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land degradation & development
Land Degrad. Develop.
12
: 519±539 (2001)
DOI
: 10.1002/ldr.472
SOIL DEGRADATION BY EROSION
R. LAL*
School of Natural Resources, The Ohio State University, Columbus, Ohio, USA
Received 5 July 2001; Accepted 30 August 2001
ABSTRACT
Soil degradation by accelerated erosion is a serious problem and will remain so during the 21st century, especially in developing
countries of the tropics and subtropics. Yet, its extent, severity, and economic and environmental impacts are debatable.
Estimates of global and regional land area affected are tentative and subjective. Results of ®eld measurements are often
technique-dependent. Considerable progress has been made in modeling soil erosion, yet ®eld validation of these models
remains to be done for principal soils and ecoregions. Similar to the land area affected, estimates of erosional impacts on crop
yield, productivity and soil quality are tentative and subjective. Further, erosion-induced losses on crop yield are scale-
dependent because of the compensatory bene®cial effects on yields from depositional sites, and technology-dependent because
of the masking effects of input such as fertilizers and irrigation. Erosion caused changes in soil carbon dynamics and non-point
source water pollution are important environmental impacts. While erosion (e.g., detachment and transport) can lead to
emission of trace gases into the atmosphere, deposition can bury and sequester some of the carbon. In addition to improving the
database on the land area affected, there is also a need to assess erosional impacts on productivity and soil Cbalance at the
watershed, regional, and global scale. Copyright
#
2001 John Wiley & Sons, Ltd.
key words
: erosion and productivity; soil Cdynamics; deserti®cation; crop yields
INTRODUCTION
Soil degradation implies long-term decline in soil's productivity and its environment moderating capacity
(Lal, 1994a; 1997). In other words, it means decline in soil quality, or reduction in attributes of the soil in
relation to speci®c functions of value to humans (Doran and Parkin, 1994; Doran and Jones, 1996; Johnson
et al.
,
1997; Karlen
et al.
, 1997; Lal, 1997). It has plagued the earth since the dawn of settled agriculture. In ancient times,
soil degradation caused the downfall of several thriving ancient civilizations, e.g., the Harappan and Kalibangan
cultures in the Indus Valley, the Mesopotamian and Lydian kingdoms in the Mediterranean region, and the Mayan
civilization in Central America (Lowdermilk, 1953; Olson, 1981). During the 20th century, the increase in
population has drastically accentuated the risks and extent of soil degradation (Richards, 1991). Available statistics
on the extent and severity of soil erosion, at regional and global scales, are questionable. There is a wide range of
methods used in data collection and extrapolation. Therefore, data accuracy, reliability, and credibility leave a lot
to be desired. Yet, some statistics show that the land area prone to soil degradation is estimated at about 2 billion ha,
of which 562 Mha (29
7 per cent) is agricultural land, 685 million ha (34
8 per cent) is permanent pastures and
719 Mha (35
5 per cent) is forest and woodland (Oldeman
et al.
, 1991). Therefore, soil degradation is a serious
issue of the modern era (Blakie and Brook®eld, 1987), and will remain so during the 21st century.
Soil degradation is a biophysical process exacerbated by socio-economic and political factors. There are three
principal soil degradative processes, i.e., physical, chemical, and biological (Figure 1). Soil physical processes
involve decline in soil structure leading to an increase in bulk density, decrease in total and macroporosity,
Copyright
#
2001 John Wiley & Sons, Ltd.
Correspondence to: Prof. R. Lal, School of Natural Resources, The Ohio State University, 2021 Coffey Road, Columbus, OH 43210-1085,
USA. E-mail: Lal.1@osu.edu
Page 2
Figure
1.
Mani
festation
of
soil
de
gr
adation
.
520
R. LAL
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#
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: 519±539 (2001)
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Page 3
reduction in in®ltration, increase in runoff, and exacerbation in erosion by water and wind. The rate of soil
degradative processes is governed by numerous natural and anthropogenic factors. Natural factors include soil,
climate, vegetation, and other ecoregional characteristics. Important among anthropogenic factors are land use,
soil management, farming/cropping systems, land tenure, marketing, and institutional support. Soil degradation is
accentuated by poverty, and can cause more serious problems on soils managed for subsistence agriculture with no
external input than on those managed for commercial agriculture with science-based input.
Soil degradation is a relative term because, within certain limits, some soil properties can be restored through
management. The ability of the soil to restore itself is called soil resilience (Lal, 1994a, 1997). Some soils are more
resilient than others. Irreversibly degraded soils have low resilience and cannot be restored. The objective of this
paper is to describe soil degradation by erosion and conceptualize erosional impacts on soil productivity, emission
of greenhouse gases, and environment quality.
SOIL DEGRADATION AND ACCELERATED SOIL EROSION
Soil erosion exacerbates soil degradation, and vice versa. In some cases decline in soil quality, especially
the weakening of structural units, precedes erosion. In others, erosion may lead to decline in soil quality and set in
motion the degradative trend. In fact, soil erosion can be a manifestation of soil degradation because it involves
physical removal of soil in a vertical and/or horizontal direction and degrades soil quality. It is a natural
process that has shaped the landscape and led to formation of fertile alluvial and loess soils. However, the
acceleration of the process through anthropogenic perturbations can have severe impacts on soil and environmental
quality.
Soil erosion involves `work' through a three-stage process: (1) detachment, (2) transport, and (3) deposition of
soil. Energy for the work is supplied by agents of erosion, and source of the energy determines the type of the
erosional process (Figure 2). There are four principal sources of energy: physical such as wind and water, gravity,
chemical reactions, and anthropogenic perturbation such as tillage. Tillage implements can be an important source
of energy and lead to a substantial transport of soil downslope (Govers
et al.
, 1999). The magnitude and rate of
energy dissipation from these four sources determines the severity of erosional processes.
Soil erosion begins with detachment (Ellison, 1947), which is caused by breakdown of aggregates (i.e., organo-
mineral complexes leading to formation of domain, microaggregates, and aggregates) by raindrop impact, shearing
or drag force of water and wind, or dissolution of cementing agents through chemical reactions. Detached particles
and microaggregates are transported by ¯owing water (overland ¯ow and inter¯ow) and wind and deposited when
the velocity of water or wind decreases by the effect of slope or ground cover. Prior to their transport downslope (or
down wind), sediments must be detached from the soil mass or be in a detached state. The distance of physical
displacement may range from a few millimeters to thousands of kilometers, and the time lapse from detachment to
eventual deposition may range from a few seconds to thousands of years.
Because erosion begins with detachment and transport of soil particles, it can be either detachment-limited or
transport-limited. Erosion is detachment-limited if all available particles are transported downslope, and it is
transport-limited if detached particles accumulate at the site of origin. Some particles are deposited at a short
distance from the site of origin (Foster, 1982; Rose, 1985).
The detachment-limited process may occur either in soils of high structural stability in which aggregate strength
exceeds that of the raindrop (or wind shear) impact or where slope gradient is too steep to allow sediment
accumulation. Deposition happens when the carrying capacity of the overland ¯ow or wind is reduced by decrease
in velocity and surface roughness, and presence of vegetation cover or an obstruction. Decrease in stream power
may also happen due to loss of turbulence by reduction in slope gradient. Both erosion and deposition are
preferential or selective processes, and the size of particles transported or deposited is determined by the Stoke's
Law. While the rate and size of particle transported is directly proportional to the velocity, that of deposition is
directly related to the concentration and density of sediment size and indirectly to the ¯ow velocity (Hairsine and
Rose, 1991).
SOIL DEGRADATION BY EROSION
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Page 4
Figure
2.
Types
or
manifest
ations
of
soil
erosio
n
depe
nd
on
the
sourc
e
of
ener
gy
.
522
R. LAL
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#
2001 John Wiley & Sons, Ltd.
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: 519±539 (2001)
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Page 5
Deserti®cation
There is a difference between erosion and deserti®cation. Accelerated erosion by wind and water in drylands is one
of many causes of deserti®cation. It refers to a special form of soil degradation in arid, semiarid and dry subhumid
areas resulting mainly from adverse human impact (Stewart
et al.
, 1991; UNEP, 1991; Dregne and Chou, 1992;
UNDP, 1994; Balba, 1995; Steiner, 1996; Lal
et al.
, 1999). Total drylands area of the world is estimated at 6
08
billion ha. Irrigated land occupies 3 per cent of the drylands, rain-fed cropland about 9 per cent and rangeland
about 88 per cent. Dregne and Chou (1992) estimated that about 25 per cent of irrigated land, 50 per cent of the
rainfed cropland, and 75 per cent of the rangeland in dry regions is already deserti®ed. Deserti®cation is
accentuated by land misuse and soil mismanagement.
Processes Affecting the Rate of Soil Erosion
The natural rate of soil erosion is accelerated by several processes that exacerbate decline in soil structure. These
processes are:
(1) Slaking or dispersion: It implies breakdown of soil aggregates.
(2) Compaction: It refers to increase in bulk density or densi®cation.
(3) Crusting: It involves formation of a thin, dense, laminated, and a relatively impermeable layer on the soil
surface.
These processes decrease structural stability, reduce soil strength, exacerbate erodibility and accentuate suscept-
ibility to transport by overland ¯ow, inter¯ow, wind, or gravity. These processes are accentuated by soil
disturbance (by tillage, vehicular traf®c), lack of ground cover (bare fallow, residue removal or burning), and
harsh climate (high rainfall intensity and wind velocity).
Factors of Soil Erosion
The effect of processes is modi®ed by biophysical environment comprising soil, climate, terrain and ground cover,
and interaction between them (Figure 3). Susceptibility of soil to agents of erosion, soil erodibility, is determined
by inherent soil properties, e.g., texture, structure, soil organic matter content, clay minerals, exchangeable cations,
and water retention and transmission properties. Soil erodibility is a dynamic property and is in¯uenced by
management. Erosivity is in¯uenced by environmental factors primarily climate including drop size distribution
and intensity of rain, amount and frequency of rain, runoff amount and velocity, and wind velocity. Another non-
climatic environmental factor affecting erosivity is chemical reaction leading to solution weathering. Terrain
characteristics have a signi®cant impact on rate of soil erosion by water and gravity agents. Important terrain
characteristics include slope gradient, length, aspect, and shape. Ground cover exerts a strong moderating impact
on dissipating the energy supplied by agents of soil erosion.
The effect of biophysical processes governing soil erosion is in¯uenced by economic, social and political causes
(Figure 3b). Social causes that can accentuate the rate of erosion-induced soil degradation include subsistence or
resource-based agriculture, poverty and illiteracy, poor health and malnutrition, political instability, and high
demographic pressure (Figure 3b). Social, economic, and policy causes in¯uence mainly the type of land use and
management. These causes then in¯uence the rate of soil erosion which determines the severity of soil degradation
(Figure 4).
GLOBAL EXTENT OF SOIL DEGRADATION BY EROSION
The total land area subject to human-induced soil degradation is estimated at about 2 billion ha (Table I; GLASOD,
1990; Oldeman
et al.
, 1991). Of this, the land area affected by soil degradation due to erosion is estimated at
1100 Mha by water erosion and 550 Mha by wind erosion (Table I). There are several hotspots of erosion-caused
soil degradation. Important among these are South Asia including the Himalayan±Tibetan ecoregion, subSaharan
Africa, Central America and the Caribbean, and the Andean region in South America. South Asia is one of the
regions where soil erosion by wind (Venkateswarlu, 1994) and water (Singh
et al.
, 1992) is a severe problem. Total
SOIL DEGRADATION BY EROSION
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Page 6
Figure
3.
(a)
Factors
of
soil
erosion,
(b)
cau
ses
of
soil
erosion,
and
(c)
interaction
betwee
n
them
.
524
R. LAL
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2001 John Wiley & Sons, Ltd.
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: 519±539 (2001)
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Page 7
land area affected by water and wind erosion respectively is 32
8 Mha and 11 Mha in India, 26
4 Mha and 35
4
Mha in Iran, 11
2 Mha and 2
1 Mha in Afghanistan, 7
2 Mha and 10
7 Mha in Pakistan, and 81
7 Mha and 59
0
Mha in South Asia (Table II).
The extent and severity of soil erosion depend on ecological factors. Soil erosion by both wind and water are
severe in semiarid and subhumid low and mid-latitudes. Risks of soil degradation are accentuated in hot and dry
climates in comparison with cold and humid climates (Stewart
et al.
, 1991). Deserti®cation is also severe in arid,
semiarid and subhumid climates.
Figure 4. Interaction between biophysical factors and socio-economic and policy causes on rate of soil erosion and severity of soil degradation.
Table I. Global extent of human-induced soil degradation (modi®ed from Oldeman
et al.
, 1991)
Total land area
Human-induced
Soil erosion (10
6
ha)
World regions
(10
6
ha)
soil degradation (10
6
ha)
Water
Wind
Africa
2966
494
227
186
Asia
4256
748
441
222
South America
1768
243
123
42
Central America
306
63
46
5
North America
1885
95
60
35
Europe
950
219
114
42
Oceania
882
103
83
16
World total
13013
1965
1094
548
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Page 8
MEASUREMENT AND PREDICTION OF SOIL EROSION
Measurement of Soil Erosion
Erosion measurement may involve assessment of the current rate of soil erosion or that of the historic cumulative
soil erosion. Choice of the method for ®eld or
in situ
assessment of the current rate of soil erosion depends on the
type of soil erosion to be monitored (Table III), and there is a wide range of techniques used (Lal, 1994b). Field
runoff plots are widely used to monitor hillside erosion (Mutchler
et al.
, 1994). However, the design of runoff plots
(in terms of plot dimension, runoff and erosion collection system, methods to monitor sediment concentration, etc.)
are not standardized. Therefore, results are often technique-dependent. A major problem lies in the design of the
collection system, which needs to be based on the return period of the rainstorm, i.e., 50- or 100-year storm. A
collection system for runoff plots established at IITA, Ibadan, Nigeria was designed for a rainstorm with a 100-year
return period (Lal, 1976). An example of the data collected for excessive rain received in June 1972 is shown in
Table IV. Despite the large capacity of the collection system, there was an over¯ow and the runoff was more than
the capacity of the collection system. It is because of the over¯ow that the runoff from plots with bare fallow
treatments for all slopes was equal (e.g., 54
6 mm). There was no over¯ow in other treatments. In contrast to
runoff, there were differences in soil erosion among plots of different slope gradients for the bare fallow treatment.
Soil erosion for the bare fallow treatment was 0
8 Mg ha
1
for 1 per cent slope, 4
3 Mg ha
1
for 5 per cent and
10 per cent slopes and 29
8 Mg ha
1
for 15 per cent slope. Soil erosion from maize (plow) treatment was 0
2 Mg
ha
1
for 1 per cent slope, 0
3 Mg ha
1
for 5 per cent slope, 2
6 Mg ha
1
for 10 per cent slope, and 5
5 Mg ha
1
for 15 per cent slope. There were also differences in erosion due to slope gradient under maize (no-till) treatment in
which case soil erosion was 0
03 Mg ha
1
for 1 per cent slope, 0
22 Mg ha
1
for 5 per cent slope, 1
4 Mg ha
1
for 10 per cent slope and 1
6 Mg ha
1
for 15 per cent slope plots.
Runoff and sediment collection systems can be different for large agricultural watersheds, which may involve a
¯ume and water stage recorder and sediment sample, or a weir, water stage recorder and sediment sampler. The
data in Table V show land-use and management effects on runoff and soil erosion. Mean annual runoff ranged from
Table II. Land area affected by soil erosion by water and wind in South Asia (modi®ed from FAO, 1994)
Country
Water erosion (Mha)
Wind erosion (Mha)
Total land area (Mha)
Afganistan
11
2
2
1
65
3
Bangladesh
1
5
0
14
4
Bhutan
0
04
0
4
7
India
32
8
10
8
328
8
Iran
26
4
35
4
163
3
Nepal
1
6
0
14
7
Pakistan
7
2
10
7
79
6
Sri Lanka
1
0
0
6
6
Total
81
74
59
0
677
4
Table III. Methods of ®eld assessment of soil erosion
Process
Area (m
2
)
Method
Reference
Interrill (splash)
10
4
±10
3
Splash cup
Ellison (1947), Elliott
et al.
(1989),
Wang (1988), Young and Wiersma (1973)
Rill
10
1
±10
3
Field plots
Brown
et al.
(1989), Meyer (1994),
Nearing
et al.
(1994)
Gully
10
3
±10
5
Field survey
USDA-SCS (1984),
La¯en
et al.
(1986)
Agricultural watershed 10
5
±10
6
Flumes, runoff samplers
Mutchler
et al.
(1994), Meyer (1994),
River basin
10
6
±10
15
Weir, bedload and sediment sampler
Walling (1994)
526
R. LAL
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Page 9
5
6±113
1 mm and soil erosion from 0
1±8
3 Mg ha
1
. The magnitude of runoff and soil erosion decreased with
increase in land area due to deposition over the landscape. The delivery ratio is an important factor to be considered
in evaluating soil erosion for watersheds.
There are several techniques for measuring the past or historic soil erosion. The application of radioactive fallout
cesium-137 (
137
Cs) is one such technique (Tamura
et al.
, 1964a,b; McHenry
et al.
, 1973; Ritchie
et al.
, 1975;
Ritchie and McHenry, 1990; Hasholt and Walling, 1992; Higgit, 1995). As a result of nuclear weapons testing
between the mid-1940s and early 1970s,
137
Cs and other radionuclides were deposited on earth's surface as
radioactive fallout where these are concentrated in the surface soil layer. Any subsequent movement of
137
C s is
attributed to tillage and erosion/deposion processes. The relationship between
137
Cs concentration in the surface
layer and past soil erosion may be linear, logarithmic or polynomial (Solieau
et al.
, 1990; Oztas
et al.
, 1997;
Bajracharya
et al.
, 1998; Hao
et al.
, 2001). Thus,
in situ
validation of the methodology is important. In addition to
radionuclides, other tracers have also been used. Important among these are Cu (common ingredients in some
pesticides) and charcoal or ¯yash deposited along the railway lines (Hussain
et al.
, 1998).
Table IV. Effect of a 105
4 mm rainfall with a maximum intensity of 142
4 mm hr
1
on runoff and soil erosion from plots on
different slope gradients and under a range of soil and crop management practices (adapted from Lal, 1976)
Slope (%)
Management
Runoff (mm)
Erosion (Mg ha
1
)
1
Bare fallow
54
6
0
80
Maize (mulch)
0
0
0
0
Maize (plow)
34
2
0
2
Maize (no-till)
6
6
0
03
Cowpea (plow)
1
8
0
0
5
Bare fallow
54
6
4
3
Maize (mulch)
7
0
0
01
Maize (plow)
31
3
0
27
Maize (no-till)
31
4
0
22
Cowpea (plow)
34
1
0
1
10
Bare fallow
54
6
4
3
Maize (mulch)
4
1
0
09
Maize (plow)
34
3
2
6
Maize (no-till)
34
2
1
4
Cowpea (plow)
5
6
0
03
15
Bare fallow
54
6
29
8
Maize (mulch)
4
0
0
02
Maize (plow)
34
3
5
5
Maize (no-till)
35
5
1
6
Cowpea (plow)
7
3
0
01
Table V. Land-use and management effects on runoff and soil erosion from agricultural watersheds of 2±4 ha each in western
Nigeria (Lal, 1996)
Treatment
Runoff (mm)
Erosion (Mg ha
1
)
Manual clearing, plow-till
43
5
0
6
Manual clearing, no-till
5
6
0
1
Shear blade, no-till
43
2
1
5
Tree pusher, plow-till
113
1
8
3
Tree pusher, no-till
68
3
5
1
Traditional farming
5
7
0
2
Rainfall
1319
9
Note
: Runoff and soil erosion are annual means for three years from 1979 to 1981.
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Page 10
There are other techniques of estimating soil erosion on a landscape scale using ®eld surveys, buried nail or plate
techniques, exposure of plant roots, and depth of deposition of sediments along fence lines, etc. Details of such
techniques, along with their potential and constraints, are given in Lal (1990) and other reviews.
In addition to soil degradation by water erosion, large areas of arid and semiarid regions are prone to wind
erosion (Oldeman
et al.
, 1991). Areas susceptible to wind erosion include much of North Africa and the Near East,
parts of southeastern Africa, parts of southern and eastern Asia, the Siberian Plain, South America, semiarid and
arid parts of North America, and the drylands of Australia. Factors and causes affecting wind erosion are similar to
that of water erosion (Figures 3 and 4). However, ®eld and laboratory methods of measuring wind erosion are not
as advanced as that for water erosion. Indirect methods of measuring wind erosion include assessment of soil
properties (e.g., texture, wind resistance of soil aggregates) microrelief, ground cover, wind velocity, and climate
aridity. There are standard techniques of measuring wind erosivity and soil erodibility (Skidmore
et al.
, 1994).
Satellite imagery can also be used to assess extent of wind erosion hazard. In contrast, direct methods of assessing
wind erosion are based on techniques involving measurement of drift and the quantity of particles entrained by
wind (Lal, 1990).
Modeling Soil Erosion
Development of USLE
Soil erosion prediction has been a challenge to scientists since the 1930s and several models have been developed
(Table IV). Baver (1933) proposed an empirical equation for estimating soil erosion involving parameters such as
soil dispersion, in®ltration rate, soil permeability, and particle size. Zingg (1940) related soil erosion to slope
gradient. Similar empirical equations were developed by Horton (1945) and Ellison (1947). Smith and Whitt
(1948) added more factors into Zingg's equation to incorporate the in¯uence of cover and management. The major
forerunner of the universal soil loss equation (USLE) was a model proposed by Musgrave (1947). He developed a
parametric model relating erosion to soil erodibility (
K
), vegetation cover (
C
), slope gradient (
S
), slope length (
L
)
and the maximum 30 minute intensity (
I
30
). It was this equation that was then modi®ed by Wischmeier and Smith
(1958) that became known as the USLE.
The USLE has been widely used, and several adaptations and changes have been made since its development.
The USLE was developed as a regression equation to predict soil erosion from cropland on a hillslope. It was
modi®ed during 1970s and 1980s to predict soil erosion under other conditions. The modi®ed universal soil loss
equation (MUSLE) was developed to estimate erosion on a single storm basis (Foster
et al.
, 1977a,b), sediment
yield from watersheds (Williams and Berndt, 1972; Williams, 1975; Cooley and Williams, 1985), erosion and
Table VI. Empirical models for predicting soil erosion
Parameter
Reference
Slope gradient
Zingg (1940)
Slope, rainfall, and soil factor
Musgrave (1947), Smith and Whitt (1948)
Slope, rainfall, soil and management (USLE)
Wischmeier and Smith (1958, 1978)
Soil loss tolerance
Stamey and Smith (1964)
RUSLE (revised universal soil loss equation)
Renard
et al.
(1991)
MUSLE (modi®ed universal soil loss equation)
Williams and Berndt (1977), Williams (1982)
CREAMS (chemical, runoff, and erosion from
Knisel (1980), Foster
et al.
(1980)
agricultural management systems)
SPUR (simulation of production and utilization of rangeland)
Wight and Skiles (1987)
SWRRB (simulator for water resources in rural basins)
Williams
et al.
(1985)
ANSWERS (areal nonpoint source watershed
Beasley
et al.
(1980)
environment response simulator)
Kineros (kinematic runoff and erosion model)
Woolhiser
et al.
(1990)
AGNPS (agricultural non-point source)
Young
et al.
(1987)
SLEMSA (soil loss estimation system for southern Africa)
Elwell (1977), Elwell and Stocking (1982)
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Page 11
sediment yield from rangelands (Renard
et al.
, 1974), soil erosion from forest lands (Dissmeyer and Foster, 1985)
and from ¯atlands (Mutchler and Murphree, 1981).
In addition, USLE was revised to update the database. The revised universal soil loss equation (RUSLE) updates
the information on data acquired after the 1978 USLE release, and incorporates several concepts from process-
based erosion models. Incorporation of these concepts provides basis for estimating values of different factors
including
L
,
S
and
C
(Renard
et al.
, 1991, 1994, 1996, 1997). Equations used to arrive at the factor values in
RUSLE are different than those used in USLE. Similar to USLE, however, RUSLE remains to be a regression
equation.
A
R
K
L
S
C
P
. . .
1
A principal modi®cation is in factor
R
which includes both rainfall and runoff erosivity factor (runoff erosivity also
includes snow melt where such runoff is signi®cant). There are also changes in factor
C
which is based on
computation of subfactor called soil loss ratios (SLR). The SLR depends on
S
subfactors: prior land use, canopy
cover, surface cover, surface roughness and soil moisture (Renard
et al.
, 1996).
Development of other parametric models
In addition to USLE some regional parametric models have also been developed to predict soil erosion from
cropland. Hudson (1961) proposed a parametric prediction equation similar to USLE (Equation 2),
A
TSLPMR
. . .
2
where
A
is erosion,
T
is the soil type,
S
is the slope gradient,
L
is the slope length,
P
is the agricultural practice,
M
is
the mechanical protection and
R
is the rainfall. Speci®c modi®cation in erosivity index were made to substitute this
factor
R
for
KE
>
l by Hudson (1961),
AI
m
by Lal (1976) and
p
2
=
P
by Fournier (1956) and Fournier and Henin
(1959) and FAO (1979). Elwell (1977) and Elwell and Stocking (1982) developed yet another empirical model
called the soil loss estimation for Southern Africa (SLEMSA) (Equation 3),
A
KCX
. . .
3
where
A
is the predicted mean annual soil loss,
K
is the mean annual soil loss from a standard ®eld plot 30
10 m
on a 4
5 per cent slope with soil of known erodibility under fallow,
C
is the crop management factor and
X
is the
slope length factor.
Process-based models of predicting soil erosion
Empirical models (e.g., USLE and SLEMSA) have constraints of applicability limited to ecological conditions
similar to those from which data were used in their development. Further, because USLE cannot deal with
deposition, its application precludes large areas and watersheds. Based on these considerations, several process-
based models have been developed (e.g., EUROSEM, LISEM). A process-based model widely used is the Water
Erosion Prediction Project (WEPP) (Foster, 1991; Nearing
et al.
, 1994; Renard
et al.
, 1996). The WEPP model
computes erosion by rill and interrill processes. The sediment delivery to rills from interrill areas is computed by
Equation 4,
D
i
K
i
I
2
e
G
e
C
e
S
f
. . .
4
where
D
i
is the delivery of sediment from interrill areas to rill (kg m
2
s
1
),
K
i
is the interrill erodibility (Kg m
4
s
1
),
I
e
is the effective rainfall intensity (m s
1
),
G
e
is the ground cover adjustment factor,
C
e
is the canopy cover
adjustment factor, and
S
f
is the slope adjustment factor calculated as per Equation 5.
S
f
1
05 0
85 exp
4 sin
a
. . .
5
SOIL DEGRADATION BY EROSION
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Page 12
where
a
is the slope of the surface toward nearby rill (Renard
et al.
, 1996). In comparison, rill erosion is the
detachment and transport of soil particles by concentrated ¯owing water (Eq. 6),
D
c
K
r
c
. . .
6
where
K
r
is the rill erodibility (s
1
m
1
),
is the hydraulic shear of ¯owing water (Pa) and
c
is the critical
hydraulic shear that must be exceeded before rill detachment can occur (Pa).
Rose (1994) adopted a different basic approach to sediment erosion and transport. He computed sediment (
q
s
)
and water (
q
) ¯uxes across a sloping, planar land surface using the mass balance approach. He calculated soil loss
from the land area during an erosion event by Equation 7,
q
s
qc
. . .
7
where
q
s
is the soil loss (kg m
1
s
1
),
q
is water ¯ux (m
3
m
1
s
1
) and
c
is the sediment concentration (mass of
sediments per unit volume of suspension). Therefore, assessment of soil erosion involves the quanti®cation of
hydrology of surface ¯ow and of the erosional processes that increase sediment concentration
C
and the
depositional processes that decrease it.
Similar to the water erosion, process-based models have also been developed for wind erosion (Skidmore,
1994), which is a signi®cant factor in arid and semiarid regions, and reliable predictive models can be useful in
developing mitigation strategies. Comparable to the USLE, a wind-erosion model was proposed by Woodruff and
Siddoway (1965) as shown in Equation 8.
E
f
I
;
K
;
C
;
L
;
V
. . .
8
where
I
is the soil erodibility index,
K
is the soil ridge-roughness factor,
C
is the climatic factor,
L
is the
unsheltered median travel distance of wind across a ®eld, and
V
is the equivalent vegetative cover. This equation
has been widely adopted and used for estimating erosion hazards in drylands (Wilson, 1975; USDA, 1984).
EROSIONAL EFFECTS ON PRODUCTIVITY
Accelerated soil erosion has adverse economic and environmental impacts (Lal, 1998). Economic effects are due
to loss of farm income due to on-site and off-site reduction in income and other losses with adverse impact on crop/
animal production. The environmental impacts are due to pollution of natural waters. Productivity effects of soil
erosion are both on-site and off-site. The on-site productivity loss due to soil erosion is attributed to three
interacting effects (Figure 5): short-term productivity effects, long-term productivity effects and reduction in soil
quality. The most severe effect is due to loss of topsoil depth in soils with a root-restrictive layer. In addition to
erosion by water and wind, tillage erosion is also an important determinant of loss of topsoil depth in cultivated
landscapes of rolling terrain (Lewis and Nyamalinda, 1996; Turkelboom
et al.
, 1997; Thapa
et al.
,1999; Quine
et
al.
, 1999). On-site loss of productivity caused by all types of erosion can be partly compensated by additional input
of fertilizers (especially N) and supplemental irrigation. Increase in input reduces pro®t margin even though crop
yield remains the same. Off-site effects on productivity may be positive or negative. Crop yields are an expression
of historical production, whereas productivity is a measure of potential yield (Tengberg and Stocking, 1997). Yield
can remain constant or even increase as soils become degraded (Stocking, 1994; Dregne, 1995). Soil degradation
may also in¯uence quality of crop produced (Johnson and Lewis, 1995). Numerous ®eld experiments have
documented increase in crop yields on depositional sites (Fahnestock
et al.
, 1995), especially in seasons with
below-normal rains and periods of prolonged drought stress. Reduction in crop yields on depositional sites is often
due to crop burial, runon of pesticides, and inundation leading to anaerobiosis. Similar to the on-site effects, the
off-site effects of accelerated erosion also comprise short-term effects, long-term effects, and changes in land/soil
quality (Figure 6).
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Page 13
It is dif®cult to assess the loss of agronomic productivity due to soil erosion, because of the confounding effects
of rainfall and other climatic factors during the growing season, and that of management. There is also a scaling
problem for extrapolation of data from plot scale to regional, national or global scale. There are few estimates of
productivity loss at regional scales. Qualitative estimates of the effects on irreversible soil productivity losses from
water erosion were reported for several regions of Africa (Dregne, 1990), Asia (Dregne, 1992a,b), Australia and
New Zealand (Dregne, 1995), and North America (Den Biggelaar
et al.
, 2001). Lal (1995) estimated yield losses
due to past erosion in Africa, and reported that losses may range from 2±40 per cent, with a mean loss of 9 per cent
for the continent and 6
6 per cent for subSaharan Africa (Table VII). It was estimated that if accelerated erosion
continues unabated, yield reductions by 2020 may be 16
5 per cent for the continent and 14
5 per cent for
subSaharan Africa. Annual reduction in total production for 1989 due to accelerated erosion in the continent of
Africa was 8
2 million Mg of cereals, 9
2 million Mg of roots and tubers, and 0
6 million Mg of pulses. The
reduction in total production for 1989 for subSaharan Africa was estimated at 3
6 milion Mg for cereals, 6
5
million Mg for roots and tubers, and 0
36 million Mg for pulses (Lal, 1995). Lal (1998) reported risks of global
loss of crop yields due to erosion by 10 per cent in cereals, 5 per cent in soybeans, 5 per cent in pulses, and
12 per cent in root and tubers (Table VIII). Accordingly, total production losses on a global scale in 1995 may have
been 192 million Mg for cereals, 6 million Mg for soybeans, 3 million Mg for pulses, and 73 million Mg for roots
and tubers. Estimates of regional losses in crop production showed total loss in food production at 31 million Mg
for Africa, 190 million Mg for Asia, and 18 million Mg for tropical America (Table IX). Den Biggelaar
et al.
(2001) estimated erosional losses on crop yields in North America. The amount of production decline resulting
from erosion in North America was estimated at 234
5
10
3
Mg yr
1
for maize, 60
2
10
3
Mg yr
1
for
soybeans, 75
0
10
3
Mg yr
1
for wheat and 1
9
10
3
Mg yr
1
for cotton. The total economic value of erosion-
induced loss in crop yields (using 2000 prices) was US $41
2 million, of which US $37
9 million were in the
United States and the remainder in Canada.
Figure 5. On-site effects of soil erosion on productivity are due to short-term and long-term effects, and on decline in soil quality.
SOIL DEGRADATION BY EROSION
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Page 14
These estimates of global loss of crop yields are to be considered with caution. These are merely the estimates of
yield losses that may occur under the worst case scenario. The actual yield loss during any growing season due to
past erosion may depend on numerous factors already described in this section.
In addition to productivity loss, there are other off-site economic impacts of accelerated soil erosion (Figure 6).
These economic impacts are due to siltation of waterways and reservoirs, and adverse impacts on aquatic
ecosystems and recreational facilities (Lal, 1998; Pimentel
et al.
, 1995).
Table VII. Erosion effects on grain production in Africa in 1989 (Lal, 1995)
Commodity
Actual production in 1989
a
Expected production if there was no soil erosion
Yield
Total production
Yield
b
Total production
(kg ha
1
)
(10
6
Mg)
Mg ha
1
)
(10
6
Mg)
African continent (51 countries)
Cereals
1228
91
1338
99
Roots and tubers
7231
103
7882
112
Pulses
584
7
637
7
SubSaharan Africa
c
(44 countries)
Cereals
1140
55
1215
59
Roots and tubers
6460
98
6886
105
Pulses
514
5
548
6
a
Actual production data were obtained from the Food and Agriculture Organization (1989).
b
Estimated yield reduction due to erosion is 9% for continent of Africa and 6.6% for subSaharan Africa.
c
SubSaharan Africa does not include Algeria, Egypt, Libya, Mauritania, Morocco, South Africa and Tunisia.
Table VIII. World food production in 1995 with and without erosion (Lal, 1998)
Commodity
Net production
a
(10
6
Mg)
Estimated
Estimated production
production loss (%)
if there were no erosion (10
6
Mg)
Cereals
1896
10
2086
Soybeans
126
5
132
Pulses
56
5
59
Roots and tubers
609
12
682
Total
2687
32
2959
a
Net production ®gures are from FAO (1995).
Table IX. Regional food production statistics for 1995 with (A) and without (B) soil erosion
Cereals (10
6
)
Soybeans (10
6
)
Pulses (10
6
)
Roots and tubers (10
6
)
Region
A
B
A
B
A
B
A
B
North Central America
358
376 (5)
61
64 (5)
6
6 (5)
28
29 (5)
Europe
268
281 (5)
1
1 (5)
6
6 (5)
80
84 (5)
Oceania
27
28 (10)
Ð
Ð
2
2 (15)
3
3 (10)
Africa
100
110 (10)
0
5
0
6 (20)
7
8 (20)
135
155 (15)
Asia
929
1068 (15)
21
23 (10)
27
31 (15)
248
293 (18)
South America
90
99 (10)
41
45 (10)
4
4 (10)
46
51 (12)
Others
124
130 (5)
3
3 (5)
4
4 (5)
69
72 (5)
Total
1896
2092
126
136
50
61
609
687
Notes
: Numbers in parenthesis indicates the assumed percent decline in production for each region.
Production statistics from FAO (1995).
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Page 15
SOIL EROSION AND THE GREENHOUSE EFFECT
The atmospheric concentration of CO
2
and other radiatively-active gases is steadily increasing. Concentration of
CO
2
has risen from 280 ppmv from about 1850 to 365 ppmv in 1995, and is increasing at the rate of 0
5 per cent
yr
1
(IPCC, 1995). At this rate of increase, CO
2
concentration is expected to reach 600 ppmv during the 21st
century. The concentration of CH
4
has increased from 0
8 to 1
74 ppmv and is increasing at the rate of
0
75 per cent yr
1
, and that of N
2
O has increased from 288 ppbv to 311 ppbv and is increasing at 0
25 per cent
yr
1
(IPCC, 1995). In addition to fossil fuel, another recognized anthropogenic source of atmospheric increase in
CO
2
concentration is land-use change and agricultural activities. These activities lead to emission of Cfrom the
soil to the atmosphere.
The soil Cpool is the third largest global Cpool. The oceanic pool is estimated at 38 000 Pg C, the geologic pool
containing fossil fuel at 5000 Pg, the soil Cpool at 3200 Pg (and likely to be as high as 3500 Pg), the atmospheric C
pool at 760 Pg and the biotic Cpool at 560 Pg (Batjes, 1996; Schlesinger, 1997). The soil Cpool comprises both
soil organic carbon (SOC) and soil inorganic carbon (SIC) components. The SOC component to 2 m depth is
estimated at about 2450 Pg, and the SICcomponent to 1 m depth at 750 Pg. Thus, the soil Cpool is about 4
2 times
the atmospheric pool and 5
7 times the biotic pool. The atmospheric Cpool is increasing at the rate of
3
3 Pg C yr
1
primarily at the expense of the soil and the biotic pools. The historic Closs is estimated at 50±100
Pg Cfrom soils, and 100±150 Pg from the biotic Cpools. The depletion of soil Cpool is accentuated by soil
erosion and other degradative processes (Lal, 1999a).
Soil erosion is a multi-stage process involving detachment, redistribution over the landscape, deposition in
depressional sites, and transport to aquatic ecosystems in water erosion and to depressional sites in wind erosion
(Figure 8). Disruption or breakdown of aggregates exposes the Cto climatic elements and microbial decomposi-
tion which was previosuly encapsulated within the aggregates and buffered against mineralization. Some of the C
Figure 6. Off-site effects of soil erosion on productivity may be short term or long term and due to decline in land/soil quality.
SOIL DEGRADATION BY EROSION
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Page 16
Figur
e
7.
O
ff-site
economi
c
impact
of
accelerated
soi
lerosio
n.
534
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Page 17
is redistributed over the landscape and is also prone to mineralization. The Cdeposited in depressional sites and
carried to aquatic ecosystems may be buried and sequestered (Stallard, 1998). Redistribution of Ctransported with
eroded sediments affects several processes that accentuate emission of greenhouse gases from landscape to the
atmosphere. It is often assumed that as much as 20 per cent of the carbon displaced by soil erosion is emitted into
the atmosphere and 10 per cent deposited in depressional sites (Lal, 1995). It is estimated that as much as 1.14 Pg C
may be emitted into the atmosphere due to water erosion and related processes. Lal
et al.
(1999) estimated annual
Closs due to deserti®cation of drylands at the rate of 0
2±0
9 Pg C yr
1
.
It is dif®cult to asses the historic loss of Cdue to past erosion. Using the available data on land area affected by
erosion (Oldeman
et al.
, 1991), Lal (1999b) estimated that 21 Pg Chas been lost due to water erosion and 4 Pg due
to wind erosion (Table X). These estimates, crude as these may be, provide a reference point with regard to the
potential of Csequestration through erosion control and restoration of degraded soils. It is assumed that 60±75
per cent of the historic Closs can be recovered through adoption of soil restorative techniques.
Figure 8. Soil erosion and gaseous emissions.
Table X. World soil Cdepletion due to accelerated soil erosion (Lal, 1999b)
Water erosion
Wind erosion
Severity
Area
Cdepletion
Closs
Area
Cdepletion
Closs
Total loss
(Mha)
(Mgha)
(Pg)
(Mha)
(Mgha)
(Pg)
(Pg)
Light
343
5
1
7
269
2
5
0
7
2
4
Moderate
527
20
10
5
254
10
2
5
13
0
Strong and extreme
224
40
9
0
26
20
0
5
9
5
Total
Ð
Ð
21
2
Ð
3
7
24
9
Ð
SOIL DEGRADATION BY EROSION
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Page 18
Erosion-induced changes in soil Cdynamics are complex and not very well understood. An important factor
affecting soil Cdynamics is the activity and species diversity of soil fauna which in¯uence microbial processes.
Microbial processes affect humi®cation, aggregation, and mineralization. Effects of erosion/deposition cycles on
these processes need to be studied.
CONCLUSIONS
Soil degradation by accelerated erosion has been a chronic problem ever since the dawn of settled agriculture. The
problem has been exacerbated by increase in human and animal population, cultivation of marginal lands, and
adoption of extensive, resource-based and subsistence farming methods. Soil erosion is a physical process driven
by socio-economic, cultural, and political causes. Effectiveness of biophysical measures to mitigate soil erosion
can be enhanced by due consideration to soil, political and cultural variables.
Estimates of the global extent of soil erosion are tentative and subjective, and need to be improved by using
remote sensing, GIS and other modern techniques. Developing a credible data base is crucial to identifying
management strategies. Erosional hot spots of the world (Himalyan±Tibetan ecoregion, subSaharan Africa,
Central America and the Caribbean, and the Andean region) are in need of coordinated efforts at the global scale to
restore degraded ecosystems.
The measurement of soil erosion is still more of an art than science, and a wide range of techniques are used to
monitor soil erosion. There is a strong need to standardize methods of measurement of soil erosion rates at ®eld,
hillside and watershed scales.
Accelerated soil erosion has adverse economic and environmental impacts. Yield reduction by erosion-induced
decline in soil quality may be high in regions with predominantly subsistence agriculture based on low external
input.
Soil degradation in general and accelerated erosion in particular also affect water quality and emission of
radiatively-active gases to the atmosphere. The strategy of controlling soil erosion and restoring degraded soils and
ecosystems has a potential to sequester carbon in vegetation and soil, and reduce the rate of enrichment of
atmospheric concentration of CO
2
and other gases.
Technology for mitigating soil erosion is known, and have been proven effective on experimental plots.
However, adoption of these technologies requires ®ne tuning at local scales with due consideration to socio-
economic, political and cultural factors.
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