Major ions composition of the groundwater and surface water systems and their geological and geochemical controls in the Ethiopian volcanic terrain

Broad hydrochemical survey has been carried out to study the spatial variation of the major ions composition of the surface and groundwater systems in the Ethiopian volcanic terrain and associated Plio-Quaternary sediments. The result revealed wide hydrochemical variations controlled by geological, geochemical, geomorphological and climatological factors. The total ionic concentration increases dramatically from the highlands towards the rift valley following the regional groundwater flow directions to low-lying regions characterized by low annual rainfall and high evapotranspiration. In the rift the total dissolved solids (TDS) variation is dramatic (in places more than 50 fold). The highland waters are predominantly fresh with low total ionic concentrations, TDS varying often from 50 to 1200 mg/l; with dominant ions of Ca, Mg and HCO3. The rift valley waters have very high TDS (varying in a wide range between 200 and 73,015 mg/l) characterized by high Na and HCO3, in places highly enriched with F. Waters of the southern and central Main Ethiopian Rift (MER) are dominated by HCO3 tending to SO4 and Cl type in northern and central Afar mainly due to changes in lithology related to evaporite deposits and due to the natural chemical evolution of waters as they move from highland recharge to low-lying rift discharge areas. Deep geothermal waters, alkaline terminal lakes of the MER and small highly evaporated saline and brackish lakes of the Afar have the highest ionic concentrations. Keywords: Ethiopian rift, hydrogeology, ionic concentration, TDS, volcanic terrain SINET: Ethiopian Journal of Science Vol. 28(2) 2005: 171-18

Bathymetric survey and estimation of the water balance of Lake Ardibo, northern Ethiopia

Quantification of the water balance components and bathymetric survey is very crucial for sustainable management of lake waters. This paper focuses on the bathymetry and the water balance of the crater Lake Ardibo, recently utilized for irrigation. The bathymetric map of the lake is established at a contour interval of 10 meters based on depth measurements at 176 points. The depth-area relationship shows that only less than 25 percent of the area is greater than 50 meters deep. The maximum-recorded depth is 62.4 m. Input from direct rainfall and evaporation are the most important components of the water balance of the lake. The annual input from rainfall and inflow from surface runoff is 18.43 and 5.53 million cubic meters, respectively. The average annual open water evaporation, estimated from Colorado Class-A Pan records and Penman modified method is 23.49 million cubic meters. The net groundwater flux estimated as a residual of the other water balance components is ­5.03 million cubic meters, indicating that groundwater outflow is greater than groundwater inflow. The recent abstraction for irrigation appears to affect the water balance and lead to grave environmental consequences on the fragile lacustrine ecosystem. Key words/phrases: Bathymetry, crater lake, hydrology, irrigation, water balance SINET: Ethiop. J. Sci. Vol.27(1) 2004: 61-6

Hydrodynamics of Selected Ethiopian Rift Lakes

The Main Ethiopian Rift Valley Lakes suffer from water level fluctuations due to several natural and anthropogenic factors. Lakes located at terminal positions (no surface water outflow) are highly affected by the fluctuations. These fluctuations are disturbing the stability of the ecosystems, putting very serious impacts on the lives of many animals and plants around the lakes. Hence, studying the hydrodynamics of the lakes was found to be very essential. The main purpose of this study is to find the most significant factors that contribute to the water level fluctuations and also to quantify the fluctuations so as to identify the lakes that need special attention. The research methodology includes correlation and least squares regression of lake levels on rainfall, discharge and evaporation, multi-temporal satellite image analysis and land use change assessment. The results of the study revealed that much of the fluctuations in the lake water levels are caused by human activities especially for the lakes in the Central Ethiopian Rift. Lakes Abiyata, Chamo, Ziway and Langano are declining while Abaya and Hawassa are rising. Among the studied lakes, Abiyata is drastically reduced in size (about 28% of its area in 1986) due to both human activities (most dominant ones) and natural factors.  The other seriously affected lake is Chamo with about 11% reduction in its area between 1986 and 2010. Lake Abaya was found to be relatively stable during this period (showed only a 0.8% increase in its area). KEYWORDS: Correlation, Fluctuation, Lake Level, Landsat Satellite Images, Land Use Change, Least Squares Regression

REVIEW OF HYDROGEOLOGY OF TEKEZE RIVER BASIN: IMPLICATIONS FOR RURAL AND URBAN WATER SUPPLY IN THE REGION

This paper reviews the available published and unpublished data on the geology, hydrology and hydrogeology of the Tekeze River Basin (trb) with the aim to better conceptualize the hydrodynamics of the basin and its implications on the water resources potential for the development of rural and urban water supply system.  The work also highlights some of the knowledge gaps to fully understand the hydrogeology of the basin and proposes better scientific approach that will lead to the understanding of the movement and occurrence of groundwater in the complex river basin. The trb constitutes different rock types with ages ranging from Precambrian to Quaternary. These rocks are affected by different sets of faults, folds and lineaments with different orientations. The groundwater occurrence and flow is strongly controlled by the geomorphology, geological structures, type of rock and their hydraulic characteristics. The major aquifers in the basin are the Trap Basalts, the Antalo Limestone Formation, the Adigrat Sandstone and Quaternary sediments. An increase in permeability in shales and marls is observed due to the presence of dolerite dykes and sills. The Neoproterozoic basement rocks and associated intrusive bodies, and the Paleozoic sediments are characterised by shallow and localised aquifers. In many places the igneous intrusive bodies act as aquicludes. The major faults and associated drag folds in the basin form favourable conditions for groundwater recharge and groundwater localisation irrespective of lithology

WEAP-MODFLOW dynamic modeling approach to evaluate surface water and groundwater supply sources of Addis Ababa city

The integration between WEAP and MODFLOW models coupled via LinkKitchen helps to create a dynamic link between surface water and groundwater supply sources of Addis Ababa city. Possible impacts of natural and anthropogenic stresses on surface water reservoirs volume and groundwater storage have been assessed through water supply scenario analysis. Besides, contrary to other surface water hydrological models, the unique nature of WEAP adds water demand assessment by simulating Addis Ababa city near future water demand coverage under three population projection scenarios. The water demand projections of Addis Ababa city indicates 100% water demand coverage will not be achieved for high (4.6%), medium (3.8%) and low (2.8%) population growth rate projections, even with all the emerging and planned water supply projects start production up until 2025. Supply scenario projections indicate, as surface water reservoirs are highly sensitive to climate change and variability, the city groundwater supply sources will be noticeably affected by the emerging and planned groundwater supply expansion schemes. If groundwater abstraction continues to reach to zero unmet demand, more than 30-meter groundwater level decline can be registered in 2025. To foresee the combined effect of both natural and anthropogenic stresses on Addis Ababa city water supply sources, best case (considering conditions which improve Addis Ababa city water supply) and worst case (considering conditions stressing Addis Ababa city water supply) scenarios were tested. The best case scenario results zero unmet water demand in Addis Ababa city in most wet months of future projection years up to 2025, with likely decline of about 6 meter on the groundwater level. The worst case scenario to the contrary shows, Addis Ababa city water demand coverage will potentially be reduced to a maximum of 35% in 2025, with seasonal and annual variability. The dynamic link between surface water reservoirs and groundwater supply sources helps to gain insight into the potential consequences of continuously changing natural and anthropogenic conditions on Addis Ababa city water supply sources. Consequently, the significant predicted near future pressure on Addis Ababa city surface water and groundwater supply clearly indicate planning and developing alternative water supply sources outside of the boundary (Upper Awash basin) where the city is located should be immediately started in order to endure the pressure from the ever increasing demand. Otherwise, not only Addis will continue suffering unmet water demand for the years to come, but also the water supply sources will be severely impacted. Nonetheless, wherever the water supply sources, minimizing water loss, recycling and improving water use efficiency should be given at most priority

Groundwater potential mapping using GIS and remote sensing with multi-criteria decision-making in Shinile sub-basin, eastern Ethiopia

The main challenge for water resources development as well as food security in arid and semi-arid regions of Ethiopia is the hydroclimatic variability. Groundwater resources are largely the main sources of water supply in such regions, alleviating the pressure of hydroclimatic variability on water resources. The present study delineated the potential groundwater zones in the Shinile sub-basin by using geospatial techniques. The criteria used were: geology, geomorphology, slope, soil, lineament density, drainage density, land use land cover, topographic wetness index, topographic roughness index, and rainfall. The relative weights were given by the analytic hierarchy process. A validation was done using the area under the curve (AUC=0.941) of the receiver operating curve (ROC) from borehole data. The study region was partitioned to low, moderate, and high potential groundwater zones having respectively 1.5%, 43%, and 55% of the total area. The high potential areas are concentrated in the central part where alluvial and lacustrine sediment is the dominant geologic unit. The validation results suggest that the geospatial identification of groundwater potential zones effectively performed well in the study area. This study is very important for water management experts as well as for stakeholders and policymakers in the study region

Numerical groundwater flow modeling of the central main Ethiopian Rift lakes basin

A three dimensional steady-state finite difference numerical groundwater flow model (modflow) was used to study the groundwater-surface water interactions in the Central Main Ethiopian Rift lakes basin. Special emphasis is given to quantifying the groundwater fluxes and the subsurface hydraulic connection of the rift valley lakes. The result shows that despite the geographic proximity of the lakes the groundwater flux is highly variable, mainly owing to geologic factors. The net groundwater flux ranges from 7,011 m 3 /day for Lake Abiyata to 651,022 m 3/day for Lake Shala. The groundwater flow converges from all sides of the basin to the center of the rift where the lakes are clustered, and ultimately to the terminal Lake Shala. Local, intermediate and regional flow systems were identified. The direction and magnitude of the faults have strong bearing on the occurrence and movement of the groundwater. The major flow systems and groundwater flux into the lakes are controlled strongly by rift faults. SINET: Ethiopian Journal of Science Volume 24, No. 2 (December 2001), pp. 167-184 Key words/phrases: Groundwater flow, groundwater modeling, hydrogeology, Rift lakes, water balanc

Surface kinetic temperature mapping using satellite spectral data in Central Main Ethiopian Rift and adjacent highlands

Remotely sensed thermal-infrared spectral data can be used to derive surface temperature of any object if the optical and thermal properties are known. In this study TM band six has been used to assess the spatial variability of the kinetic temperature of the central Ethiopian rift lakes and adjacent highlands. NOAA-AVHRR data have been used to understand the relative differences of the monthly lake surface kinetic temperatures. The result revealed that despite the limited topographic differences of the rift lakes and their proximity, the surface kinetic temperature difference is high, mainly due to groundwater and surface water fluxes. From thermal signature analysis two hot springs below the lake bed of Ziway were discovered. The various hot springs around three of the lakes have limited role in the overall water budget of these lakes as compared to the total surface water and cold groundwater fluxes. Temperature from the land surface shows also large variations with altitude and land cover types. SINET: Ethiopian Journal of Science Vol. 24, No. 1 (June 2001), pp. 51-68 Key words/phrases: Kinetic temperature, rift lakes, spectral data, thematic mapper, thermal infra-re