Fjölrit RALA - 05.12.1999, Síða 81

Tamirie Hawando
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year. Taking an average value of nitrogen loss of 30 kg/ha/year, organic matter loss of
200 kg/ha/year and phosphorus loss of 75 kg/ha/year the corresponding loss of the
three plant nutrients amounts to 15.6, 2.16 and 5.85 million tons per year of organic
matter, nitrogen and phosphorus respectively from 780,000 km2 of land (Table 4).
The EHRS study (FAO 1984) estimated that about 10% of the soil crosses interna-
tional boundary as a sediment load in rivers.
Table 4. Annual loss of organic matter, nitrogen and phosphorous associated with the loss of top
soil under various land use systems (Hawando 1985).
Nutrient documented range of annual loss, kg/ha
OM 15 50 100 200 500 1000
Land area N 5 10 15 30 50 65
Land use type million ha P 15 30 50 75 100 150
Amount of nutrient loss, million kgs
1. Cultivated land 18 OM 270 900 ' 1800 3600 9000 18000
N 90 1800 270 360 900 1170
P 270 360 900 1350 1800 2700
2. Pasture & rangelands 60 OM 900 3000 6000 12000 30000 60000
N 300 600 900 1800 3000 3900
P 900 1800 3000 4500 6000 9000
Total 78 OM 1170 3900 7800 15600 39000 78000
N 90 780 1170 2160 3900 5070
P 1170 2160 3900 5850 7800 11700
Climate as a natural cause of desertiftcation
Low and erratic rainfall and recurring droughts
Analysis of mean annual rainfall in Ethiopia showed that large areas in the highlands
are characterized by more than 30% coefficient of variation (Degeffu and Haile 1990).
The situation is further aggravated by the narrow intervals of drought occurrences.
Analysis of the chronological events of Ethiopian drought and famine years revealed
that there were over 305 drought years during the last 2310 years (NMSA 1990). The
frequency of droughts in Ethiopia is variable but has been occurring in narrower inter-
vals during the last 50 years, especially towards the end of 20th century. The most
publicized drought and famine years were those of 1972/73 and 1984/85 when thou-
sands of human lives were lost and millions of livestock died. The dry sub-humid,
semi-arid and arid zones are the most affected areas during the years of low rainfall
and droughts. The loss to grain production ranged ffom 500,000 tons during the nor-
mal years to 1,8 million tons in severely drought affected years. The figures estimated
by the official govemment sources (RRC) on the loss of grain production ranged up to
1.4 million tons during the severe drought years.
Moisture regimes in Ethiopia - rainfall and evapo-transpiration
The analysis of rainfall and evapo-transpiration data from 250 stations throughout the
country across topographic features and latitudinal zonation indicated that desertifica-
tion threats are equally wide-spread in Ethiopian highlands and lowlands (Figure 1
and Table 1). The number of stations with ratio of RR/PET less than 0.65 which fall