This blog shows mean annual and mean seasonal rainfall distribution in Ethiopia. The data employed to depict these records is a mean average that ranges from five to thirty years.
The inter-annual oscillation of the surface position of the Inter-Tropical Convergence Zone (ITCZ) causes a variation in the wind flow patterns over Ethiopia.
In its oscillation to the north and south of equator, the ITCZ passes over Ethiopia twice a year and this migration alternately causes the onset and withdrawal of winds from north and south.
As it drifts towards the north, equatorial jet streams from the south and southwest invade most parts of Ethiopia while the Trade Winds from the north retreat.
It southward drift marks the onset of the Trade Winds from the north and causes the equatorial monsoons to retreat. This periodical anomaly of winds causes rainfall to be variable and seasonal in Ethiopia.
The annual rainfall map shows the mean annual distribution of rainfall over the entire country.
The mountainous areas are of heavier rainfall amount compared with the surrounding lowlands. An increase of precipitation with altitude is also governed to continue up to a certain height known as the zone of maximum precipitation.
As the density and type of natural vegetation and the intensity of human activity is affected by the amount and distribution of rainfall, this map provides a more accurate explanation of the patterns of land use and land cover in the country.
The map shows that southwestern Ethiopia is the region of heaviest rainfall. It is the wettest part of the country with only two to four dry months in the year.
The mean annual rainfall for this region is about 1,500mm, but it is much higher in specific localities.
For instance, it is over 2,800mm in the southwestern part of Gore Awraja in Ilubabor and the western part of Arjo Awraja in Welega. Parts of Gimira, Kafa and Limu Awrajas in Kefa, Gore, Buno Bedele and Sor and Geba Awrajas in Ilubabor and Arjo and the southern extreme of Gimbi Awraja in Welega receive over 2,000mm of annual rainfall.
The western flat low lying region which is on the windward side of the mountains receives over 1,000mm of rainfall annually.
Mean annual rainfall gradually decreases towards the northeast and east. In central and north-central Ethiopia, the annual amount is moderate, about 1,100mm. Here again, there are some pockets where it amounts to over 2,000mm.
These include the western parts of Agew Midir, the extreme southeastern part of Metekel and the extreme north-central parts of Kola Dega Damot Awrajas in Gojam. In parts of northern Gonder, in central Wegera and central Simen Awrajas, mean annual rainfall is over 1,600mm.
In southeastern Ethiopia, the mean annual rainfall is about 700mm, but this amount varies from over 2,000mm in northern Jemjem Awraja in Sidamo and over 1,200mm in parts of Genale and Delo Awrajas in Bale and northeastern Webera in Harerge, to less than 400mm over most of the Ogaden.
For northern Ethiopia the mean annual rainfall is about 500mm, but there is a pocket in the western part of Mitsiwa Awraja where it is more than 800mm. In the area of Fifill, in the same region, it rises to over 1,200mm.
The discussion has to far concentrated on the general pattern of annual rainfall distribution in the country, but rainfall in seasonal, varying in amount, space and time. There is the long and heavy summer rain, normally called the big rain or Keremt, and there are short and moderate rains in spring, autumn and winter. These are known as the little rains or Belg.
However, southwestern Ethiopia gets rain for a long period at a stretch usually from eight to ten months. The other regions, southeastern Ethiopia for instance, receive rain twice a year, designated to have bi-modal rainfall which does not coincide with the periods of Keremt and Belg.
The seasonal rainfall maps are, therefore, included to demonstrate these variabilities. One of these maps shows rainfall distribution in summer (June-September) when the ITCZ is to the north of Ethiopia. In this season the whole country, with the exception of the northern tip, is under the influence of the southwest equatorial westerlies and southerly winds from the Indian Ocean.
As these winds originate from the South Atlantic Ocean, blowing over the humid regions of the Gulf of Guinea, the Congo basin and across Central Africa, they are moisture laden by the time they arrive in Ethiopia. When ascending over the highlands, they cause very heavy rain in southwestern Ethiopia (Ilubabor, Kefa, Gamo Gofa).
The rainfall amounts gradually decrease as they move to the northeast. The summer or Keremt rain has a wide coverage and all of highland Ethiopia receives rain though in varying degrees. It is the major sowing season, locally known as Meher.
Southwestern Ethiopia gets 40% of its rain at this time. As the rain advances and retreats through the southwest, this region has the longest duration of Keremt rains.
The shortest duration is in the extreme northeast where it lasts for only two or three months in Tigray.
The eastern escarpments of the northwestern high lands and the associated lowlands, being rain shadow areas are dry. The southeastern highlands and the associated lowlands which come under the influence of the southerly winds are also dry.
This is mainly because the southerly winds which originate from the Indian Ocean, having lost their moisture over the East African highlands, are dry when they reach Ethiopia.
The second map shows rainfall distribution mainly in winter when the ITCZ has shifted to the south. Most of Ethiopia at this time comes under the influence of the continental air currents from the north and northeast.
These winds originate from North African and West Asian high pressure centres, and, as they are cold and dry, they carry little rain.
Southwestern Ethiopia, which still falls under the influence of the southwest equatorial westerlies, though weaker at this time, receives moderate rain. However, generally speaking, winter is the season of lowest rainfall in Ethiopia.
In spring, (March, April, May), the ITCZ, as it drifts towards the north, lies across southern Ethiopia. At this time a strong cyclonic cell develops over the Sudan, a lowland country.
Winds from the Gulf of Aden and the Indian Ocean highs are drawn towards this center and blow across central and southern Ethiopia. These moist, easterly and southeasterly winds produce the main rain in southeastern Ethiopia and the little rains of spring to the east central part of the northwestern highlands.
The little rains on the highlands are known as Belg rain, referring to the second most important sowing season of the region.
However, in the southeastern highlands and associated lowlands (Ogaden, Borena, the southern part of Sidamo and Gamo Gofa), spring is the major rainfall season, the second coming in autumn (September, October, February).
For instance, the annual total for Moyale and Kelafo being about 1,000mm and 500mm, they receive 50% and 60% in spring and 37% and 33% in autumn, respectively.
(Source: National Atlas of Ethiopia)