Facts about Ethiopia

The mean annual water surplus shown by the map is the sum of monthly excesses of rainfall over potential evapotranspiration when soil moisture storage is assumed to be at field capacity.

Basically the water surplus is considered to be approximately equal to surface runoff since the determination of the surplus is derived from the inflow-outflow hydrological equation which may be stated as follows:

Surface runoff (Surplus) = Precipitation

– Evapotranspiration – Subsurface drainage – Change in soil moisture storage

From the above, subsurface drainage may be ignored since annual changes in ground water storage are normally small. If the soil is assumed to be saturated, in this case after a storage of 100mm, the runoff is the excess of precipitation over evapotranspiration.
The hydrological equation therefore, becomes:

Surface Runoff = Precipitation – Evapotranspiration.

However, for various reasons, it should be noted that the water surplus values shown on the map indicate only broad and relative values and not absolute amounts of surface runoff.

First, the assumed soil field capacity of 100mm is only an approximate value for average soils. The amount of precipitation that would bring an area to saturation varies widely depending on the type of soil and other physical characteristics of the area.

Second, the term potential evapotranspiration represents the amount of full evaporation and transpiration from plants and other surfaces for available net energy.

However, local conditions of air temperature and vegetation characteristics may play important roles in the amount of actual evapotranspiration that takes place.

Thirdly, how much runoff actually takes place from a region also depends, in addition to evapotransporation, rainfall, soil and vegetation, on terrain and other physical characteristics of the region.

Lastly, it is assumed in the present estimation of water surplus that runoff occurs under conditions of ground saturation only, while in actual fact, runoff does occur even before saturation occurs because of slope and other ground characteristics. Also runoff can vary significantly from year to year with variation in rainfall.

While the above statements are meant as cautions against reading absolute values of runoff into what is shown on the water surplus map, the map itself is useful in indicating the relative abundance or scarcity of water in various parts of the country.

As may be expected, high surpluses occur in the southwest and west. If surpluses of over 900mm are considered to be heavy runoff, then the upper basins and tributaries of the Didessa-Anger, Dabus, Baro, Omo, Beles and Bir in the southwest and west seem to belong to this category.

Portions of the Debre Tabor and Gayint highlands that drain into the Abay and the Beshilo also have heavy runoff. Gore and Lekemt areas have over 1,300mm mean annual water surplus, the highest in the country.

In contrast to the southwestern and western highlands where most of the upper basins of streams have over 700mm mean annual water surplus, the southeastern highlands of Ethiopia, those of Sidamo, Bale, Arsi and Harerge have very light runoff with 100 to 300mm water surplus.

Most of Tigray, Welo, Harerge, Bale and Sidamo as well as southern Gamo Gofa and the Lakes Region have little, (less than 100mm), or no water surplus.

(Source: National Atlas of Ethiopia)

The record showing the mean annual water deficit broadly indicates the regional variation in the magnitude of the mean annual moisture deficiency for plant growth and development.

The amount of moisture deficit is the excess of potential evapotranspiration over rainfall when soil moisture storage is assumed to be nil. The calculations are made on a monthly basis and totaled for the year.

While the amounts show the relative magnitudes of stress placed on plants during the dry seasons, they do not show the duration of the dry season.

Thus two places can have about the same amount of mean annual moisture deficit but vary greatly in the duration of the dry season.

For example, Gambela and Bati have about the same mean annual water deficit, 687mm and 689mm respectively. However, Gambela has moisture deficiency for six months while at Bati the dry season lasts for nine months Both Gambela and Bati, on the other hand, can at best support only scanty woodland vegetation although their mean annual rainfalls are 1,327 and 810mm respectively.

Similarly, two places can have about the same mean annual moisture surplus but vary widely in the amount of moisture deficit and the duration of the dry season.

Thus, at Jimma, the mean annual moisture deficit is 122mm and the dry season lasts for three months, while at Addis Zemen the mean annual moisture deficit is 779mm and the dry season lasts for seven months.

The mean annual moisture surplus at Jima and Addis Zemen however, is about the same 463mm and 492mm respectively. Because of the difference between the two places in the amount of moisture deficit and the duration of the dry season, Jima area can support much more luxuriant forest vegetation than Addis Zemen region.

The amounts of moisture deficit also show the relative magnitudes of the annual amount of water which needs to be supplied artificially through irrigation at various localities for optimum crop growth.

However, this does not mean irrigation requirements in absolute terms since the figures only indicate annual rather than seasonal amounts, and also since irrigation water requirements are affected by many other factors.

The map shows that the highest moisture deficits of over 900mm a year occur in most of the northern, northeastern, southeastern and southern parts of the country where potential evapotranspiration amounts are high and the rainfall amounts are low. At Gode the deficit is 1,546mm, at Gewane 1,216mm and at Humera 1,149mm.

In parts of the southern, southwestern and southeastern highlands of Ethiopia there is practically no moisture deficit (less than 100mm).

These are the highlands of Ilubabor, Kefa, Gamo Gofa, northern Sidamo, central Bale and central Arsi. At Wushwush the deficit is zero, at Gore 76, at Chencha 42, at Yirga Chefe 58, at Goba 89 and at Ticho 15.

(Source: National Atlas of Ethiopia)

The record, which shows the moisture regions of Ethiopia, is based on moisture indices determined by working out monthly water surpluses and deficiencies for some one hundred and forty climatic stations in Ethiopia.

The amounts of water surplus and deficit calculated on a monthly basis, represent, respectively, positive and negative values of the difference between rainfall and potential evapotranspiration, taking into account maximum soil field capacity of 100mm.

The mean monthly potential evapotranspiration data for the stations used for the water budget calculation are as determined by the Penman method and processed for the Landuse Planning and Regulatory Department of the Ministry of Agriculture by the Food and Agriculture Organization of the United Nations (FAO) in 1982.

The moisture index chosen is that of Thornthwaite (the early or 1948 version) which is found to correlate very well with the distribution fo the major types of natural vegetation in East Africa.

The moisture indes (1m) is defined as (100S-60D) /PE where S is water surplus, D is water deficit and PE is the potential evapotranspiration.

The deficit is given less weight in determining the index since plants can withstand periods of moderate rainfall by drawing on stored moisture during periods of moisture surplus.

The index implies that where rainfall is always equal to potential evapotranspiration there is neither water surplus nor deficiency and the climate is neither humid nor dry. As deficiency increases the climate becomes more arid and as surplus increases the climate becomes more humid.

Examples of water budget calculation to obtain the moisture index are given for three stations.

From the mean annual surpluses and deficits and the mean annual potential evapotranspiration for the three stations, the moisture index for Jimma is +34, for Kibre Mengist – 13 and for Mekele – 33. Jimma area is therefore described as humid, Kibre Mengist area as dry sub-humid and Mekele area as semi-arid.

The map and the indices in general show the relative magnitude of moisture availability for plant growth and other purposes in different parts of the country.

In general, more humid regions with a moisture index of over +40 are confined mostly to the administrative regions of Kefa, Ilubabor, Welega and Gojam. Smaller areas with this index are found in Gonder, Gamo Gofa and northern Sidamo.

Substantial portions of Tigray, Welo and Harerge are classified as arid with a moisture index of less than – 40. Most of Bale and Sidamo may be described as semi-arid and dry sub-humid with indices of 0 to – 40.

The lakes region is mostly dry subhumid, having indices of – 20 to 0. Portions of northwestern Welo extending into Tigray (parts of Lasta, Wag and western Inderta awrajas) may actually be arid with a moisture index of less than – 40.

The comparison of the moisture regions map with the map of the natural vegetation shows that there is general relationship between the moisture regions and the distribution of major vegetation types.

Broadly speaking, moisture indices of over +40 indicate areas of climatic climax vegetation of broadleaved evergreen forests and indices of +20 to +40 communities of mixed broadleaf and coniferous forests up to about 3,300 meters elevation.

Indices of -20 to +20 in general show areas which support coniferous forest in their more humid parts at higher elevations (up to about 3,200m.) and various kinds of woodland vegetation in their drier parts and at lower elevations (lower than 2,200m).

Indices of -40 to -20 in general coincide with areas of acacia woodland and savannah and similar vegetation regions, while indices of less than -40 represent areas of steppe and semi-desert climatic climax vegetation.

(Source: National Atlas of Ethiopia)

The daily measure of the mean surface wind speed and direction of ten minutes average at 1200 (noon) local time at the various stations are grouped in accordance with the seasonal wind prevailing directions.

The wind roses diagrams are drawn to obtain the complete picture of the seasonal wind for selected stations.

The wind speed is expressed in knots and the wind direction is also expressed according to the eight sectors of the wind roses from where the wind is blowing. That is:

North  (340-020)
North East (030-060)
East  (070-11)
South East (120-150)
South  (160-200)
South West (210-240)
West  (250-290)
North West (300-330)

The mapping of the prevailing wind direction is computed by expressing the recorded events of each of the main wind directions in percentage frequencies. Such mapping of the wind roses gives general information on the wind system at ground level for the specified stations.

From these wind rose diagrams, seasonal variations of the prevailing wind flow pattern is observed.

This seasonal variation is best explained by reference to the position of the Inter-Tropical Convergence Zone (ITCZ), a low pressure area of convergence along which equatorial wave disturbances take place.

The ITCZ oscillates seasonally within the tropics, its surface position is influenced by topography and local eddies. Thus this seasonal oscillation of the ITCZ causes a variation in the pattern of wind flows over Ethiopia.

Between June and September the ITCZ is located north of Ethiopia and pronounced cyclonic cells along the ITCZ are over North Africa and the Arabian Peninsula.

With the effect of topography on surface temperature the ITCZ meanders parallel to the Red Sea Coast. The rest of the country comes under the influence of the Atlantic equatorial westerlies and southerly winds from the equatorial Indian Ocean.

The south-west equatorial westerlies ascend over the south-western highlands and produce the main rainy season over most parts of highland Ethiopia.

The southerly winds from the Indian Ocean, despite the fact that they lose their moisture over the East African highlands, are blowing over the eastern lowlands of Ethiopia where their influence on rainfall is minimized due to the föhn effect.

From October to May is the longest period of the year during which the effect of the ITCZ is progressively located in central and southern Ethiopia both in spring (September, October and November) when it shifts northwards and in autumn (March, April and May) when it shifts southwards.

At these times there is a strong low pressure cell over central Sudan. The anticyclone system over north Africa and Arabian Peninsula weakens and a strong anticyclone cells develops over the Gulf of Aden and the Indian Ocean.

The Gulf of Aden-Indian Ocean high pressure system generates moist, easterly air current over south eastern Ethiopia and south easterly air currents along the Red Sea Coast.

In northern Ethiopia, the prevailing air currents at these seasons are the dry, subsiding continental air currents from the east and north. In south-western Ethiopia, the surface air current are the Atlantic equatorial westerlies.

The Gulf of Aden-Indian Ocean moist air currents form a zone of convergence with the northerly air currents along the northern half of the western escarpment of the Ethiopian rift system. Thus the ITCZ separates the Atlantic equatorial westerlies from the tropical easterlies from the north and east.

In December, January and February, the ITCZ is located in the southern hemisphere, south of Ethiopia.

High pressure systems develop over North Africa and Arabia. The main pressure systems which determine air circulation over Ethiopia are therefore, the anticyclones in northern Africa and Arabia and the low pressure systems in southern Africa.

The Arabian High also extends southwards to eastern Ethiopia, the Horn and the Indian Ocean. The dry, subsiding tropical north easterlies dominate the northern and most of the western half of the country while easterly air currents from the Indian High dominate the south-eastern part.

(Source: National Atlas of Ethiopia)

Previously in Ethiopia, as the awareness of the need to collect climatic data increased, climatological stations were established by various organizations such as the Water Resource Development Authority, to meet their particular needs.

Most of these stations are located in large towns. In some of these stations, the lack of continued observation has created serious data gaps. Data, collected by more than one organization was not centralized and therefore, could not be fully utilized.

Moreover, in remote regions there is a considerable gap in network of stations. These are areas which appear to have provoked little interest in the past, but for which development projects are now proposed or are foreseable.

The need to make established stations continuously operational, to centralize and store data, to ensure a rational distribution of stations and to make effective use of the available data, led to the establishment of the National Meteorological Service Agency in 1973 E.C. (1980).

The establishment of this Agency has led to improvements in data acquisition. Though data acquisition is the primary function, the most important and ultimate objective is the production of information that assists in planning and management for current and future national development projects.

These include making available summarized and analysed data which interprets climate for all users.

As can be seen from the map, rainfall stations are the most numerous. In Ethiopia, as is true for most of Africa, rainfall is by far the most important element of climate, for it is a basic resource in the increase of agricultural production.

Meteorological information not only promotes increased production but also reduces vulnerability to extreme climatic conditions.

Rainfall is a climatic element liable to considerable variability and since it is critical to food and water supplies it requires a dense network of stations for short and long term monitoring. This high degree of variability from year to year requires a long period of observation.

Usually data of 30 years or more, is needed to establish norms and frequency distribution and also for research into possible trends and cycles of drought.

The Meteorological Service Agency is continuously establishing higher grade climatological stations with improved standardization of instruments and methods of observation to increase the range of climatic data.

(Source: National Atlas of Ethiopia)

The climatic map drawn according to the Koppen System, shows the various climatic regions of Ethiopia.

This system is based on the annual and monthly means of temperature and rainfall also on seasonal changes of rainfall and types of native vegetation associated with them.

The climatic types range from equatorial desert to hot and cool steppe, and from tropical savannah and rain forest to warm temperate and cool highland.

It is also broadly divided into highland and lowland regions with elevations ranging from around 110 metres below sea level in the Dallol Depression and rising to 4,620 metres in Ras Dejen in the Simen Mountain Massif.

Climate ranges not only from arid areas to areas of rather plentiful moisture, but also from hot lowland to cool highlands.

The climate is grouped into three types;
I. Dry climate
II. Tropical rainy climate
III. Temperate rainy climate

These three main climatic regions are each subdivided into three or more types, making a total of nine principal climatic types, as follows:

1. BWH – hot arid climate:- Barren to sparse vegetation. In this region the mean annual temperature is between 270C and 300C; and mean annual rainfall is less than 450mm.

It is usually characterized by strong wind, high temperature, low relative humidity and little cloud. Evaporation is twenty or more times in excess of rainfall in some places.

2. BSH – hot semi-arid climate:- Steppe type vegetation. In this region the mean annual temperature is between 180C and 270C; and mean annual rainfall is 410-820mm.

The rainfall is highly variable from year to year. Since evaporation exceeds rainfall, permanent streams do not originate.

Grasses are less tall and coarse and edible, but during the dry seasons they are not very palatable, so that wild game and cattle at such times rely largely on the tender fresh grasses along water courses. This region is intermediate between the arid desert and the humid climate.

3. BSK – cool semi-arid climate:- Steppe type vegetation mainly over the adjacent highlands of Tigray. The mean annual temperature is 120C to 180C; and mean annual rainfall is between 400 and 620mm. Evaporation is less than BSH, due to the low temperature of this region and therefore it is less arid than BSH.

4. AS – tropical climate I: This region has dry months in summer. The mean temperature of the coldest month is above 180C and mean annual rainfall is between 680 and 2,000mm. This region is found in a very small area between Asmera and Mitsiwa.

5. AW – tropical climate II:- This climate prevails up to an elevation of 1,750 metres above mean sea level. The lengths of the wet and dry periods vary considerably from the western part to the northern and eastern parts of the country. Tall grass characterizes this climate type. Usually grass and trees are intermingled.

6. AM – tropical climate III:- It is known as tropical rainforest climate. Temperature of the coldest month is above 180C and mean annual rainfall is between 1,200 and 2,800mm.

It differs from AW climate by the total amount that falls during the driest month. It supports an evergreen rainforest. It prevails up to elevations of 1,750 metres above mean sea level.

7. CWB – warm temperature climate I:- This has distinct dry months in winter. The mean temperature of the coldest month is below 180C, and for more than four months it has mean temperatures above 100C.

The annual rainfall in mm is also greater than twenty times the annual mean temperature plus fourteen [20 (t+14)].

Rainfall distribution and amount varies considerably from area to area. In areas of heavy rainfall, forests predominate and grass covers the areas of moderate rainfall. It prevails from an elevation of 1,750 metres to 3,200 metres above sea level.

8. CFB – warm temperate climate II:- This has more soil moisture than CWB climate. It is a humid temperate climate without dry season and mean temperature of the coldest month is less than 180C.

Rainfall of the driest summer month is greater than one-third of the wettest winter month, and rainfall of the driest winter month is also greater than one-tenth of the wettest summer month.

The area experiences adequate rainfall at all seasons. This climate is suitable for abundant forest cover.

9. CWC – cool highland climate:- This is characterized by dry months in winter and 100C or less of mean temperature of the warmest month; the annual rainfall is between 800 and 2000m.

This area is situated at an altitude equal to, or more than 3,500 metres above mean sea level. It is found on small isolated high areas.

(Source: National Atlas of Ethiopia)

The sun is the source of life on earth. It is the prodigious inflow of solar energy in the form of sunshine and radiation that produces, through photosynthesis, all the food, fuel and free oxygen upon which life on earth depends.

The total amount of solar energy reaching the earth is about 7X1017KW hr. per year, more than 30,000 times as much as is used in all man-made devices. Unlike the conventional sources of energy, solar energy is inexhaustible; it is also the least exploited.

Solar radiation has a direct effect on the day-to-day life of mankind. The availability of adequate sunshine in the crucial phase of the growing season is decisive in crop production.

But sunshine is intermittent in availability and variable in direction. In the technologically developed world, this effect of variability of sunshine is compensated by temperature modification devices in the operation of heating or cooling system.

But in countries like Ethiopia such technological capability is lacking and is too expensive to acquire.

One possibility of development is the acquisition of more adequate knowledge of climate parameters and a greater ability to predict changes so that adjustments can be made through earlier or later sowing.

Moreover, the understanding of the duration and intensity of sunshine would enable farmers to select crops that are better suited to a given environment and, in this way, an acceptable yield in most years would have greater assurance.

More information on solar radiation intensity would also contribute to a greater understanding of water loss through evaporation.

Solar radiation is, however, an important source of energy. In the present world, energy plays a pivotal role in technological and social development. But the cost of conventional fuel is rising and its availability is shrinking day by day.

Realizing this, man has been attempting for some time to make use of the sun’s radiant energy. Obviously his success has been limited as the economic utilization of solar energy requires a level of technological development that has not yet been attained.

But the world is witnessing very rapid technological advances. Within a few years it is likely that the cost of solar energy tapped directly will become competitive with that of the conventional sources of energy.

When this technology becomes global, what will matter then will be the availability of sunshine and large areas of land where solar power generating stations can be built. Ethiopia, for that matter Africa, is naturally abundantly endowed with both the prodigious energy of the sun and the necessary expanse of land.

Generally, northern Africa including Ethiopia, is the area where there is the maximum intensity and duration of sunshine over the globe, and as much as 4,000 bright sunshine hours are receive in a year.

Most parts of Ethiopia receive over 7 hours of bright sunshine daily, and in the south eastern, northern and north eastern lowlands, it reaches 10 to 12 hours duration.

It is only during the summer rainy season in the highlands of central and south western Ethiopia, when there is much cloud cover, that the daily duration of sunshine hours is less than 5 hours.

Ethiopia receives 7.4 to 7.6 KWh/m2 of solar radiation per day. This abundant solar energy of the order of 7-8 KWh/m2, received daily for over 6-8 hours day in most parts of Ethiopia for the major portion of the year, could be utilized for economic and social development.

The primary step to exploitation begins with adequate knowledge of the various elements of these resources. Unlike other resources, climate requires a long period of observation and accumulation of data to analyse and interpret patterns and variabilities.

To derive a detailed solar radiation climatology for a region, as well as to estimate its solar energy potential, extensive radiation data of high accuracy at a large number of stations covering all major climatic zones of the region is essential.

The successful application of solar energy to increase the capacity of this nation and to support its growing population depends on an increasing knowledge of the amount of available solar radiation.

The four maps showing the mean monthly hours of bright sunshine in January, April, July and October are representative of the relatively high and low sun periods over Ethiopia. These maps can give a general impression of the distribution of sunshine and solar radiation potential in Ethiopia.

(Source: National Atlas of Ethiopia)

Latitude, altitude, winds and humidity, with varying magnitude have significant impacts on temperature conditions in Ethiopia.

As the sun’s radiation is the source of all heat on earth, latitude is the primary consideration in the understanding of temperature.

Ethiopia lies within the tropics (30N to 180N latitude), a zone of maximum insolation where every place has overhead sun twice a year. However, as it is a highland country, tropical temperature conditions are not experienced everywhere. They are limited to the lowlands in the peripheries.

Away from the peripheries, the land begins to rise gradually and considerably, culminating in peaks in various parts of the country. The highlands form the heartland of the country.

Thus temperature, as it is affected by altitude, decreases towards the interior. Mean annual temperature varies from over 300C in the tropical lowlands to less than 100C at very high altitudes.

Ethiopia is, therefore, a country where both extremes of temperature are experienced. Altitude is the most important temperature controlling factor in the country.

Environmental influences have their own traditional expressions in Ethiopia and there are local terms denoting temperature zones.

Though it is a country located in the tropics, a good part of Ethiopia enjoys a temperature climate. However, unlike places in the middle latitudes, its temperature shows typical tropical characteristics.

In the tropics, daily range of temperature is high, and annual range is small, whereas the reverse is true in the temperate latitudes. In Ethiopia, as in all places in the tropics, the altitude of the sun is always high, making solar radiation intense.

The variation in the amount of solar radiation received daily, is small throughout the year. Temperature is high during the day, over 400C in some places, and is considerably reduced at night causing the daily range of temperature to be large.

But in the case of monthly averages, variation is minimal and the annual range of temperature is small. This holds true in both the highlands and lowlands.

As the sun is always high in the tropics, seasonal variation is not distinctly observable. However, there is a slight temperature increase in summer. In most places in Ethiopia, the highest temperatures are experienced between March and September as this is the high sun period.

As the relative position of the sun shifts, southern Ethiopia has its highest temperatures in autumn and spring when the sun is vertically overhead (Kelafo, Gode and Awasa).

However, some deviations need to be considered. Summer being the big rainy season, temperature is reduced considerably in the southern and southwestern parts where humidity is high and cloud cover frequent.

In these places, the highest temperatures are experienced at other periods than when the sun is overhead, or even during the low sun period (Sodo in February and Dila in January).

The position of slopes in relation to the direction of rain bearing winds, (leeward or windward side), is also another consideration for this variation, as is aspect in determining temperature variations in mountainous regions.

(Source: National Atlas of Ethiopia)

Climatic variabilities among others have caused frequent drought and famine that have ravaged the people of Ethiopia for generations.

It is a paradox that this country is on the “world hunger list”. However, the maps on hail, frost, rainfall patterns and variability reveal the vagaries of nature that intermittently cause disruption to normal life.

The graph showing rainfall patterns in different zones, shows the frequent variations of rainfall from the mean.

Hardly a year passes when farmers in some parts of the country do not suffer from either too little, or too much rainfall. Even when the amount is normal, they suffer when it comes too early or too late.

Agriculture is also adversely affected, from time to time, by frost and hail. The two maps on the left show those parts of the country which are frequently affected by these climatic adversities.

Frost occurs for a very short duration usually in the early morning just before sunrise. The lowest temperature of the day is recorded at this time, when the stored heat from the sun is lost through radiation.

Frost occurs at high altitudes where the atmosphere is thin and radiation faster. It is a very serious hazard to crops between October and February, but, due to the absence of any quantification of the damage done, our understanding remains inadequate and largely conjectural.

Temperature inversion at the base of escarpments and on deep gorges is also a factor in frost occurrence.

The third map on the right shows the coefficient of rainfall variability. By comparing this map with the map of mean annual rainfall, the high but inverse correlation between annual amount and variability of rainfall can be seen.

The higher the annual rainfall, the lower the coefficient of variability, or conversely, the lower the annual rainfall the higher the coefficient of variability.

Variability or unreliability of rainfall is very high in the areas of low rainfall; the drier lowlands of the country.

The regions of abundant rain are more secure. The precarious existence of the people in the drier semi-arid lands is frequently made worse by adverse climatic changes.

Thus a substantial decrease of the rains from the mean, say up to 30%, does little damage in the wet regions, whereas a slight variation of about 5% causes catastrophic famine in drier areas. The gradual encroachment of the menace is traced from these drier areas towards the wet regions.

From the graph showing rainfall patterns over 25 years, 1961-1975 E.C. (1969-1983), the years with annual rainfall much below the mean can be noted. Immediately after the failure of these rains, disastrous famines followed.

The Tigray famine of 1950 E.C. (1958) (not included in the graph). The Wag-Lasta famine of 1958 E.C. (1966), the Welo famine of 1965 E.C. (1973) and the latest famine of 1977 E.C. (1984/85), that covered the whole country except seven Awrajas, are significant examples.

(Source: National Atlas of Ethiopia)

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)