Tenalem Ayenew

Evapotranspiration estimation using thematic mapper spectral satellite data in the Ethiopian rift and adjacent highlands

Quantification of actual evapotranspiration from spectral satellite data requires determination of the surface energy balance components with the support of field observations. In this paper an approach called Surface Energy Balance Algorithm for Land (SEBAL) is presented to calculate daily evaporation of some of the Ethiopian rift lakes and the surrounding landscape from the Thematic Mapper spectral satellite data on the basis of the energy balance approach. The result has been used in part to support long-term evapotranspiration estimation made using conventional methods. Conventional open-water evaporation estimation methods usually assume that the surface temperatures of lakes remain uniform and that evaporation proceeds more or less at the same rate over the total lake surface. However, this study clearly demonstrated that evaporation from a single lake varies depending upon surface temperature, albedo and on the incoming fluxes from groundwater and surface waters. The daily evaporation of the rift lakes is highly variable, ranging from 4.9 to 5.9 mm/day. On the land surface, actual evapotranspiration increases with altitude following the availability of moisture in the soil instead of the temperature gradient. On average it ranges from 0.2 mm/day in rift lacustrine soils and salty lake shores to around 4.5 mm/day in the highland mountain peaks covered with afro-alpine vegetation. The relative spatial differences of the SEBAL result from the land surface is in good agreement with the conventional evapotranspiration estimates.

Recent changes in the level of Lake Abiyata, central main Ethiopian Rift

Abstract The spatial and temporal variations in the level of Lake Abiyata and controlling natural and manmade factors are presented. This study has been made by combining evidence from hydrometeorological and lake level records, water budget analyses, aerial photograph and satellite imagery interpretations, and numerical groundwater flow modelling. The most important components of the water balance of the lake are precipitation, river inflow and evaporation. The lake level has been fluctuating considerably over a wide range (by 6 m during the last 60 years) strongly controlled by the precipitation trends in the adjacent highlands. Climatic changes and consequent reduction in the surface water inputs have resulted in the reduction of its size. Recent abstraction of water for irrigation and soda ash production have drastically changed both the lake level and its hydrochemistry. This change appears to have grave environmental consequences on the fragile rift lacustrine ecosystem.

Application of a spreadsheet hydrological model for computing the long-term water balance of Lake Awassa, Ethiopia

Abstract The water balance of the closed freshwater Lake Awassa was estimated using a spreadsheet hydrological model based on long-term monthly hydrometeorological data. The model uses monthly evaporation, river discharge and precipitation data as input. The net groundwater flux is obtained from model simulation as a residual of other water balance components. The result revealed that evaporation, precipitation, and runoff constitute 131, 106 and 83 × 106 m3 of the annual water balance of the lake, respectively. The annual net groundwater outflow from the lake to adjacent basins is 58 × 106 m3. The simulated and recorded lake levels fit well for much of the simulation period (1981–1999). However, for recent years, the simulated and recorded levels do not fit well. This may be explained in terms of the combined effects of land-use change and neotectonism, which have affected the long-term average water balance. With detailed long-term hydrogeological and meteorological data, investigation of the subsurface hydrodynamics, and including the effect of land-use change and tectonism on surface water and groundwater fluxes, the water balance model can be used efficiently for water management practice. The result of this study is expected to play a positive role in future sustainable use of water resources in the catchment.

Environmental isotope-based integrated hydrogeological study of some Ethiopian rift lakes

Environmental isotope (δ2H and δ18O) techniques supported by hydrochemical surveying, water balance and conventional hydrogeological mapping were used to study the hydrogeological system of four major lakes located in a closed central Ethiopian rift basin. The focus of attention is the groundwater and surface water interaction and the subsurface hydraulic connection of the lakes. The result shows that despite their geographic proximity, the lakes have highly variable isotopic and hydrochemical signature owing to geological and geomorphological factors. Two of the lakes are alkaline and found to be terminal from both surface water and groundwater point of view. They are in the state of high evaporation with respect to present day precipitation. The groundwater flow converges from all sides of the basin and plays very important role in the water balance of all the lakes. Major flow system in the rift and the hydraulic connection of the lakes is strongly controlled by the direction and extent of the extensional normal faults.

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.

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.

Overview of the Hydrogeology and Groundwater Occurrence in the Lake Tana Basin, Upper Blue Nile River Basin

The Blue Nile (Abay) River is originating in the Lake Tana basin where many perennial and seasonal streams feed the largest lake of Ethiopia (Tana). The basin is characterized by different volcanic formations covered by thick alluvial and residual Quaternary sediments at the center of the basin around Lake Tana. In this study, an attempt was made to outline the hydrogeology of the basin based on limited information from well lithologic logs and previous relevant works. The Lake Tana basin is considered as a potential area for surface water and groundwater development corridors. The hydrogeology and groundwater occurrence is complex due to tectonism and existence of different volcanic rocks covered with thick alluvial sediments at the center of the basin. The shallow aquifer systems in the middle of the basin from the alluvial sediments are tapped by local communities using shallow wells fitted with hand pumps. The volcanic aquifers are multi-layer and tapped from different volcanic layers. The latter are highly productive depending up on the degree of fracturing and weathering. In few cases, as in the case of Kola Diba area there are artesian wells. The groundwater system converges towards the center of the basin. The Lake Tana basin leaks groundwater to the adjacent Beles basin and through the Blue Nile outlet.

Morphometric Characteristics and Hydrology of Selected Ethiopian Rift Lakes

In the Main Ethiopian Rift (MER), intra-rift faulting and associated volcanic activities resulted in the formation of volcano-tectonic structural depressions, responsible for the formation of many lakes on the rift floor. These lakes are bordered to the east and west by elevated highlands where the main tributary feeder rivers originate. The MER is occupied by seven major lakes, namely, from south to north, Chamo, Abaya, Awasa, Shala, Abijata, Langano, and Ziway. Most of these lakes are closed basins and interconnected by groundwater, through a NE–SW-aligned regional fault system, whereas Abijata Lake receives surface water from Ziway and Lagano lakes by two small streams. The geomorphic characteristics and size of these lakes changed on time scales of decades to millennia in response to different factors such as volcanic activity, tectonics, climate change and, very recently, human impact (mainly water withdrawal, deforestation, and irrigation). The most prominent example is the dramatic change of Abijata, the size of which decreased dramatically in the last decades.