Seifu Kebede

The Ziway–Shala lake basin (main Ethiopian rift, Ethiopia): a revision of basin evolution with special reference to the Late Quaternary

Abstract The Ziway–Shala basin, in the Main Ethiopian Rift (MER), is a reference site for regional to global paleoclimatic reconstructions. We undertook and interpreted a stratigraphical, pedological and geomorphological study, including a new geological map scale 1:250,000, to provide a Late Quaternary-centred revised geological history of the basin. 1 We mapped several Late Quaternary sedimentary units and arranged them in four major unconformity-bounded stratigraphic units (synthems), recording equivalent phases of geomorphic change. A new, extensive, soil survey allowed us to establish a pedostratigraphic unit, the T’ora geosol, as a distinctive marker of landscape stability and instability in the area during the Holocene. Climate change was a major control on geo-morphologic evolution of this area during the intense climate fluctuations of the last 100,000 years. Extensive lake systems developed during relatively humid Last Glacial interstadials and in the early-mid Holocene; this last was characterized by short, but high-amplitude, regressions during arid pulses. Major lakes’ lowering occurred in the terminal Pleistocene and in the last 5000 years. Evidences for high or very high terminal Pleistocene lake levels suggest possible non-climatic controls on changes in lakes’ extension and volumes between Late Pleistocene and Holocene. We suggest that modifications of hydrological thresholds, due to activity of structures parallel and transversal to the MER, established new lakes’ boundaries between terminal Pleistocene and early Holocene, setting the maximum level of Holocene lake systems at about 1670 m a.s.l. The integrated analysis of lacustrine, fluvial, slope and soil systems provided a basis for a general interpretation of relations between climatic changes and geomorphic processes at a basin scale.

Lake — Groundwater Relationships, Oxygen Isotope Balance and Climate Sensitivity of the Bishoftu Crater Lakes, Ethiopia

Oxygen isotope data from groundwater, streams and crater lakes in central Ethiopia provide a basis for modelling lake hydrological and isotopic budgets. The environmental parameters δ A(isotopic composition of vapor) and e* (equilibrium fractionation factor) were determined by a semi-empirical approach using one lake as a terminal index lake. The models show that lake oxygen-isotope composition is more sensitive to rainfall-related parameters (humidity and δ A) than to the isotopic composition of inflow (δ in). The isotopic composition of the lacustrine sedimentary carbonates should therefore provide a record of past rainfall variation.

Estimating ungauged catchment flows from Lake Tana floodplains, Ethiopia: an isotope hydrological approach

The isotope balance approach, which used 18O content of waters, has been used as an independent tool to estimate inflow to Lake Tana of surface water flows from ungauged catchment of Lake Tana (50% of the total area) and evaporative water loss in the vast plains adjoining the lake. Sensitivity analysis has been conducted to investigate the effects of changes in the input parameters on the estimated flux. Surface water inflow from ungauged catchment is determined to be in the order of 1.698×109 m3a−1. Unaccounted water loss from the lake has been estimated at 454×106 m3a−1 (equivalent to 5% of the total via surface water). Since the lake is water tight to groundwater outflow, the major error introduced into the water balance computation is related to evaporative water loss in water from the flood plains. If drained, the water which is lost to evaporation can be used as an additional water resource for socio-economic development in the region (tourism, agriculture, hydropower, and navigation). Hydrological processes taking place in the vast flood plains of Lake Tana (origin of salinity, groundwater surface water interaction, origin of flood plain waters) have been investigated using isotopes of water and geochemistry as tracers. The salinity of shallow groundwaters in the flood plains is related to dissolution of salts accumulated in sediments covering former evaporation pools and migration of trace salt during recharge. The waters in the flood plains originate from local rainfall and river overflows and the effect of backwater flow from the lake is excluded. Minimum linkage exists between the surface waters in the flood plains and shallow groundwaters in alluvio lacustrine sediments suggesting the disappearance of flood waters following the rainy season, which is related to complete evaporation or drainage than seepage to the subsurface. There is no groundwater outflow from the lake. Inflow of groundwater cannot be ruled out. Discharge of groundwater to the lake is presumed to take place along rocky bottom in southern sector from Quaternary volcanics covering the southern sector of the catchment.

Integrated assessment of hydrogeology and water quality for groundwater-based irrigation development in the Raya Valley, northern Ethiopia

Ambitious plans for expanding the area under irrigation in Ethiopia have not been adequately underpinned by assessments of the actual potential. A detailed hydrogeological study was conducted in one of the focal areas for expansion, in the northern part of the country. The study revealed that there are large groundwater reserves within the valley floors Quaternary alluvial sediments and underlying Tertiary fractured volcanic rocks. Levels of salinity and sodicity do not pose constraints for agricultural water use. In the valley-floor aquifers there would appear to be sufficient water to support large- and small-scale irrigation development.

Quaternary glacial activity on the Ethiopian mountains

Publisher Summary Central Ethiopia comprises a high plateau at 2000-3000m a.s.l., formed from successive lava flows and bisected by the eastern African rift. Ten mountains rise from this to altitudes of just over 4,000 m a.s.1 and though the reports of various workers are not in complete agreement it appears that five of these were glaciated at the last glacial maximum (LGM), but without clear evidence of earlier glaciations. The chapter explores that in moist central Ethiopia, on the Arsi mountains, there are large, clear terminal moraines on Mount Bada at 3200–3700m with the glaciated area estimated to be 85 km 2 with equilibrium line altitudes (ELAs) of 3700m a.s.1 on the east and 3900 m a.s.1 on the west. This appears to represent the LGM. Moraines are reported less authoritatively on Mounts Kecha and Chilalo, but are not visible on the air photographs. It also discusses that Kara Tuluke and other minor volcanic cones south of Tullu Dimtu and just outside the Big Boulder Moraine are the eroded remnants of small volcanic plugs. The boulder slopes around their bases display remarkable features dissimilar to any described elsewhere in the world. An ecological monograph on the Bale mountains has a useful review of climate history and photographs of the plateau.

Origin and Isotopic Composition of Aragonite Laminae in an Ethiopian Crater Lake

Millimetre-scale white aragonite laminations alternating with dark diatom-rich organic layers are present in the uppermost sediments of the crater lakes Hora and Babogaya, at Debre Zeit, Ethiopia. The sediment accumulation rate calculated from lamina counts matches that estimated from a 210Pb chronology, indicating that the laminations were deposited annually. The oxygen-isotope composition of the surface white layer is equivalent to that of surface water, which shows that white-layer aragonite is formed in isotopic equilibrium with the lake water, and suggests that isotopic analyses of these layers may provide valid paleoclimatic information. Because the aragonite is probably precipitated during dry-season mixing, aragonite δ18O values for individual laminae reflect the composition of the entire lake, integrated over its water-residence time of about 10 years. The sedimentary record of oxygen-isotope variations should therefore be interpreted as a proxy-climate record with decadal rather than annual resolution. Comparisons between δ18O values for the laminae and climate data for equivalent years show no clear relationships, so calibration of the sedimentary record requires a more detailed understanding of the climatic controls on the isotopic composition of these groundwater-fed lakes. An isotopic mass-balance model of the lake’s response to rainfall variation shows (1) that the oxygen isotope composition of the lake waters varies by about 1 ‰, which is comparable to the range of δ18O values determined from the individual laminae; and (2) that modelled lake level is a reasonable match to observed levels, confirming that climate changes can interpreted from the oxygen-isotope record.

Conceptual groundwater flow model of the Mekelle Paleozoic–Mesozoic sedimentary outlier and surroundings (northern Ethiopia) using environmental isotopes and dissolved ions

A wide range of lithologic units and tectonic disturbances by cross-cutting faults and folds has resulted in the quite complex hydrogeological setting of the sedimentary outlier and its surroundings at Mekelle, northern Ethiopia. The environmental isotopes of oxygen and hydrogen and patterns of dissolved ion concentrations in the groundwater, coupled with understanding of the three-dimensional geological framework, are used to conceptualize the groundwater flow model and recharge–discharge mechanisms in the area. In agreement with the piezometric-surface map, recharge areas are determined to be the highlands (northwest, north, east and south of the study area), characterized by relatively more depleted isotopic compositions, higher d-excess, and lower concentrations of dissolved ions in the groundwater samples; the narrow major river valleys of Giba, Illala, Chelekot and Faucea Mariam are discharge areas. The groundwater divide between the Tekeze and the Denakil basins coincides with the surface-water divide line of these two basins. In most cases, groundwater feeds the semi-perennial streams and rivers in the area. However, isotopic signatures in some wells indicate that there are localities where river flow and seepage from micro-dams locally feed the adjacent aquifers. The lithostratigraphic, geomorphologic, isotopic and hydrochemical settings observed in this study indicate that three groundwater flow systems (shallow/local, intermediate and deep/semi-regional) can exist here. Tritium data indicate that the groundwater in the study area has generally short residence time and is dependent on modern precipitation.RésuméUne large gamme d’unités lithologiques et de perturbations tectoniques par des failles et plis entrecroisés créent un schéma hydrogéologique relativement complexe de formations sédimentaires aux environs de Mekelle, nord de l’Éthiopie. Les isotopes environnementaux de l’oxygène et de l’hydrogène et le schéma des concentrations en ions dissous dans les eaux souterraines, couplés avec la compréhension du schéma géologique tridimensionnel, ont été utilisés pour conceptualiser le modèle des écoulements souterrains et les mécanismes de recharge et décharge dans cette région. En accord avec la carte piézométrique, les zones de recharge ont été délimitées dans les montagnes (nord-est, nord, est et sud de la zone d’étude), caractérisées par une composition isotopique relativement appauvrie, un excès en deutérium élevé et de faibles concentrations en ions dissous dans les eaux souterraines; les étroites vallées des rivières principales Giba, Illala, Chelekot et Faucea Mariam sont les lieux de décharge. La zone de partage des écoulements souterrains entre les bassins de Tekeze et Denakil coïncide avec la zone de partage des eaux de surface de ces deux bassins versants. Dans la plupart des cas, les eaux souterraines alimentent les fleuves et rivières semi-pérennes de cette région. Toutefois, les signatures isotopiques de quelques puits indiquent qu’il y a des localités où les flux de rivières et les infiltrations de micro-barrages alimentent localement les aquifères limitrophes. Le cadre litho-stratigraphique, géomorphologique, isotopique et hydrochimique décrit dans cette étude indique que trois systèmes de flux d’eau souterraine (peu profond/local, intermédiaire, profond/semi-régional) peuvent coexister ici. Les données de tritium indiquent que les eaux souterraines de cette région d’étude ont généralement un court temps de résidence et dépendent des précipitations actuelles.ResumenUn amplio rango de unidades litológicas y de perturbaciones tectónicas por fallas transversales y pliegues ha resultado en una configuración hidrogeológica muy compleja del afloramiento sedimentario y sus alrededores en Mekelle, en el norte de Etiopía. Se usaron los isótopos ambientales de oxígeno e hidrógeno y sus patrones de concentraciones de iones disueltos en el agua subterránea, conjuntamente con la comprensión del marco geológico tridimensional, para conceptualizar el modelo de flujo de agua subterránea y los mecanismos de recarga – descarga en el área. De acuerdo con el mapa de la superficie piezométrica, las áreas de recarga se determinaron para las tierras altas (norte, este, y sur del área de estudio), caracterizadas por una composición de isótopos relativamente más empobrecidas, mayores excesos de deuterio, y bajas concentraciones de iones disueltos en las muestras de agua subterránea; los estrechos valles de los ríos principales de Giba, Illala, Chelekot y Faucea Mariam son áreas de descarga. La divisoria de agua subterránea entre las cuencas de Tekeze y Denakil coincide con las líneas de divisorias de agua superficial de estas dos cuencas. En la mayoría de los casos, el agua subterránea alimenta a corrientes y ríos semipermanentes en el área. Sin embargo, las firmas isotópicas en algunos pozos indican que hay localidades donde el flujo del río y la filtración desde micro presas alimentan localmente a los acuíferos adyacentes. Las configuraciones litoestratigráfica, geomorfológica, isotópica e hidroquímica observadas en este estudio indican que allí pueden existir tres sistemas de flujo de agua subterránea (somero / local, intermedio y profundo / semi-regional) pueden existir aquí. Los datos de tritio indican que el agua subterránea en el área de estudio tiene generalmente un tiempo de residencia corto y depende de las precipitaciones actuales.摘要大范围的岩性单元及横切断层和皱褶造成的构造扰动使埃塞俄比亚北部沉积外露层及周边地区的水文地质背景相当复杂。地下水中的氧和氢环境同位素及溶解离子含量,加上对三维地质结构的认识,用来概念化地下水流模型和本地的补给-排泄机理。与压力水面图相符,补给区处于(研究区西北部、北部、东部和南部的)高地,特点就是地下水样品中同位素组分相对枯竭,氚含量较高以及溶解离子较低;Giba, Illala, Chelekot 和 Faucea Mariam狭窄的主要河谷是排泄区。Tekeze流域和Denakil之间的地下水分水岭与这两个流域的地表分水线相一致。在大部分情况下,地下水补给本地区的半常流河。然而,一些井中的同位素特征显示有些地方河水水流和小型水坝的渗流局部上补给毗邻的含水层。本研究中观测到的岩性地层、地貌、同位素和水化学背景表明,这里可能存在着三个地下水流系统(浅层/局部系统、中间系统和深层/半区域系统)。氚资料显示研究区的地下水通常停留时间较短,这取决于现代降水量。ResumoUma grande gama de unidades litológicas e de distúrbios tectónicos causados por falhas e dobras que afetam as estruturas geológicas resultaram num ambiente hidrogeológico muito complexo dos afloramentos sedimentares e das unidades envolventes em Mekelle, no norte da Etiópia. Os isótopos ambientais de oxigénio e hidrogénio e os padrões das concentrações dos iões dissolvidos na água subterrânea, acoplados ao conhecimento da estrutura geológica tridimensional, são usados para concetualizar o modelo de fluxo e os mecanismos de recarga-descarga na área. De acordo com o mapa da superfície piezométrica, as áreas de recarga são determinadas pelas terras altas (noroeste, norte, este e sul da área de estudo), caraterizadas por uma composição isotópica relativamente mais deficitária, maior excesso de d e menores concentrações de iões dissolvidos nas amostras de água subterrânea; os vales estreitos dos rios principais, o Giba, o Illala, o Chelekot e o Faucea Mariam, correspondem a áreas de descarga. A divisória de águas subterrâneas entre as bacias de Tekeze e de Denakil coincide com a divisória das águas superficiais destas duas bacias. Na maior parte dos casos, a água subterrânea alimenta os fluxos semi-perenes e os rios da área. No entanto, as assinaturas isotópicas nalguns poços indicam que há locais onde o fluxo dos rios e a drenância a partir de micro-barragens alimentam localmente os aquíferos adjacentes. As estruturas litoestratigráficas, geomorfológicas, isotópicas e hidroquímicas observadas neste estudo indicam que podem existir aqui três sistemas de fluxo de água subterrânea (pouca profundidade/local, intermédio e profundo/semi-regional). Os dados de trítio indicam que a água subterrânea na área de estudo tem geralmente um tempo de residência curto e que é dependente de precipitação moderna.

Geochemistry and Water Quality

Water quality is the measure of physical, chemical, radiological and biological property of water. Water quality poses limits on the suitability of water use for a particular development purpose.

Interaction of surface water and groundwater in the Nile River basin: isotopic and piezometric evidence

Past discussions around water-resources management and development in the River Nile basin disregard groundwater resources from the equation. There is an increasing interest around factoring the groundwater resources as an integral part of the Nile Basin water resources. This is hampered by knowledge gap regarding the groundwater resources dynamics (recharge, storage, flow, quality, surface-water/groundwater interaction) at basin scale. This report provides a comprehensive analysis of the state of surface-water/groundwater interaction from the headwater to the Nile Delta region. Piezometric and isotopic (δ18O, δ2H) evidence reveal that the Nile changes from a gaining stream in the headwater regions to mostly a loosing stream in the arid lowlands of Sudan and Egypt. Specific zones of Nile water leakage to the adjacent aquifers is mapped using the two sources of evidence. Up to 50% of the surface-water flow in the equatorial region of the Nile comes from groundwater as base flow. The evidence also shows that the natural direction and rate of surface-water/groundwater interaction is largely perturbed by human activities (diversion, dam construction) particularly downstream of the Aswan High Dam in Egypt. The decrease in discharge of the Nile River along its course is attributed to leakage to the aquifers as well as to evaporative water loss from the river channel. The surface-water/groundwater interaction occurring along the Nile River and its sensitivity to infrastructure development calls for management strategies that account groundwater as an integral part of the Nile Basin resources.RésuméLes discussions passées sur la gestion et le développement des ressources en eau dans le bassin du Nil n’introduisent pas dans l’équation les ressources en eau souterraine. Il y a un intérêt croissant pour intégrer les ressources en eau à part entière dans les ressources en eau du bassin du Nil. Le manque de connaissances concernant la dynamique des ressources en eau souterraine (recharge, stockage, écoulement, qualité, interaction entre les eaux de surface et les eaux souterraines) à l’échelle du bassin freine le développement. Cet article fournit une analyse complète de l’état de l’interaction entre les eaux de surface et les eaux souterraines de la tête de bassin à la région du delta du Nil. Les données piézométriques et isotopiques ((δ18O, δ2H) mettent en évidence que le Nil passe d’un état de cours d’eau drainant dans les régions de tête de bassin à un état de cours d’eau drainé principalement dans les zones basses arides du Soudan et de l’Egypte. Des zones spécifiques d’infiltration d’eau vers les aquifères adjacents sont cartographiées à l’aide de deux sources de données probantes. Jusqu’à 50% du débit d’eau de surface dans la région équatoriale du Nil provient des eaux souterraines, constituant le débit de base. Les données montrent également que la direction naturelle des écoulements et des taux d’échange dans les interactions entre les eaux de surface et les eaux souterraines sont en grande partie perturbées par les activités humaines (dérivation, construction de barrages) particulièrement en aval du Haut Barrage d’Assouan en Egypte. La diminution du débit du Nil le long de son cours est attribuée à des infiltrations vers les aquifères ainsi qu’à la perte d’eau par évaporation à partir du lit du fleuve. Les interactions entre les eaux de surface et les eaux souterraines qui se produisent le long du Nil et leur sensibilité au développement d’infrastructure nécessitent des stratégies de gestion qui prennent en considération les eaux souterraines en tant que partie intégrante des ressources en eau du bassin du Nil.ResumenLas discusiones pasadas sobre la gestión y el desarrollo de los recursos hídricos en la cuenca del río Nilo desprecian los recursos hídricos subterráneos de la ecuación. Existe un creciente interés por la influencia de los recursos hídricos subterráneos como parte integral de los recursos hídricos de la Cuenca del Nilo. Esto se ve obstaculizado por la falta de conocimientos sobre la dinámica de los recursos hídricos subterráneos (recarga, almacenamiento, flujo, calidad, interacción agua superficial / agua subterránea) a escala de cuenca. Este artículo ofrece un análisis exhaustivo del estado de la interacción agua de superficie / agua subterránea desde las cabeceras hasta la región del delta del Nilo. Las evidencias piezométricas e isotópicas (δ18O, δ2H) revelan que el Nilo cambia de una corriente ganadora en las regiones de cabeceras a perdedora en la mayor parte de las tierras bajas áridas de Sudán y Egipto. Las zonas específicas de la filtración del agua del Nilo hacia los acuíferos adyacentes se mapean utilizando dos fuentes de evidencia. Hasta el 50% del flujo superficial de agua en la región ecuatorial del Nilo proviene del agua subterránea como flujo base. La evidencia también muestra que la dirección natural y la tasa de interacción agua de superficie / agua subterránea está en gran medida perturbada por las actividades humanas (desvío, construcción de presas) particularmente aguas abajo de la presa de Aswan en Egipto. La disminución en la descarga del río Nilo a lo largo de su curso se atribuye a la filtración hacia los acuíferos, así como a la pérdida de agua por evaporación del canal del río. La interacción agua de superficie / agua subterránea que se produce a lo largo del río Nilo y su sensibilidad al desarrollo de infraestructuras requiere estrategias de manejo que consideren el agua subterránea como una parte integral de los recursos de la Cuenca del Nilo.摘要过去有关尼罗河流域的论述及水资源管理和开发忽视了地下水资源这一部分。把地下水资源作为尼罗河流域水资源不可分割的一部分越来越得到了人们的重视。但由于缺乏流域尺度上对地下水资源动力学(补给量、储存量、水流量、质量、地表水/地下水相互作用)方面的了解而受到阻碍。本文综合分析了河流源头到尼罗河三角洲地区地表水/地下水相互作用的状态。压力水面和同位素(δ18O和 δ2H)证据显示,尼罗河在河头地区为潜水补给河,而到苏丹和埃及的干旱低地通常变为渗失河。利用两个证据来源绘出了尼罗河河水渗流到毗邻含水层的特定区域图。在赤道地区,多达50%的尼罗河地表水流来自作为基流的地下水。证据还显示,地表水/地下水相互作用的自然方向和强度很大程度上受到人类活动的影响(引水及大坝建设),特别是受到下游阿斯旺水坝的影响。尼罗河沿河道的排泄量减少主要是因为渗漏至含水层以及河道的蒸发损失。沿尼罗河出现的地表水/地下水相互作用及其对基础设施建设的敏感性迫切需要采纳新的管理策略,即把地下水作为尼罗河水资源不可分割的一部分。ResumoDiscussões passadas sobre a gestão dos recursos hídricos e desenvolvimento na bacia do Rio Nilo desconsideraram os recursos hídricos subterrâneos da equação. Existe um interesse crescente sobre consignar os recursos hídricos subterrâneos como uma parte integrante dos recursos hídricos da Bacia do Nilo. Isso é dificultado pela falta de conhecimento sobre a dinâmica dos recursos hídricos subterrâneos (recarga, armazenamento, escoamento, qualidade, interação águas superficiais/subterrâneas) na escala da bacia. Esse artigo apresenta uma análise abrangente do estado da interação entre águas superficiais/subterrâneas da cabeceira a região do delta do Nilo. Evidencias piezométricas e isotópicas (δ18O, δ2H) revelaram que o Nilo muda de uma corrente de ganho em suas cabeceiras para praticamente correntes de perda nas planícies áridas do Sudão e do Egito. Zonas específicas de infiltração das águas do Nilo aos aquíferos adjacentes são mapeadas usando as duas fontes de evidencia. Mais de 50% do escoamento das águas superficiais da região equatorial do Nilo vem das águas subterrâneas como escoamento de base. A evidencia também mostra que a direção natural e taxa de interação entre as águas superficiais/subterrâneas é amplamente perturbada por atividades humanas (diversões, construções de barragens) particularmente a montante da barragem Aswan High no Egito. A diminuição da descarga do Rio Nilo ao longo do seu curso é atribuída à infiltração aos aquíferos como para perdas evaporativas pelo canal fluvial. A interação das águas superficiais/subterrâneas ocorrendo ao longo do Rio Nilo e sua sensitividade a desenvolvimentos de infraestrutura clamam por estratégias de gestão que considerem as águas subterrâneas como uma parte integrante dos recursos da Bacia do Nilo.

Groundwater Occurrence in Regions and Basins

The broad volcanic plateau (Fig. 1.2) accounts for about 25 % of Ethiopian landmass. The Ethiopian volcanic plateau is a thick monotonous, rapidly erupted pile of locally deformed, flat lying basalts consisting of a number of volcanic centers with different magmatic character and with a large range of ages.