Hydro-Geology of Ethiopia

Hydro-Geology of Ethiopia

outline General facts about Ethiopia: Physiographic and climate: WATER RESOURCE POTENTIAL (surface water and Ground water) Groundwater quality: Recharge Groundwater development and utilization:

1. Generally description of Country: 1.1.introduction Ethiopia is a landlocked country with a total surface area of an approximately 1.12 million km 2 (110,430,000ha). The country characterize by topography that consist of complex massive highlands ; rugged terrain ; low plane and rift valley. At present time Ethiopia has a population of over 100 million.

Cont There is variable rainfall distribution (annual rainfall range around 3000mm to less than 50 mm) uneven and erratic distribution of enormous water resources potential (groundwater and surface water) Different groundwater recharge sources directly from rainfall, and indirectly from lakes and reservoirs Extremely variable geology (litho logy and structure) that allow for Existence of different aquifers type which consolidated and unconsolidated, fractured and karastified deep and shallow both homogeneous and heterogeneous aquifer

1.2.Geography: and climate: Ethiopia is located north eastern part of Africa ; bordered by Eritrea to north, Djibouti and Somalia to the west,and Kenya to the south. has high central plateau that that vary from 1,290 to 3,000 m above sea level with the highest mountain ranges flat-topped plateaus, deep gorges, and rift valley

There are Four physiographic regions can be identified in Ethiopia as shown Fig. 1 below (1).Northwestern plateau. It covers the tigray and amhara region includes the highest peak mountain in Ethiopia which named Ras dasheshin about 4620m abov e sea level.lake tana the source of Blue Nile also lie in the northwestern portion of the these highlands. (2).southeastern plateau. These are the second physiographic region covers south eastern part of the country. (3).Ethiopia rift valley. The Ethiopia rift valley is an extend part of great African rift system cut across the uplift Ethiopia plateau which is stretches from north east to south west It is Dominated by large, discontinuous normal fault that give rise to major tectonic escarpments separating the rift floor from the surrounding plateaus and producing lacks that occupy the floor of the rift valley between the two highland (northwestern, southeastern).

4. Afar triangle (also called Afar depression); The afar depression is other physiographic region catego ry in Ethiopia It is a geological depression which is the product of a tectonic triple rift junction where Gulf of Aden and red sea emerge on land and meet forming lowest area (126 below sea level at the Danakil depression.

fig1

1.2.2.Climate The climatic spectrum of the country ranges from equatorial desert to hot and cool steppe ; from tropical savannah and rain forest to warm temperate; from hot lowland to cool highlands. It is geographically diverse due to equatorial positionin g and variable topography (block 2008). The Mean annual temperature varies from over 30 0 C in the tropical lowlands to less than 10 0 C in very highlands of the country.

Based on the altitude,the climate can be classified in to five groups. Cold to moist(wurchi) average temperature 10 or less than 10 C. The altitude is above 3300 meters above sea level. cool to humid temperature(dega) This is typical cool highlands where average temperature range from 11.5 to 17.5 ;at 2300 to 3300 m above sea level. cool sub humid climate (Weina Dega ) Average annual temp ranges between 17.5 C and 20 c and comprises much of the highlands between 1,500-2300m above sea level. Warm semiarid(kola), temperature ranges between 20 C and 27.5 C and comprises much of the highlands between 500-1500 meters above sea level Hot arid (berha); average temperature greater than 27,comprise hot lowlands less than 500m altitude.

1.2.3. Rainfall The geographical location of Ethiopia and its endowment with favorable climate provides a relatively higher amount of rainfall in the country. Sometimes it falls either ahead of time or comes too late or even stops short in mid-season and varies special and seasonal

Based on the annual rainfall distribution patterns, three major rainfall regimes can be identified. The south-western and western areas of the country which are characterized by monomial (single peak) rainfall pattern. The central, eastern and north-eastern areas of the country experience a nearly bimodal rainfall distribution from February to may and from June to September locally called Belg and Kiremt season respectively The southern and southeastern areas of the country are also bi-modal (two peak) rainfall pattern (September to November and March to May )

2. WATER RESOURCE POTENTIAL 2.1.Surface Water Resources the country constitutes 99.3% of land area and the remaining 0.7% is covered with water bodies(source :Mowe 2013) Ethiopia has 12 major basins; which are Blue Nile; Tekeze; Mereb; Baro, Akobo; Gibeomo; Rift Valley (Lakes); Awash; Genale Dawa; Wabeshebelle; Aysha; danakel; Ogaden.map2 below However, three of the major basins(aysha; danakel; Ogaden) are dry basins, which do not have significant stream flows (Berhanu B.K, et.al, 2014). These twelve major river basins categorized into four major drainage systems: The Nile basin system (including Abbay or Blue Nile, Baro-Akobo, Tekeze and Mereb); covers 39 percent of the land mass of the countr y and drains the northern and central parts to westwards It account for about 69.83% of the estimated surface water flow in this basin system

Cont.. The Rift Valley system (including Denakil, Awash, Omo- Gibe and Rift valley Lakes) covers 21 percent of the country and drain south towards Kenya The Wabi-Shebelle and Genale-Dawa system.it covers 33 percent of the country and drains the southeastern mountains towards Somalia The East dray system including the Ogaden ; Ayesha covers 7 percent of the country.\ Ethiopia is endowed with quite a substantial amount of water resources potential from these rivers.

Most of the rivers are trans-bound ary and a significant amount of the runoff flows to surrounding countri es except awash Source :Ethiopian Valleys Development Studies Authority (1989),

s/n Table 2: River basins water resources potential study by Integrated River basin Master Plans s F;ow dirn/locati on River basin Area(km2) Water resource (BCM) % share 1 Abay 199,776 52.60 54.40 54.80 2 West Baro-Akobo 75,825 23.23 23.23 23.60 69.83 3 Tekeze 82,453 7.63 8.20 8.20 4 Mereb 5,854 0.26 0.72 0.65 5 East GenaleDawa 172,889 5.88 6.00 5.88 7.58 6 WabiShebele 192,960 3.40 3.40 3.16 7 South Rift Valley 53,034 5.64 5.64 5.60 Omo-Gibe 77,827 17.94 8 17.96 16.60 16.60 9 NE Awash 121,962 4.63 4.90 4.90 3.95 10 Dry or no water Ayesha 1,962 - - - 11 flow Denakil 58,268 0.86 0.86 0.86 0.69 12 Ogaden 89,051 - - - Table:1 (Source: MOWIR,2016) Total 1,131,860 *122.09 **123.95 ***124.25 100 It shows that 90% of the annual runoff goes to the rivers that flow into Sudan, Egypt, Somali and keneya.. Between 80-90% of Ethiopia's water resources is found in the four river basins namely, Abay (Blue Nile), Tekeze, Baro Akobo, and Omo Gibe in the west and south-western part of Ethiopia. On the other hand, the water resources available in the east and central river basins is only 10 to 20 per cent. According to Integrated River basin Master Plans surface water potential as identified and estimated about 124.4 Bcm.

3. Regional Hydrogeology The geology of Ethiopia comprises a mixture of ancient crystalline basement rocks, volcanic rocks associated with the East African Rift system and sediments of various ages from Precambrian to Quaternary age: Quaternary Sediments, Tertiary and Quaternary volcanic, Mesozoic Sedimentary rocks Precambrian basement rocks (dominantly metamorphic and intrusive Basement cover 18 Paleozoic and Mesozoic cover 25 Tertiary volcanic 40 Quaternary sediments and volcanic 17

Table General geologic proportion of Ethiopia (extracted from BGS, 2016 Regional Geology Coverage Basement cover 18% Paleozoic and Mesozoic cover 25% Tertiary volcanic 40% Quaternary sediments and volcanic 17%. map 2.geolocal map of Ethiopia (Geological Surveys of Ethiopia, 1999.

All the four major categories of rocks hold ground water at different specific capacities. According to different study implies volcanic rock s forms the most accessible aquifers in central Ethiopia while Sedimentary rock forms aquifers in areas where they are exhumed by erosion of the volcanic caps (such as in the Blue Nile basin,) or where the volcanic rocks didn t exist initially (such as the Ogaden lowlands. Metamorphic rocks as a whole can be considered to have low storage capacity in comparison with intrusive rocks.

Generally in Ethiopia the most important aquifers are formed by unconsolidated Quaternary sediments; Tertiary-Quaternary volcanic rocks; and Mesozoic consolidated sedimentary rocks. Basement aquifers are also important locally in some place. All the four hydro geologic settings studied in the country merely describes the possible yield at a very specific location. The yield of unconsolidated sediment quaternary aquifers characterize by low to high productive varies from 0.05 to 20 l/ s on average at different depth while Tertiary-Quatern ary volcanic rocks are highly productive and yielding from 5 to 150l/s ;Good water quality: generally low salinity, low flu oride. The aquifer thickness range from 50 to 1000m(( Keb

3.1 Ground Water Resources Different studies exist on Ethiopian water resources. Most of these studies are on surface water resources and little study has been conducted so far are related to local characteristics identification than regional and more wide characterization of the flow system.. Similar to surface water resources, the country is also endowed with substantial amount of groundwater resource which is not quantified accurately due to lack of sufficient hydro geological data. However, the nation wide preliminary water resources master plan study estimates groundwater of Ethiopia to be 2.6 billion cubic meters, and to date, only a small fraction of this resource is in use, mainly for local water supply purposes (MoWR, 2002). According to the Ministry of Water and Energy study the annual recharge rate is about 28 billion cubic meters (MoWE, 2012) and some others study groundwater renewable to be about 36 billion cubic with estimation of total ground water storage varying 1 to 10 billion m3 (source;afirica Ground Atlas > hydrogeology of Ethiopia).

4.Recharge The recharge to the groundwater system is also variable in Ethiopia both in space and time. The main source of recharge for the vast groundwater system is rainfall on the highlands and indirect (eg from river beds) also common. variable recharge, which is high on the plateau and low along the dry lands The major recharge occurs in the northeastern and southwestern plate au where annual rainfall is high The Ethiopian rift acts as a discharging zone, which contains numerous perennial rivers, fresh and salt lakes, cold and thermal springs (Alemay ehu, 2006).

5.Water Quality Groundwater quality is highly variable across Ethiopia, from fresh waters in many of the springs flowing from basement aquifers, to more saline waters in volcanic aquifers in parts of the Rift Valley and sedimentary aquifers of the plains. Fluoride and salinity has long been a recognized water quality concern in Ethiopia. The Concentrations of fluoride in groundwater that are higher than the WHO guideline value of 1.5 mg/l have been found across different part of Ethiopia, but are concentrated in the Rift Valley, linked to the volcanic geology. High concentration of Salt exist in sedimentary aquifers in the south, southeast and northeast of the country, is linked to the dissolution of evaporate minerals while in volcanic aquifer in rift valley is due to geothermal waters.(bgs/wateraid. 2001. Groundwater Quality: Ethiopia

6. Groundwater development and utilization groundwater development in ethiopia is complicated by highly variable hydro geological conditions besides techni cal; economical; challenge and other factors that hinder th e water sector to develop. As the result water resources development is still infancy in Ethiopia. Groundwater utilization has been highly confined to comm unity water supply using shallow well ;hand dug wells and unprotected springs, Limited deep boreholes were drilled in few rural areas, mainly in the rift valley, in some periph eral semi-arid regions and in the vast highland volcanic terrain. about 80 percent of the existing drinking water comes fro m groundwater (sources: (EPA and UNEP, 2008) Borehole ;hand gwell; natural spring traditional well etc are the source of ground water in the Ethiopia.

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