Fjölrit RALA - 05.12.1999, Blaðsíða 80

78
Desertification in Ethiopian highlands
Table 2. Annual soil movement (loss) documented in Ethiopia under various land use systems
and topographic features (Hawando 1995).
Land area Documented range of annual soil loss, ton/ha/year
Land use type million ha 16 50 100 200 300
Annual soil movement, million tons
1. Cultivated land 18 288 900 1800 3600 5400
2. Pasture & rangelands 60 960 3000 6000 12000 18000
Total 78 1248 3900 7800 15600 23400
In the dryland areas in Ethiopia, 30.9 million ha, consisting of twenty five major
soil great groups, are shallower than 50 cm. Further erosion will cause severe damage
if rehabilitation measures are not taken in time. Out of the 30.9 million ha, 20.2 mil-
lion ha occur in areas with xeric SMR, 5.2 million ha in aridic SMR and 4.6 million ha
occur in ustic SMR zones (Hawando 1995).
Soil conservation research project (SCRP;
Humi 1988) and National Conservation Secre-
tariat (1992) claim that soil losses may have been
severely over-estimated by FAO (1984). The fig-
ures for the soil loss by erosion from 6 SCRP
sites range from 18-214.8 tons per ha per year
(Table 3).
When the soil loss figures obtained from
SCRP sites are used to calculate the magnitude of
soil loss from cultivated fíelds, pasture and
rangelands, the amount of annual local soil
movement ranges from 1,248-23,400 million
tons per year (Table 2).
It is reasonable to state that the soil movement fígures reported by FAO (1984)
were not over-estimated. Rather, the figures estimated by National Conservation Se-
cretariate and the Soil Conservation Research Project can be considered to grossly un-
der-estimate the magnitude of land degradation in Ethiopia mainly because of low
land area figures used in their calculations. Study by Lund University Geography
Group (Helden 1987) concluded that land cover is the most important factor control-
ling soil erosion, while precipitation characteristics and slope are also important.
Therefore, for conservation and rehabilitation programme, we have to make sure that
land cover is given the top priority.
LOSS OF PLANT NUTRIENTS
Associated with the soil movement is the loss of organic matter, nitrogen, phosphorus,
potassium and other essential plant nutrients. Data in Table 4 shows that organic mat-
ter loss associated with the removal of surface soil ranges from 15-1000 kg/ha/year
which amounts to 1.17-78 million tons of organic matter lost per year from 78 million
ha of cultivated and grazing lands. The loss of soil nitrogen ranged from 0.39-5.07
million tons per year and that of phosphorus ranged from 1.17-11.7 million tons per
Table3. Soil erosion loss on 6 SCRP
sites in various parts of Ethiopia
(World Aragaye Berehe 1996).
Site Soil loss (tons/ha/year)
South Wollo 36.5-53.8
Sidamo 41.2-49.5
Harar 25.5-27.8
North Showa 152.4-214.8
Gojam 40.2-199.2
Illubabur 18.0-135.3