Ethiopia occupies most of the Horn of Africa. The country covers approximately 1,221,900 square kilometers and shares frontiers with Sudan, Kenya, Somalia, and Djibouti. Its Red Sea coastline is about 960 kilometers long. The major physiographic features are a massive highland complex of mountains and plateaus divided by the Great Rift Valley and surrounded by lowlands along the periphery. The diversity of the terrain is fundamental to regional variations in climate, natural vegetation, soil composition, and settlement patterns.
Except for the Red Sea coastline, only limited stretches of the country's borders are defined by natural features. Most of Ethiopia's borders have been delimited by treaty. The Ethiopia-Somalia boundary has long been an exception, however. One of its sectors has never been definitively demarcated, thanks to disputed interpretations of 1897 and 1908 treaties signed by Britain, Italy, and Ethiopia. This sector was delimited by a provisional "Administrative Line" that was defined by a 1950 Anglo-Ethiopian agreement, when the United Nations (UN) established Somalia as a trust territory. After it became independent in 1960, Somalia refused to recognize any of the border treaties signed between Ethiopia and the former colonial powers. The Somali government also demanded a revision of the boundary that would ensure self-determination for Somali living in the Ogaden. Consequently, the frontier became the scene of recurrent violence and open warfare between Ethiopia and Somalia.
Figure 6. Topography and Drainage
Much of the Ethiopian landmass is part of the East African Rift Plateau. Ethiopia has a general elevation ranging from 1,500 to 3,000 meters above sea level. Interspersed on the landscape are higher mountain ranges and cratered cones, the highest of which, at 4,620 meters, is Ras Dashen Terara northeast of Gonder. The northernmost part of the plateau is Ethiopia's historical core and is the location of the ancient kingdom of Aksum. The national capital of Addis Ababa ("New Flower") is located in the center of the country on the edge of the central plateau (see fig. 6).
Millennia of erosion have produced steep valleys, in places 1,600 meters deep and several kilometers wide. In these valleys flow rapid streams unsuitable for navigation but possessing potential as sources of hydroelectric power and water for irrigation.
The highlands that comprise much of the country are often referred to as the Ethiopian Plateau and are usually thought of as divided into northern and southern parts. In a strict geographical sense, however, they are bisected by the Great Rift Valley into the northwestern highlands and the southeastern highlands, each with associated lowlands. The northwestern highlands are considerably more extensive and rugged and are divided into northern and southern sections by the valley of the Abay (Blue Nile).
North of Addis Ababa, the surface of the plateau is interspersed with towering mountains and deep chasms that create a variety of physiography, climate, and indigenous vegetation. The plateau also contains mountain ranges such as the Chercher and Aranna. Given the rugged nature of these mountains and the surrounding tableland, foreigners receive a false impression of the country's topography when Ethiopians refer to the landform as a plateau. Few of these peaks' surfaces are flat except for a scattering of level-topped mountains known to Ethiopians as ambas.
Southwest of Addis Ababa, the plateau also is rugged, but its elevation is slightly lower than in its northern section. To the southeast of Addis Ababa, beyond the Ahmar and Mendebo mountain ranges and the higher elevations of the southeastern highlands, the plateau slopes gently toward the southeast. The land here is rocky desert and, consequently, is sparsely populated.
The Great Rift Valley forms a third physiographic region. This extensive fault system extends from the Jordan Valley in the Middle East to the Zambezi River's Shire tributary in Mozambique. The segment running through central Ethiopia is marked in the north by the Denakil Depression and the coastal lowlands, or Afar Plain, as they are sometimes known. To the south, at approximately 9Ý north latitude, the Great Rift Valley becomes a deep trench slicing through the plateau from north to south, its width averaging fifty kilometers. The southern half of the Ethiopian segment of the valley is dotted by a chain of relatively large lakes. Some hold fresh water, fed by small streams from the east; others contain salts and minerals.
In the north, the Great Rift Valley broadens into a funnel-shaped saline plain. The Denakil Depression, a large, triangle-shaped basin that in places is 115 meters below sea level, is one of the hottest places on earth. On the northeastern edge of the depression, maritime hills border a hot, arid, and treeless strip of coastal land sixteen to eighty kilometers wide. These coastal hills drain inland into saline lakes, from which commercial salt is extracted. Along the Red Sea coast are the Dahlak Islands, which are sparsely inhabited.
In contrast with the plateau's steep scarps along the Great Rift Valley and in the north, the western and southwestern slopes descend somewhat less abruptly and are broken more often by river exits. Between the plateau and the Sudanese border in the west lies a narrow strip of sparsely populated tropical lowland that belongs politically to Ethiopia but whose inhabitants are related to the people of Sudan (see Ethiopia's Peoples, this ch.). These tropical lowlands on the periphery of the plateau, particularly in the far north and along the western frontier, contrast markedly with the upland terrain.
The existence of small volcanoes, hot springs, and many deep gorges indicates that large segments of the landmass are still geologically unstable. Numerous volcanoes occur in the Denakil area, and hot springs and steaming fissures are found in other northern areas of the Great Rift Valley. A line of seismic faults extends along the length of Eritrea and the Denakil Depression, and small earthquakes have been recorded in the area in recent times.
All of Ethiopia's rivers originate in the highlands and flow outward in many directions through deep gorges. Most notable of these is the Blue Nile, the country's largest river. It and its tributaries account for two-thirds of the Nile River flow below Khartoum in Sudan. Because of the general westward slope of the highlands, many large rivers are tributaries of the Nile system, which drains an extensive area of the central portion of the plateau. The Blue Nile, the Tekezā, and the Baro are among them and account for about half of the country's water outflow. In the northern half of the Great Rift Valley flows the Awash River, on which the government has built several dams to generate power and irrigate major commercial plantations. The Awash flows east and disappears in the saline lakes near the boundary with Djibouti. The southeast is drained by the Genale and Shebele rivers and their tributaries, and the southwest is drained by the Omo.
Diverse rainfall and temperature patterns are largely the result of Ethiopia's location in Africa's tropical zone and the country's varied topography. Altitude-induced climatic conditions form the basis for three environmental zones-- cool, temperate, and hot--which have been known to Ethiopians since antiquity as the dega, the weina dega, and the kolla, respectively.
The cool zone consists of the central parts of the western and eastern sections of the northwestern plateau and a small area around Harer. The terrain in these areas is generally above 2,400 meters in elevation; average daily highs range from near freezing to 16ÝC, with March, April, and May the warmest months. Throughout the year, the midday warmth diminishes quickly by afternoon, and nights are usually cold. During most months, light frost often forms at night and snow occurs at the highest elevations.
Lower areas of the plateau, between 1,500 and 2,400 meters in elevation, constitute the temperate zone. Daily highs there range from 16ÝC to 30ÝC.
The hot zone consists of areas where the elevation is lower than 1,500 meters. This area encompasses the Denakil Depression, the Eritrean lowlands, the eastern Ogaden, the deep tropical valleys of the Blue Nile and Tekezā rivers, and the peripheral areas along the Sudanese and Kenyan borders. Daytime conditions are torrid, and daily temperatures vary more widely here than in the other two regions. Although the hot zone's average annual daytime temperature is about 27ÝC, midyear readings in the arid and semiarid areas along the Red Sea coast often soar to 50ÝC and to more than 40ÝC in the arid Ogaden. Humidity is usually high in the tropical valleys and along the seacoast.
Variations in precipitation throughout the country are the result of differences in elevation and seasonal changes in the atmospheric pressure systems that control the prevailing winds. Because of these factors, several regions receive rainfall throughout most of the year, but in other areas precipitation is seasonal. In the more arid lowlands, rainfall is always meager.
In January the high pressure system that produces monsoons in Asia crosses the Red Sea. Although these northeast trade winds bring rain to the coastal plains and the eastern escarpment in Eritrea, they are essentially cool and dry and provide little moisture to the country's interior. Their effect on the coastal region, however, is to create a Mediterranean-like climate. Winds that originate over the Atlantic Ocean and blow across Equatorial Africa have a marked seasonal effect on much of Ethiopia. The resulting weather pattern provides the highlands with most of its rainfall during a period that generally lasts from mid-June to mid-September.
The main rainy season is usually preceded in April and May by converging northeast and southeast winds that produce a brief period of light rains, known as balg. These rains are followed by a short period of hot dry weather, and toward the middle of June violent thunderstorms occur almost daily. In the southwest, precipitation is more evenly distributed and also more abundant. The relative humidity and rainfall decrease generally from south to north and also in the eastern lowlands. Annual precipitation is heaviest in the southwest, scant in the Great Rift Valley and the Ogaden, and negligible in the Denakil Depression.
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