Accurate characterization of aquifers remains challenging for large-scale systems because of the spatial heterogeneity of hydraulic properties and temporal variability of hydrologic inputs. This study highlights the importance of integrating geological, hydrogeological and geophysical approaches to characterize an aquifer and evaluate groundwater productivity. Data from geological maps, drill logs, a pumping test, vertical electrical soundings (VES) and different field hydrogeological studies were combined and applied to a heavily extracted aquifer system—Lake Haramaya watershed, Ethiopia. From the geological characterization, the aquifer was found to be a single heterogeneous and anisotropic unconfined unit. Significant differences were found between the three-dimensional geological models of the aquifer developed from the drill logs and VES data; the VES data were likely affected by moisture content. The pumping-test and VES data were combined to estimate transmissivity (T; 126.5 ± 25.8 m²/day) and hydraulic conductivity (K; 4.1 ± 1.0 m/day). This combined use allowed for a reduction in uncertainty (40.1% for T and 33.3% for K) compared with values estimated from the VES data alone. The combined approach also allowed for much greater spatial coverage and a higher resolution characterization of the aquifer. The available volume of groundwater resource in the system was estimated at ~0.62 ± 0.09 km³. The groundwater extraction rate was ~30,120 m³/day, approximately double the estimated sustainable yield of the aquifer (15,720 m³/day). This showed that the current exploitation rate could exhaust groundwater resources in 27–32 years and should be reduced by 50% to ensure sustainability of the groundwater resource.

Sustainable development and management of groundwater resources require application of scientific principles and modern techniques. An integrated approach is implemented using remote sensing and geographic information system (GIS)-based multi-criteria evaluation to identify promising areas for groundwater exploration in Raya Valley, northern Ethiopia. The thematic layers considered are lithology, lineament density, geomorphology, slope, drainage density, rainfall and land use/cover. The corresponding normalized rates for the classes in a layer and weights for thematic layers are computed using Saaty’s analytical hierarchy process. Based on the computed rates and weights, aggregating the thematic maps is done using a weighted linear combination method to obtain a groundwater potential (GP) map. The GP map is verified by overlay analysis with observed borehole yield data. Map-removal and single-parameter sensitivity analyses are used to examine the effects of removing any of the thematic layers on the GP map and to compute effective weights, respectively. About 770 km²(28 % of the study area) is designated as ‘very good’ GP. ‘Good’, ‘moderate’ and ‘poor’ GP areas cover 630 km²(23 %), 600 km²(22 %) and 690 km²(25 %), respectively; the area with ‘very poor’ GP covers 55 km²(2 %). Verification of the GP map against observed borehole yield data shows 74 % agreement, which is fairly satisfactory. The sensitivity analyses reveal the GP map is most sensitive to lithology with a mean variation index of 6.5 %, and lithology is the most effective thematic layer in GP mapping with mean effective weight of 52 %.

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.

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.

The groundwater resources potential of Ethiopia is estimated to be about 40 billion cubic meters. Groundwater has been used as the main source of water supply since the 1970s for the main cities, towns and dispersed rural communities across the country, where provision of reticulated surface-water schemes is often expensive because of initial project construction costs and poor water quality. The exponential growth of the urban population and agriculture-led industrial development have resulted in greater attention to groundwater as the potentially cost-effective water supply source. As part of the growing focus on the use of groundwater, the Ethiopian government is currently implementing irrigation projects. One plan involves nine irrigation projects covering an estimated area of 8,000 ha, being developed on a pilot scale, with 9,000 test wells, 28,000 monitoring wells and 14,657 spring improvements. If this unprecedented Ethiopian groundwater-centred development plan is implemented successfully at such a scale, it is highly likely that its success will persuade other Sub-Saharan developing nations to put in place the necessary policies, regulations and investment for infrastructure and capacity development for exploring, exploiting and managing their groundwater resources.

Limited scientific information has been published on the application of tools in the geographic information system (GIS) environment to understand factors that influence groundwater recharge. The objectives of the research reported here were to understand the spatial variability of factors that influence groundwater recharge using GIS, and to estimate the amount of groundwater recharge and its spatial distribution in Illala catchment, northern Ethiopia. Reconnaissance surveys coupled with satellite imagery were used to collect data related to water, dry-deposition and hydrogeology from the study catchment. The data analysis involved geo-statistics. The chloride mass balance (CMB) method was applied to estimate mean groundwater recharge. The study catchment is distinguished by a semi-arid climate (average aridity index value of 0.35) and it is dominated by limestone-shale-marl intercalation. Mean chloride concentration in rainwater ranges from 0.4 to 1.28 mg/L, while values in dry deposition vary from 1.78 to 1.82 mg/m². Groundwater and runoff chloride concentration ranges are 1.4–31.96 mg/L and 0.60–1.56 mg/L, respectively. Mean annual groundwater recharge estimated by the CMB method varies from 6.1 to 288.3 mm, and the mean groundwater recharge represents 11.7% of the 548 mm mean annual rainfall. The CMB-derived groundwater recharge estimation showed a nearly comparable value with the recharge estimated by other approaches. More effort should be made to boost groundwater recharge using various recharge enhancing techniques such as constructing artificial recharge wells and water harvesting structures, targeting areas with the lowest recharge.