Geoscience Reference
In-Depth Information
to prevent deforestation in the earliest years of the park's existence. He wrote despair-
ingly of the resultant destruction of the once prime forest:
Seen close, the destruction was incredible. The place looked like a First World War battle-
ground. Everywhere the ground was strewn with hot ash, smoking debris, charred stumps, and
partially burned tree trunks, lying about willy-nilly. Some of the felled trees had been sixty
feet [about 20 m] and more in height, and now they lay burned on the ground. I could hardly
believe that a fewmen with simple, blunt iron axes had felled so many huge trees. Cedar, olive,
hagenia, Podocarpus, euphorbia and many others. This year alone, Bogale and his sons and
followers had cut and burned about three square kilometres of forest. (Nicol,
1971
, p. 262)
Removal of forest can alter the local hydrological balance, leading to increased run-
off, reduced infiltration and the drying up of springs and stream headwaters once
fed by base flow and by the subsurface lateral flow of the infiltrating soil water, as
discussed in
Chapter 10
. A related effect is acceleratedmovement of soil downslope by
slopewash, soil creep and landslides. The Swiss geographer Hans Hurni investigated
rates of soil erosion in the Ethiopian Highlands (Hurni,
1999
). He found that mean
rates of soil loss amounted to 40 t/ha/year on uncultivated mountain slopes and rose
to more than 300 t/ha/year when cultivated, or five to ten times more than those in
cultivated areas beyond the mountains. He also observed a reduction in the length
of fallow to virtually zero and an expansion in the area under cultivation. In part of
Gojjam Province, the area cultivated rose from 40 per cent in 1957 to 77 per cent in
1995, while natural forest decreased from 27 per cent to 0.3 per cent (Hurni,
1999
).
It is instructive to compare current rates of soil loss in the Ethiopian headwaters
of the Blue Nile and Atbara (known in Ethiopia as the Abbai and Tekazze rivers,
respectively) with long-term geological rates of denudation, expressed as a mean rate
of surface lowering. The mean rate of surface denudation over the past 20million years
amounts to 0.01 mm/year (McDougall et al., 1974; Williams and Williams,
1980
),
which is an order of magnitude lower than the mean rate of 0.12-0.24 mm/year in the
seventy years before completion of the Aswan High Dam in 1970. In the 1970s, soil
loss from parts of the Ethiopian Plateau amounted to 0.4-1.0 mm/year (Nyssen et al.,
2004
).
The effects of such accelerated soil erosion have repercussions well beyond the
local area. For example, before completion of the Sudanese Roseires Dam on the
Blue Nile close to the Ethiopian border in 1965, Blue Nile suspended sediment
load amounted to 50-100
10
6
t/year when measured at Khartoum but had fallen to
×
10
6
t/year from 1967 to 1969, largely as a result of silt trapped upstream of
the dam (El Badri,
1972
). By 1996, the capacity of the Roseires Reservoir had been
reduced by nearly 60 per cent and that of the Khashm el Girba Reservoir on the Atbara
by 40 per cent (Swain,
1997
). (The Atbara rises close to the Blue Nile headwaters in
Ethiopia, where it is known as the Tekazze). It appears that deforestation in and around
the Semien Mountains during the past half-century has initiated a pulse of accelerated
40-70
×
