in various geographical locations. The scientific community offered substantial attention for the alteration in extremes and possible relations with climate variability and/or change since climate is a key driver to various natural and managed systems. Hydrological cycle and the climate system have close interactions, subsequently local and regional hydrology will bear the immediate consequences of climate variability/change. Changes in hydrological extremes have considerable implications on the planning and management of water resources of a given basin. Accordingly, quantification of possible hydrological impacts due to climate alteration and incorporation of the outcomes in engineering designs and policy formulations is crucial. Recent changes in climate observed as global warming are commonly attributed to anthropogenic activities such as the burning of fossil fuels that increase the concentration of greenhouse gases in the atmosphere. Nevertheless, changes in climate may be due to natural internal processes or external forcings in addition to persistent anthropogenic changes in the composition of the atmosphere or in land use. The most advanced tools such as Global Climate Models (GCMs) are being used to project the future climate and provide information for impact investigators in various aspects of the environment and society. Quantifying the impact of climate change on the future hydrological extremes is undoubtedly important. Nonetheless, prior to such estimations it is wise to retrospectively investigate the historical variability of hydro-climatic extremes to better understand their patterns, causes and effects. This research was aimed at studying the observed climate variability and the impact of future climate change on hydrological extremes of one of the most important river basins in Africa, the upper Blue Nile basin. The importance of this study is guided by the location of the study area which is highly vulnerable to climate variability and change as well as its large discharge contribution to the entire Nile basin system. This study was supported by the readily available data from climate models as well as from selected long-term observations. The study commenced with addressing its first research questions which required assessment of historical patterns and trends in hydro-climatic extremes. This analysis aimed at seeking evidence for a certain pattern in historical records and unravelling whether the recent changes are statistically significant. This analysis provides information on whether certain periods are wet or dry compared to the long-term natural variability. Furthermore, it provides the possibility of separating natural variability patterns from long-term trends in extremes that could be attributed to anthropogenic causes. Long-term records of rainfall extremes, high and low flows from the basin were considered for this analysis. The outcome of this study was that there is an evidence of (multi-) decadal oscillation on hydro-climatic extremes of the basin and a statistically significant negative anomaly period during the 1980s. However, the recent years did not show consistent increasing or decreasing trend. Hence, it is intricate to conclude the recent changes in the extremes on anthropogenic climate change. Alternatively, as a possible cause for the historical variability pattern in the hydro-climatic extremes, large scale atmospheric variables from Pacific and Atlantic Oceans were found to have strong correlations. Identification of the primary causes of the observed variability patterns in the hydro-climatic extremes was conducted making use of lumped conceptual hydrological models and statistical analysis on the simulated results. The hydrological models were used to simulate a long-term time series with the assumption that if changes in catchment behaviour had significant influence on the temporal dynamics of the basin s extreme flows, a clear difference would be detected between the simulated and observed variability results. The outcome of the analysis revealed that there is no discernible trend of the difference between the variability pattern of the simulations and that of the observations over time. Additionally there is no perceptible change in the catchment response behaviour between different periods. Hence, the temporal dynamics of extreme flows of the Blue Nile River are much more influenced by (multi-) decadal climate variability rather than the changes in land use or other catchment characteristics. Considering that the basin is one of the data scarce regions and the presence of a multi-decadal oscillation pattern, it was important to assess the effect of the variability patterns on the derivation of flow-duration-frequency (QDF) predictions when short-term data is used. The study showed that the QDF predictions are dependent on the period used for the analysis. For instance, high flow QDF statistics estimated using the 1980s data required bias correction of around 15% to match with the long-term period estimation. A strong correlation was found between temporal variability of high flow extremes and that of large scale atmospheric variables. This relation can be used as an indicator for correcting the bias in the estimation of high flow quantiles that utilized short term data. The final research objective sought to analyse the impact of anthropogenic climate change on the future extremes to determine the potential direction of change by also investigating the influence of different downscaling methods. The results indicated that the choice of downscaling method was an important factor to be considered and that the results based on one downscaling method may not entirely give the full picture. For the upper Blue Nile basin, the chosen downscaling methods agree in projecting mostly decreasing flow for the main rainy season. The reason isboth decreasing rainfall and increasing evapotranspiration at seasonal scale. It is worth mentioning that the uncertainty range obtained in this study can get wider if more GCMs, downscaling methods and hydrological models were used. However, despite this uncertainty, water managers are recommended to take actions that are based on no-regret strategies.status: publishe

Nexus thinking for sustainable irrigation

On the 8th International Day of Women & Girls in Science, clean water and sanitation are front and center. Within the framework of the Sustainable Development Goals, the high-level sessions at the United Nations Headquarters in New York will advocate for women in science. Importantly, the agenda also recognizes the role of women in the communities targeted by development programs. On both fronts, observes IWMI Researcher Dr Meron Teferi Taye, “There’s more awareness than before. But on the ground, it’s not always the case that gender mainstreaming has taken place.