Tesfamichael Gebreyohannes

Bio: Tesfamichael Gebreyohannes is an academic researcher from Mekelle University. The author has contributed to research in topics: Groundwater & Groundwater recharge. The author has an hindex of 10, co-authored 23 publications receiving 572 citations. Previous affiliations of Tesfamichael Gebreyohannes include Free University of Brussels & Vrije Universiteit Brussel.

Impact of soil and water conservation measures on catchment hydrological response - a case in north Ethiopia.

Abstract: Catchment management in the developing world rarely include detailed hydrological components. Here, changes in the hydrological response of a 200-ha catchment management in north Ethiopia are investigated. The management included various soil and water conservation measures such as the construction of dry masonry stone bunds and check dams, the abandonment of post-harvest grazing and the establishment of woody vegetation. Measurements at the catchment outlet indicated a runoff depth of 5 mm or a runoff coefficient (RC) of 1.6 % in the rainy season of 2006. Combined with runoff measurements at plot scale, this allowed calculating the runoff Curve Number (CN) for various land uses and land management techniques. The pre-implementation runoff depth was then predicted using the CN values and a ponding adjustment factor, representing the abstraction of runoff induced by the 242 check dams in gullies. Using the 2006 rainfall depths, the runoff depth for the 2000 land management situation was predicted to be 26.5 mm (RC = 8 %), in line with current RCs of nearby catchments. Monitoring of the ground water level indicated a rise after catchment management. The yearly rise in water table after the onset of the rains (�T) relative to the water surplus (WS) over the same period increased between 2002-2003 ( �T/WS = 3.4) and 2006 ( �T/WS >11.1). Emerging wells and irrigation are other indicators for improved water supply in the managed catchment. Cropped fields in the gullies indicate that farmers are less frightened for the destructive effects of flash floods. Due to an increased soil water content, the crop growing period is prolonged. It can be concluded that this catchment management has resulted in a higher infiltration rate and a reduction of direct runoff volume by 81 % which has had a positive influence on the catchment water balance.

Spatial analysis of groundwater potential using remote sensing and GIS-based multi-criteria evaluation in Raya Valley, northern Ethiopia

Abstract: 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 %.

Groundwater recharge and flow in a small mountain catchment in northern Ethiopia

Abstract: The hydrodynamic behaviour of a sloped phreatic aquifer in the Tigray Highlands in northern Ethiopia is described. The aquifer is situated in the soils of a plateau on top of a basalt sequence and lies on steep slopes; the latter lead to hydraulic gradients that can cause high discharge fluxes. Distinct wet and dry seasons characterize the climate of the Tigray Highlands and recharge is absent during the dry season. Because of the fertile vertisols that have developed, the plateau is heavily cultivated and thus has great local economic, and hence social, importance. Water for land irrigation is almost exclusively delivered by rainfall, which is largely restricted to the period June—September. During the dry season, the water table drops dramatically and the aquifer drains nearly completely, under the strong gravity-driven, sustained discharges. This study strives to give insights into recharge and discharge mechanisms of the aquifer, in order to improve the effectiveness of the implemented water ...

Mass movement mapping for geomorphological understanding and sustainable development: Tigray, Ethiopia

Abstract: Mass movement topography characterises the escarpments and piedmont zones of the tabular ridges in the western part of the Mekelle outlier, Tigray, Ethiopia. Several types of mass movements can be distinguished. The first type is rockfall produced by 357 km rocky escarpments and cliffs during the rainy season. In the study area, every current kilometer of Amba Aradam sandstone cliff annually produces 3.7 m3 of rock fragments. However, this is an under-estimation of the actual cliff and escarpment evolution, which is also characterised by debris slides and small rock slides. In the debris flow class, three dormant flow types are recognised. The first type comprises preferential or undifferentiated mobilisations of the so-called plateau layers, the whitish sandy-clayey lacustrine deposits and the lower and upper basalts, and more especially, the swelling clays, derived from the basalts. These debris flows start on the nearly horizontal Amba Aradam Sandstone tabular extensions, jump over cliff recessions or even spurs, and go very far into the valley. In some instances parts of Amba Aradam sandstone and Antalo limestone cliffs are transported. Secondly, some debris flows take their origin in the Antalo limestone supersequence. It concerns deeply weathered layers resting upon aquicludes/aquitards. Finally, gigantic debris flows and rock slides occur around dolerite dyke ridges. About 20% of the total surface of the study area is occupied by landslide topography. Most of the landslides affect the steep edges of the table mountains or the dykes/sills. Mapping and listing of active and dormant mass movements increases knowledge in three domains which are crucial for sustainable development of the study area. The first one is geomorphological risk assessment. The distribution map of active rockfall and dormant landslides shows the areas where potential risks are located. Land use changes which improve the water infiltration capacity of dormant landslides, should be followed up. Secondly, the impact of global climate change on these risk areas can be assessed. The second domain is the water sector, which needs attention in the study area and in many parts of Africa. Landslides mobilised by hydrostatic pressures are related to the occurrence of aquicludes and aquifers. In the study area, landslide mapping led to the location of three aquitards, not described before. The third domain is the pedological mapping. In the study area, soil distribution is very well explained by the morphology and extension of dormant landslides. Finally, mapping of dormant landslides stimulates the academic debate on the geomorphological significance of mass movements in hillslope retreat in tropical areas.

The Water Balance

Abstract: The ultimate source of water for plants is precipitation; rain falling upon soil penetrates it at a rate depending upon the physical properties of that particular soil; snow and hail do the same after melting. If the rate of rainfall or the rate of production of water by melting exceeds the infiltration rate, then surface runoff occurs and the excess water drains into streams and eventually reaches the sea. That water which penetrates the soil replenishes the soil reservoir and when this is filled to capacity (see chapter 3) the surplus drains through into the aquifers. These are strata such as sand or chalk which can hold substantial quantities of recoverable water. Water held in the soil reservoir is drawn into plant roots and up their stems to be evaporated from the leaves back into the atmosphere, where it rejoins water evaporated from the sea, lakes and rivers and from the surface of wet soil. This so-called hydrological cycle (figure 1.2) depends for its continuance upon energy derived from the sun’s radiation and as will be shown in later chapters its rate is governed largely by meteorological factors.