Ethiopia Fact

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)

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