Ethiopia Fact

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)

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