Kassa Amare

Spatial and temporal variability of river flows in the degraded semi-arid tropical mountains of northern Ethiopia

Water availability has for long been a critical issue in many developing countries. Despite its enormous potential of water resources, Ethiopia is suffering from a lack of water availability and threatened by the consequences of climate change. Well-considered planning to develop these resources is crucial. However, very few observational runoff data exist for this type of environments. Especially runoff data for catchments at the intermediate scale (100-10,000 km(2)) are lacking. This study assesses the runoff from 10 medium-sized catchments in the Geba river basin, a subcatchment of the Nile in the semi-arid degraded northern Ethiopian highlands. Flow depth records were automatically obtained every 10 minutes during the rainy seasons (July-September) of 2004-2007 and converted to continuous runoff discharge records. Cumulative annual runoff depths (46-395 mm) are mainly correlated with rainfall depth. Estimated runoff coefficients (9-47%) and are negatively correlated with the areal fraction of limestone outcrops in the catchments, indicating runoff transmission losses. Throughout the rainy season, increases in runoff depth and runoff coefficient were observed, which is partly attributed to an increase in baseflow throughout the season. The majority of the runoff occurs during flash floods, i. e. relatively short runoff events with often very high peak discharges. Characteristics of these floods are discussed with some examples, including an exceptionally large flood. Taking into account the difficult conditions for river discharge measurements, this study provides one of the most comprehensive analyses so far of the magnitude and dynamics of river discharges in Ethiopia.

Shock metamorphism of siliceous volcanic rocks of the El'gygytgyn impact crater (Chukotka, Russia)

The 18-km-diameter El’gygytgyn crater is located on the Chukotka peninsula, northeastern Russia. It represents the only currently known impact structure formed in siliceous volcanics, including tuffs. The impact melt rocks and target rocks provide an excellent opportunity to study shock metamorphism of volcanic rocks. The shockinduced changes observed in porphyritic volcanic rocks from El’gygytgyn can be applied to a general classifi cation of shock metamorphism of siliceous volcanic rocks. Strongly shocked volcanic rocks with phenocrysts converted to diaplectic quartz glass and partially melted feldspars as well as cryptocrystalline matrices are widespread in the El’gygytgyn crater. In particular, the following different stages of shock metamorphism are observed: (i) weakly to moderately shocked lavas and tuffs with phenocrysts and clasts of quartz and feldspars; (ii) moderately shocked volcanic rocks and tuffs with diaplectic glasses of quartz and feldspars; (iii) strongly shocked lavas and tuffs with phenocrysts of diaplectic quartz glass and fused glasses of feldspars in melted matrixes; and (iv) impact melt rocks and impact glasses. In addition, thin glassy coatings of voids in impact melt rocks have been observed. While the shock-induced changes of clasts of framework silicates in these volcanic rocks do not differ from respective changes in other crystalline rocks, the fi negrained matrix of porphyritic rocks is converted into fused glass at the same shock pressures as feldspar minerals. No remnants of fi ne-grained quartz are preserved in matrix converted into fused glass by shock.

Regional groundwater flow modeling of the Geba basin, northern Ethiopia

The Geba basin is one of the most food-insecure areas of the Tigray regional state in northern Ethiopia due to recurrent drought resulting from erratic distribution of rainfall. Since the beginning of the 1990s, rain-fed agriculture has been supported through small-scale irrigation schemes mainly by surface-water harvesting, but success has been limited. Hence, use of groundwater for irrigation purposes has gained considerable attention. The main purpose of this study is to assess groundwater resources in the Geba basin by means of a MODFLOW modeling approach. The model is calibrated using observed groundwater levels, yielding a clear insight into the groundwater flow systems and reserves. Results show that none of the hydrogeological formations can be considered as aquifers that can be exploited for large-scale groundwater exploitation. However, aquitards can be identified that can support small-scale groundwater abstraction for irrigation needs in regions that are either designated as groundwater discharge areas or where groundwater levels are shallow and can be tapped by hand-dug wells or shallow boreholes.RésuméLe bassin de Geba est l’une des régions les plus touchées par l’insécurité alimentaire de l’état régional de Tigray dans le nord de l’Ethiopie en raison de sécheresse récurrente résultant d’une répartition erratique des précipitations. Depuis le début des années 1990, l’agriculture pluviale a été soutenue par des projets d’irrigation à petite échelle, principalement par la collecte des eaux de surface, mais le succès a été limité. Par conséquent, l’utilisation des eaux souterraines pour l’irrigation a attiré une attention considérable. Le but principal de cette étude est d’évaluer les ressources en eau souterraine dans le bassin de Geba au moyen d’une modélisation MODFLOW. Le modèle est calibré en utilisant les niveaux d’eau souterraine observés, conduisant à une idée précise concernant les systèmes d’écoulement des eaux souterraines et les réserves. Les résultats montrent qu’aucune des formations hydrogéologiques ne peut être considérées en tant qu’aquifères permettant une exploitation des eaux souterraines à grande échelle. Cependant, des aquitards capables de supporter des prélèvements d’eaux souterraines à petite échelle pour les besoins de l’irrigation peuvent être identifiées soit dans les régions qui sont désignées comme des zones de décharge des eaux souterraines ou des zones où les niveaux d’eau souterraine sont peu profonds et peuvent être captés par des puits creusés à la main ou des forages peu profonds.ResumenLa cuenca de Geba es una de las zonas con mayor inseguridad alimentaria del estado regional de Tigray en el norte de Etiopía debido a las recurrentes sequías resultantes de la distribución irregular de las lluvias. Desde comienzos de los años noventa, la agricultura de secano ha sido apoyada mediante regímenes de riego a pequeña escala, principalmente mediante la recolección de aguas superficiales, pero el éxito ha sido limitado. Por lo tanto, el uso de agua subterránea para fines de riego ha ganado considerable atención. El propósito principal de este estudio es evaluar los recursos de agua subterránea en la cuenca de Geba mediante un enfoque de modelado MODFLOW. El modelo se calibra usando los niveles observados de agua subterránea, lo que da una idea clara de los sistemas de flujo de agua subterránea y reservas. Los resultados muestran que ninguna de las formaciones hidrogeológicas puede ser considerada como acuíferos que pueden explotarse en una explotación de agua subterránea a gran escala. Sin embargo, se pueden identificar acuitardos que pueden apoyar la extracción de agua subterránea a pequeña escala para las necesidades de riego en regiones designadas como áreas de descarga de aguas subterráneas o donde los niveles de aguas subterráneas son superficiales y pueden ser aprovechados por pozos excavados a mano o pozos poco profundos.摘要由于降雨分布极度不均从而造成周期性的干旱,致使Geba流域是埃塞俄比亚北部Tigray州粮食最得不到保障的地区之一。自从20世纪90年代开始,旱作农业主要通过地表水收集而进行的小规模灌溉计划得到支持,但成就有限。因此,使用地下水用于灌溉受到了人们的极大关注。本研究的主要目的就是通过MODFLOW模拟方法评价Geba流域的地下水资源。采用观测的地下水位校正模型,深入了解了地下水流系统和地下水储量。结果显示,没有一个水文地质层可以作为含水层进行大规模地下水的开采。然而,发现一些弱透水层,这些弱透水层在地下水排泄区或地下水位浅的地方可以进行地下水开采以满足灌溉所需,可以通过手工挖掘井或者浅的机井抽取。ResumoA bacia de Geba é uma das áreas de maior insegurança alimentar no estado regional de Tigray, Norte da Etiópia, devido às secas recorrentes resultantes da distribuição errática da precipitação. Desde o inicio da década de 1990, a agricultura dependente da chuva vem sendo mantida por sistemas de irrigação de pequena escala, majoritariamente utilizando águas superficiais, mas o êxito tem sido limitado. Desse modo, a utilização de águas subterrâneas para irrigação ganhou uma atenção considerável. O principal propósito desse estudo é avaliar os recursos hídricos subterrâneos na bacia de Geba por meio de modelagem utilizando o MODFLOW. O modelo é calibrado utilizando níveis freáticos observados, fornecendo uma visão clara sobre o sistema de fluxo e reservas de água subterrânea. Os resultados demonstram que nenhuma das formações hidrogeológicas podem ser consideradas como aquíferos que permitem a extração de água subterrânea em larga escala. Entretanto, podem ser identificados aquitardos que podem apoiar a captação de águas subterrâneas em pequena escala para utilização na irrigação em regiões que são tanto designadas como áreas de descarga como onde os níveis freáticos são rasos e podem ser captados por poços escavados a mão ou poços rasos.