Nile River Basin

This chapter discusses the hydrometeorology, land use, soils, topography, agroecological zones, extreme flows, climatic variability and climatic teleconnections of the upper Blue Nile River basin. The basin has a varied topography, rainfall and temperature resulting in different agroclimatic zones. Spatial distribution of annual rainfall over the basin shows high variation with the southern tip receiving as high as 2,049 mm and the northeastern tip as low as 794 mm annual average rainfall. The analysis of the basin’s river flow and El Niño Southern Oscillation (ENSO) index connectivity indicates that the upper Blue Nile River basin rainfall and flows are teleconnected to the ENSO index. Based on event correspondence analysis, high rainfall and high flows are likely to occur during La Niña years and dry years are likely to occur during El Niño years at a confidence level of 90%. Low and high flow analysis for selected tributaries and flow at the Blue Nile River flow shows different recurrence intervals of the high and low flows.

Mara is a transboundary river located in Kenya and Tanzania and considered to be an important life line to the inhabitants of the Mara-Serengeti ecosystem. It is also a source of water for domestic water supply, irrigation, livestock and wildlife. The alarming increase of water demand as well as the decline in the river flow in recent years has been a major challenge for water resource managers and stakeholders. This has necessitated the knowledge of the available water resources in the basin at different times of the year. Historical rainfall, minimum and maximum stream flows were analyzed. Inter and intra-annual variability of trends in streamflow are discussed. Landsat imagery was utilized in order to analyze the land use land cover in the upper Mara River basin. The semi-distributed hydrological model, Soil and Water Assessment Tool (SWAT) was used to model the basin water balance and understand the hydrologic effect of the recent land use changes from forest-to-agriculture. The results of this study provided the potential hydrological impacts of three land use change scenarios in the upper Mara River basin. It also adds to the existing literature and knowledge base with a view of promoting better land use management practices in the basin.

In recent years, few studies are presented on the water balance of Lake Tana. In these studies, the water balance is closed by unknown runoff contributions from ungauged catchments. Studies relied on simple procedures of area comparison to estimate runoff from ungauged catchments. In this study, emphasis is on regionalisation approaches by the use of physical catchment characteristics and a regional model. For runoff modelling, the HVB-96 model is selected while automated calibration is applied as based on a Monte Carlo procedure. Closure of the lake water balance was established by comparing measured to estimated lake levels. Results of daily lake level simulation show a relative volume error of 2.17% and a Nash–Sutcliffe coefficient of 0.92. Results show runoff from ungauged catchments of 527 mm/year for the simulation period 1994–2003 while the closure term only is 85 mm. Compared to previous works this closure term is smallest.

In this study remote sensing for rainfall estimation is evaluated. For the Lake Tana basin in Ethiopia the diurnal cycle of rainfall is assessed using satellite observations at high temporal resolution and ground based observations. Also convective activity of a cloud system on the lake has been observed through satellite imagery and shows a potential to observe characteristics of a cloud that produced extreme rainfall intensity. These characteristics include the cloud area and a volume index as well as temporal evolution of distance and direction of the centroid of a cloud mass from a rain gauge at the Gurer Island in Lake Tana. In this work it is concluded that remote sensing can be very helpful in estimating rainfall, assessing the diurnal cycle and monitoring heavy rainfall producing clouds. The high potential of remote sensing observations is mainly because the observations are consistently available with spatially continuous coverage.

A relatively dense station network over the Ethiopian highlands is used to evaluate the accuracy of some satellite rainfall estimates and gridded raingauge products. The satellite rainfall estimates evaluated are GPCP, CMAP, RFE, TRMM-3B42, and CMORPH. These products are evaluated at monthly, ten-daily, and daily accumulations and spatial resolutions of 2.5°, 1°, 0.5° and 0.25° lat/long. The two gridded raingauge analyses evaluated are from the Global Precipitation Climatology Centre and University of East Anglia. These monthly products were evaluated at spatial resolutions of 2.5°, 1°, and 0.5°. While satellite rainfall estimates at monthly and ten-daily time scale showed good agreements with the reference raingauge data, results for daily accumulations were not as good. However, the daily products performed reasonably well in detecting the occurrence of rainfall. Very good agreements were observed between the gridded raingauge analyses and the reference raingauge data.

The objective of this study is to compare the performances of two rainfall products (with resolutions of 3-h, 0.25°×0.25°) developed by the Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) method: TMPA 3B42RT (a real-time version that does not include any rain gauge data) and TMPA 3B42 (a research version that combines TMPA 3B42RT with global rain gauge data). These products are separately used as input into the SWAT hydrological model to simulate daily streamflow for two adjoining watersheds (Koga with drainage area of 299 km², and Gilgel Abay with a drainage area of 1,656 km²) in the Ethiopian part of the Nile basin, and the simulations are then compared to observed streamflow. Results turn the conventional notion on its head: the satellite-only TMPA 3B42RT products are found to be much better than the satellite-gauge TMPA 3B42 products in terms of their ability in reproducing daily streamflow. Nile hydrologist are advised to use TMPA 3B42RT over TMPA 3B42. Algorithm developers are advised to take a deeper look into their bias adjustment techniques especially in mountainous topography and rain gauge sparse regions.

Understanding the basic relationships between rainfall, runoff and soil loss is vital for effective management and utilization of water resources and soil conservation planning. A study was conducted in three small watersheds in or near the Blue Nile basin in Ethiopia, with long-term records of rainfall and discharge. To better understand the water movement within the watershed, piezometers were installed and infiltration rates were measured in the 2008 rainy season. We also reanalyzed the discharge from small plots within the watersheds. Infiltration rates were generally in excess of the rainfall rates. Based on this and plot discharge measurements, we concluded that most rainfall infiltrated into the soil, especially in the upper, steep and well-drained portions of the watershed. Direct runoff is generated either from saturated areas at the lower and less steep portions of the hill slopes or from areas of exposed bedrock. Using these principles, a simple distributed watershed hydrology model was developed. The models reproduce the daily discharge pattern reasonably well for the small watershed and the 10-day discharge values for the whole Blue Nile Basin in Ethiopia. The simplicity and scalability of the model hold promise for use in un-gauged catchments.

Conventional methods and remote sensing were applied for the estimation of reference evapotranspiration and actual evapotranspiration over the Fogera floodplain. Reference evapotranspiration (ET₀) by Modified Makkink (MM), Priestly-Taylor (PT) and Abtew (A) simple equations was compared to the Penman-Monteith (PM) estimations, in order to decide which method for ET₀ is the most suitable alternative to PM in data scarce conditions. A comparison was also made to a satellite based energy balance approach that estimated actual evapotranspiration. For the remote sensing approach, images from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor were selected. For the study, data has been used from Bahir Dar meteorological station at a distance of 50 km from the floodplain and from Woreta weather station that is located in the floodplain. The comparison of results from the conventional methods indicated that the MM method performed best over the floodplain as compared to the PM approach while the PT and Abtew (A) simple equations only produced fair results. The latter two approaches required calibration of site specific coefficients that may have affected the estimation results. Accumulated actual evapotranspiration from the satellite based approach for the year 2008 was about 1,519 mm for rice, while the reference evapotranspiration by the PM approach was 1,498 mm. A comparison of these results with literature values of the crop coefficient of rice indicated that rice transpired at a potential rate.

Flood is a natural disaster. However human activities in many circumstances change flood behavior. The objective of this study was to assess flood hazard and risk of Fogera woreda (district), which is one of the most severely flood affected areas in Ethiopia in general and Ribb–Gumara Catchment in particular, using Geographic information system (GIS) and remote sensing techniques. Land use/land cover change detection was done for the catchment using the 1973, 1985 and 1999 Landsat images and the general trend showed that vegetative and grassland areas were mainly changed to agricultural lands. Comparison between long year (1974–2006) annual maximum daily rainfall and annual maximum daily gauge levels (1971–2005) data of Ribb and Gumara Rivers showed that rainfall slightly decreases while gauge level increases, and this can be attributed to land cover conversion especially in the upper catchment. Flood frequency analysis was done using Ribb and Gumara Rivers annual maximum daily gauge levels by Gumbel’s, and the likely flood levels in different return periods were found. Digital elevation models (DEM) and the 100 year return period base-flood were combined in the GIS environment in order to produce flood inundation maps. More over, flood causative factors were developed in the GIS and remote sensing environment and weighted and overlaid in the principle of pair-wise comparison and Multicriteria Evaluation (MCE) technique in order to arrive at flood hazard and risk mapping. The major findings of the study from both the two methods revealed that most of the areas in the downstream part of the catchment and the different land uses in these areas were within high to very high flood hazard and risk level. The presence of risk assessment mapping will help the concerned authorities to formulate their development strategies according to the available risk to the area.

Soil erosion is a critical problem in Ethiopia. The rate of soil erosion at the Debre Mewi Watershed in the upper Blue Nile River basin has occurred at an alarming rate. Estimating soil erosion, identifying erosion hotspot areas and setting priorities are needed by agencies involved in development works of the watershed to plan and implement soil and water conservation measures. Therefore, this research has been carried out to assess soil erosion, identify erosion hotspots and to set priority for conservation measures. The study integrated remote sensing with a Geographic Information System (GIS). Soil, land use/land cover, topography, and climatic data were used to generate the RUSLE factor values. Soil erosion was calculated through overlay analysis, which ranged from 0.0046 to 192 tons/ha/year. About 68% of the watershed experiences from very low to moderate erosion rates, 31% experiences from high to extreme erosion rates and 1% experiences exceptional erosion rates that is greater than 100 tons/ha/year. The results were compared with conventionally collected plot level soil loss data and good agreement was found. Agricultural areas (crop lands) have very high soil erosion followed by grazing lands and bush lands. However, the soil loss is low in eucalyptus plantations and built up areas.

Hydrological models have a wide range of applications in water resources planning and management as well as flood forecasting and climate impact assessments. In the latter case, they are usually coupled to meteorological or climate models. In this study, two hydrological models (Hydrologiske Byrån avdeling för Vattenbalans (HBV) and Nile Forecast System (NFS)) are applied to the Atbara catchment, Nile River basin area to study the sensitivity of runoff to changes in rainfall and potential evapotranspiration. The HBV model is a conceptual lumped model while the other NFS is a conceptual distributed model. Atbara River is the last major tributary of the Nile River and has a highly seasonal pattern with very high flows during the flood season and almost zero flows during the dry season. These features pose problems to the calibration of hydrological models. However, both models were able to capture the main features of the monthly flow time series of the Atbara (Nash efficiency index reached 0.92 for HBV and 0.68 for NFS). Results indicate a very high climate sensitivity of the catchment where rainfall increases of 10, 20 and 30% result in runoff increases of 29, 62, and 97%, respectively. The catchment runoff is also sensitive to changes in potential evapotranspiration but to a lesser extent. These results are confirmed by the two used models with slight differences.

The economy of Ethiopia mainly depends on agriculture, and this in turn largely depends on available water resources. A major effect of climate change is likely to be alterations in hydrologic cycles and changes in water availability. This chapter reports the use of global climate models (GCM’s) and application of a hydrological model to investigate agricultural water resources’ sensitivity to climate change in the Lake Tana Basin, Ethiopia. Projected changes in precipitation and temperature in the basin for two future seasons (2046–2065 and 2080–2100) were analyzed using outputs from fifteen GCMs. A historical-modification procedure was used to downscale large scale outputs from four GCM models to watershed-scale climate data. The study then investigated how these changes in temperature and precipitation might translate into changes in streamflow and other hydrological components using SWAT model. We interpret the different aspects of the hydrological responses to imply that changes in runoff and other hydrological variables could be significant, even though the GCMs do not agree on the direction of the change indicating high uncertainty.

This chapter presents a review of climatic factors modulating variability of Nile River flow. Through composite analysis of seasons with high and low flow, the Atlantic zonal overturning atmospheric circulation is seen as a dominant feature. When upper easterlies and lower westerlies prevail, convection is enhanced over the highlands of Northeast Africa. This atmospheric cell is shown to be coupled with the Pacific Ocean thermocline oscillation that comprises the El Niño Southern Oscillation. At the event scale, floods are produced by an enhanced southerly monsoon over the West Indian Ocean that is reflected back toward Northeast Africa by an Arabian ridge. Diurnal forcing is evident in surface heating and a mid-day strengthening of northwesterly winds over Sudan. During the 1970s and early 1980s, droughts caused the Nile River flow to decline. This appears related to a multi-year cool phase in the North Atlantic and a southward retreat of the near-equatorial trough. Sympathetic responses of rainfall extend from Ethiopia to India and across the West African Sahel, suggesting that climatic variability in the Nile catchment is part of a global pattern. Although some predictability is uncovered, more than half of the variance in Nile River flow is apparently random and unresolved by either statistical or numerical models. Thus coping mechanisms and strategies for resource switching in wet and dry phases are needed, to put countries bordering the Nile on sound economic footing.

Within the Blue Nile River basin rainfall varies significantly with altitude and is considerably greater in the Ethiopian highlands than on the plains of Sudan. The river is the principal tributary of the main Nile River providing 62% of the flow reaching Aswan in Egypt. Flow and sediment variation are large, with unimodal peaks. Significant water development therefore requires considerable investment in water control and management to offset variability. This chapter provides an overview of the basin characteristics, hydrology and hydrological variability of the Blue Nile, as well as a brief evaluation of the current and future status of water resource development and implications for water availability.

The competition for water between different uses and users is increasing, particularly in the Nile basin where about 90% of the production systems comprise livestock. There is an ongoing debate on how to increase water productivity in these crop-livestock systems. This paper presents a comprehensive framework to provide policy guidance and promote action to improve returns from water investments through: (i) provision of sufficient watering points for livestock across the basin; (ii) improving water productivity through promoting water-saving technologies, ensuring system integration and control of transboundary flux of livestock diseases; and (iii) formulating participatory basin scale regulatory frameworks on water use and sharing. It also argues that improving water productivity through integrated technological, policy and institutional interventions offers an opportunity for smallholders in both upstream and downstream countries to adapt to climate and market risks.

The low level of access, rising fuel costs, and increasing effects of climate change are reinvigorating the policy-makers’ interest in Africa in renewable energy sources such as hydropower. Ethiopia is among countries which have very low modern energy sources, but possesses one of the highest hydropower potential, next to Congo. The topographic feature and the availability of water in Ethiopia permit a large hydropower potential. However, as the available runoff in rivers has very high hydrological variability, tapping into this potential requires investment on storage to smooth out the temporal hydrological variability. In this chapter, first the behavior of this hydrological variability and implication of water resources development are discussed. Second, various documents and reports providing varying values of hydropower potential of Ethiopia and Abbay are summarized. To improve the existing understanding of sites, topographical and hydrological evaluation of 129 hydropower potential sites has been carried out. These sites were identified in previous studies having a total capacity of 13,845 MW. After evaluations, 91 possible sites with total potential of 12,148 MW from various tributary rivers were identified and mapped. Dabus sub-basin stands first among the 16 sub-basins with 13 hydropower potential sites and a total capacity of 3,524 MW, and similarly the other sub-basins are also ranked. The ranking of these sites has been carried out based on cost per kilowatt hour of the hydropower potential (HP) sites. Furthermore, the chapter discusses the benefits and trade-offs for four priority developments identified as Eastern Nile regional fast-track projects.

The degradation of riverine ecosystems, resulting from changes in natural flow regimes, is increasingly recognized as being amongst the most significant negative effects of hydraulic structures. Environmental flows are managed releases from a reservoir intended to mitigate these impacts. Numerous techniques have been developed to estimate environmental flows but, for a variety of reasons, these methods are rarely applied in developing countries. The Ethiopian Government is planning major hydropower and irrigation development in the catchment of the Blue Nile River. This paper reports the findings of a first attempt to rigorously quantify environmental flows in the Blue Nile River. Three desktop hydrological methods, the Global Environmental Flow Calculator, the Desktop Reserve Model, and the Tennant Method, were applied at three locations. With reasonable consistency they indicate that 21–28% of the mean annual flow may be sufficient to sustain basic ecological functioning. The results, which are low-confidence estimates, need to be confirmed with much more detailed studies, but provide a basis for discussion and can contribute to the early phases of planning.

The Mara River Basin (MRB) is an international river basin between the bordering countries of Kenya and Tanzania in Eastern Africa. This study looks at several consumptive water-use factors that exist within the basin, as well as established environmental flow requirements, and quantifies the amount of water demanded. Subsequently, this quantity is compared to existing records of water availability for the dry season months of December through March. Hydrologic records, site interviews, population census data, and spatial datasets were used in combination with a geographic information system to determine water demand. Results show that the total current water demand within the basin does not appear to exceed water supply during periods of maintenance or average flow. However, current water requirements do exceed supply during periods of low flow for each of the dry season months, posing a serious threat to water resources within the MRB.

Assessing soil and water qualities for salinity and other related problems and suggesting remedies are fundamental in irrigation management decision options. The analysis reported in this chapter provides the necessary information to predict the soil and water related problems as consequences of irrigation practices undertaken at various irrigated command areas in the upper Blue Nile River basin, Ethiopia. The study presented in this chapter was conducted in five selected irrigation schemes which have been constructed before 20 years in the Upper Blue Nile basin, Ethiopia. Farmers’ perception of changes in crop yield as a result of changes in soil characteristics and water logging problems were compared with soil physical and chemical analyses. Soil profile pits were opened from selected representative sites in the respective irrigation command areas and from non-irrigated fields adjacent to the irrigated area for the purpose of comparison. The soil pH at Mendel and Tikurit schemes ranges mildly alkaline to moderately alkaline in both irrigated and non-irrigated sites and pH increased with depth due to the corresponding increase in bicarbonate. Infiltration rate and bulk densities in all schemes showed some variation between irrigated and non-irrigated sites. Farmers perceive changes in land productivity as a result of irrigation activities compared with non-irrigated plots especially, onion crops decreased from time to time. Furthermore, seasonal water logging was observed in some of the schemes during the rainy season as a result of flat topography and vertic nature of the soils of the command area. Total nitrogen, organic carbon and to some extent available phosphorus contents are generally found to be in the range of low to very low status while potassium is found to be more or less enough for the current low yield levels.

The central water management issue for the Nile River basin, as in many other river basins throughout the world, is sustainability of water supply in the context of intense population growth, recurring drought, and increasing competition for water. The issue gets complicated as a result of global climate change that is taking place at an alarming rate. A serious discussion of these and other important water resource issues and the challenges in the basin is necessary and needs our attention to seek solutions and insure sustainability of the water supply. This chapter addresses the physical and hydrological conditions of the basin as a background and present the cross-cutting issues of concern in the basin. The uncertainty of availability of water in the face of climate variability and increased land degradation will be also discussed. The challenges to obtain, protect, and manage the basin’s water supply and ecosystem will also be discussed. Even though one cannot make a meticulous coverage of all the issues and challenges in the basin, a serious attempt was made to present possible solutions at local and regional scale. The solution will be geared towards getting more of the water, using it as much, and making it sustainable.