Page 92 - Soil Degradation, Conservation and Remediation

3.3 Types of Water Erosion 79

Fig. 3.7 Riverbank erosion (Photo courtesy of Stephen Hallett of Soil-Net, Cranfield University, UK)
−1
−1
−1
−1
5–20 t ha year , about 40 % of the sites had soil losses of 1–5 t ha year , and
−1
−1
about 25 % of the sites had negligible soil loss (less than 1 t ha year ) (Foster et al.
2006 ). Erosion may remove the entire soil profile in some situation in lithosols

(shallow soils on steep slopes). Sediment yields from urban earthworks can be
−1
−1
extremely high, sometimes reaching values of 50,000 t ha year (Novotny and
−1
−1
Chesters 1981 ). Figures of 100–120 t ha year are reported by Williamson ( 1993 )
from New Zealand areas undergoing construction. In the Auckland region, a mea-
−1
−1
sured loss of 66 t ha year was obtained.
It is estimated that about 1,960 M ha of land are prone to erosion, which represents
about 15 % of the earth’s total land area, of which 50 % is severely eroded, and much
of that is being abandoned (Lal et al. 2004 ). On-site effects of water erosion include
soil loss and loss of organic matter as well as nutrients (10 mm topsoil loss equals to
−1
−1
−1
350 kg ha N, 90 kg ha P, and 1,000 kg ha K; Hicks and Anthony 2001 ). Other
on-site effects are surface sealing and soil compaction, exposure of roots, deformation
of terrain, difficulty in tillage operations, exposure of subsoil, reduction in growth and

yield of crops, loss of growing crops, decline in soil quality, and reduced capability of
ecosystem functions. Erosion can cause yield reductions of 30–90 % in some root-
restrictive shallow lands of West Africa (Mbagwu et al. 1984 ; Lal 1987 ). Yield reduc-
.
tions of 20–40 % have been measured for row crops in Ohio (Fahnestock et al 1995 )
and elsewhere in Midwest USA (Schumacher et al. 1994 ). The productivity of some
lands in Africa has declined by 50 % as a result of soil erosion and desertifi cation
(Dregne 1990 ). Yield reduction in Africa due to past soil erosion may range from 2 to
40 %, with a mean loss of 8.2 % for the continent (Lal 1995 ).
Off-site effects of soil erosion are not always easily noticed. Eroded materials are
carried to distant places and get deposited there in water bodies, including lakes,
streams, rivers, and on agricultural land, roads and highways, homesteads, and other