Agriculture and the Environment IX, Valuing Ecosystems: Policy, Economic and Management Interactions (2012)
ASSESSMENT AND EVALUATION OF SURFACE SOIL EROSION UNDER AN
AGRICULTURAL SOIL ECOSYSTEM IN THE SUDAN SAVANNAH
S Usman
Natural Resources Institute, Agriculture, Health and Environment Department, University of
Greenwich at Medway, ME4 4TB, UK, E-mail: [email protected]
SUMMARY
The focus of this assessment was to evaluate the types and impacts of soil erosion under
surface soil of the Sudan Savannah ecosystem in Kebbi State, Nigeria. Soil erosion was viewed
from the basis of different types of soil erosion and their resulting patterns. In the first
assessment, six types of soil erosion were evaluated: sheet, rill, gully, winnowing, massmovement and dry-mechanic. In the second assessment, seven different shapes of rill and gully
erosions were assessed based on their physical appearance and the nature of their surface soil
textures: I, J, S, U, V, X, and Y. The V, U and Y categories belong to gully erosions under
homogeneous sand-silt soil texture, I, J, S and X belong to rill erosion under sand-loam soil
texture. Finally, erosion was classified according to its category and degree of impact for soil
quality and land suitability descriptions.
INTRODUCTION
Soil ecosystems are widely recognised as important components of agricultural development
globally (e.g. Usman, 2007). In the 19 years since the creation of Kebbi State, Nigeria,
significant numbers of complaints have been voiced by local farmers over the occurrence and
impact of soil erosion on agricultural lands in the Sudan Savannah zone of the State (KARDA,
1997). These complaints need to be considered, because soil erosion has a serious impact on
agricultural soils worldwide (e.g. Vrieling, 2006; Symeonakis and Drake, 2010) and because of
its severe adverse economic and environmental consequences on soil ecosystems, it has been
recognised as a global issue of the 21st Century (Lal, 1998; Lal et al., 2003). Erosion gnaws
away the soil, starting with the surface layer as sheet erosion, creating small channels as rill
erosion and then extending to deep channels as gullies.
Soil erosion has been considered to decrease food production, water quality, soil quality,
promote ecological imbalance and contaminate soil (Usman, 2011). The nature and impact of
soil erosion on soil properties and natural ecosystems have been well documented (Lal, 1998;
Lal et al., 2003; Vrieling, 2006; Usman, 2007; Stavi and Lal, 2011) and provide invaluable and
educated information for researchers in soil science, crop science, environmental science and
climatology, as well as for policy makers, students, industry and governmental organisations.
However, few of these studies have looked at the aspect of classification and assessment of soil
erosion according to erosion profiles (shapes) and the degree of impact of the erosion on the
ecosystem or environment. This is important, because understanding the impact and types of
soil erosion for future sustainable soil and environmental management requires more valuable
and additional site information on how soil erosion occurs in the field. Therefore, the physical
nature of soil erosion in the field needs to be surveyed and recorded at different sites, according
to the degree of detail required (e.g. Hudson, 1987). Access to qualitative field data information
has been widely recognised as the cornerstone to meaningful soil and environmental studies,
including studies of soil erosion (FAO, 2006). Therefore, the present assessment was initiated
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as a first step to collecting qualitative field data with the general objective of studying surface
soil erosion in Kebbi State. The assessment aims to report the properties and classes of soil
erosion, focusing mainly on its categories, degree of severity and surface structure and shape.
MATERIALS AND METHODS
Study Area
The Sudan Savannah is located in sub-Saharan Africa, bordering the nations of Niger to the
west and Benin to the southwest. The region is dominated by the Hausa-Fulani people who are
largely farmers. The total land area of the State is 36,229 km2 of which 12,600 km2 is under
agriculture (KARDA, 1997). The Sudan Savannah zone has two important agricultural lands:
the fadama land and dryland. The fadama is an agricultural land which is seasonally flooded
during the period of rains: these recede during the dry season leaving a coating of alluvial clay
soil. The dryland areas of the State share similar characteristics with other dry areas of the
world. The major crops cultivated are millet, sorghum, cowpea, rice, wheat and vegetables.
Field sites were selected according to observations made during the field visit from 2008 to
2011. These sites are physically characterised by a high sand content (fine to very fine sand
60%-80%), low moisture content and low organic matter. Site information on soil erosion was
collected according to soil guidelines (FAO, 2006; USDA-NRCS, 2002). Erosion was
classified according to a standard international classification system (Table 1: USDA-NRCS,
2002; FAO, 2006).
Classification of Soil Erosion
Table 1:
Procedure used in the classification of surface soil erosion in the study area
S
Slight
M
Moderate
V
Severe
E
Extreme
Water
erosion
Sheet
Rill
Gully
Winnowing
Mass-move
Wind
erosion
1(a) Classification of erosion, by degree (FAO, 2006)
Some evidence of damage to surface horizons. Original biotic
function largely intact.
Clear evidence of removal of surface horizons. Original biotic
functions partly destroyed.
Surface horizons completely removed and subsurface horizons
exposed. Original biotic functions largely destroyed.
Substantial removal of deeper subsurface horizons (badlands).
Original biotic functions fully destroyed.
1(b) Classification of erosion, by category USDA-NRCS (2002)
Uniform removal of soil in thin layers from sloping land.
Detachment and transport of soil by concentrated flow of water.
An advance stage of rill erosion, deep like drainage channels.
Soil detachment and transport by strong air (wind).
Soil detachment and transport by strong air (wind).
Consideration was given to category, degree and shape of soil erosion in the study area.
However, physical soil quality (PSq) and land suitability (PLs) for crop production were used
to describe the surface soil condition around the erosion sites. Five sets of soil quality classes
(Sq1, Sq2, Sq3, Sq4, Sq5) and five sets of land suitability classes (Ls1, Ls2, Ls3, Ls4, Ls5)
indicating how well each affected site is suited to crop production were used (Table 2). The
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relative importance of these classes can be explained according to the specific surface soil
condition in the eroded soils of the assessment areas. Soil quality classes, which represent a
specific soil area for a specific kind of erosion impact, are attributes of surface soils. The land
suitability classes are an attribute of surface soils which can be suitable or unsuitable for crop
production.
Shapes, soil quality (P-Sq) and land suitability (P-Ls) description guide1
Table 2:
(a) Shape
Type
Upper case letters are used – I, J, S, U, V, X, and Y. U, V, Y for gully
erosion; and I, J, S, X for rill erosion (see description guide: Table 1(b)).
(b) PSq
Sq1
Sq2
Sq3
Sq4
Sq5
Very small-size channel of sheet erosion: 0–1 cm width and depth.
Small-size channel of sheet erosion: (1-2 cm width and depth.
Small-size channel of rill erosion: 2-5 cm width and depth.
Large-size channel of rill erosion: 5-20 cm width and depth.
Gully surface erosion: >20 cm width and depth.
(c) PLs
Ls1
Ls2
Ls3
Ls4
Ls5
Good land: few indications of very small-size channels of sheet erosion.
Moderately good land: few of small-size channels of sheet erosion.
Poor land: 20% of the site is affected by small channels of rill.
Very poor land: >20% of the site is affected by large channels of rill.
Bad land: significant portion of the land is affected by gullies.
1
Field work guide (2008-2011).
RESULTS
Table 3:
Class
01
02
03
04
Soil erosion by category and by degree classes in Sudan Savannah
Water
Sheet
Rill
Gully
Erosion by categories
Wind and human-induced (*)
Winnowing of sand
Mass-movement of sand
Dry-anthropogenic (mechanical) (*)
Erosion by degree
Water
Wind
Slight
Moderate
Moderate
Moderate
Severe
Moderate
Moderate (*)
Slight (*)
Table 3 shows the classes of soil erosion by category and by degree. Water erosion was
classified into slight, moderate and severe degrees of impact. Wind was also classified into
slight and moderate degrees of impact classes (refer to Table 1). For dry anthropogenic erosion,
moderate and slight impacts are classified under wind and human induced erosions.
Differences of these two degree impact classes of dry mechanical erosion are judged according
to how water and wind move soil particles from one area to another. However, soil particles
moved by water and causing slight surface impact is regarded as sheet erosion. Extension of
the sheet surface channel puts the same area into moderate impact, rill erosion. Deeper cut and
larger channels, gully erosion, are classified as severe degree of impact. The differences
accounted for by these three types of water erosion are very clear. Movement and transport of
very light soil particles by wind are common phenomena in dry periods. This is classified as
winnowing of sand. The physical soil quality and land suitability classes of the slight, moderate
and severe impact degrees of soil erosion in the affected areas are given in Table 4. Generally,
the term dry-anthropogenic or dry-mechanical is used in this classification to define the surface
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soil condition affected by human agricultural activities as a result of long term cultivation
without proper sustainable soil management practices. This long term cultivation has given
room for wind to moved soil particles from one place to another, a phenomenon called
winnowing (i.e. soil particles moving in circulation as a result of strong winds).
Table 4:
Soil quality and land suitability classes of areas affected by erosion in Sudan
Savannah
Erosion
Gully
Gully
Gully
Shape
V-shape
U-shape
Y-shape
Sq class
Sq5
Sq5
Sq4
Ls class
Ls5
Ls5
Ls4
Physical surface soil description
Well-drained sand, sandy-silt, small-size.
rocks fragments, low moist, very poor
vegetation and low organic matter.
Rill
Rill
Rill
Rill
C-shape
S-shape
X-shape
I-shape
Sq3
Sq2
Sq3
Sq2
Ls2
Ls3
Ls3
Ls1
Deep sandy-loam, well-drained, coarse sand,
low moisture, poor vegetation cover, and
low organic matter.
Sq2, Sq3, and Sq4 or Sq5 soil quality classes correspond very well to slight, moderate and
severe degrees of impact of soil erosion, respectively. This means soil erosion may be also
categorised as rill or gully even under the same surface soil condition, but that the degree of
physical impact and the severity level of its occurrence will differ greatly. The shapes of the
erosion structures may also differ. For example, the amount of soils that physically removed
from U-shaped and V-shaped gullies are much greater than the amount removed from Yshaped gullies. Similarly, the soil particles removed from X-shaped and S-shaped rill channels
is higher than that removed from J- or I-shaped channels. On the contrary, the land suitability
classes show big differences between rill and gully erosion. Most of the gully affected areas are
very poor (Ls4) and bad (Ls5) lands, not suitable for agriculture. However, in the case of rill
affected areas, most of the lands are good (Ls1) and moderately good (Ls2) for a wide range of
cereal production as physically observed during the field visit from 2008 to 2011.
DISCUSSION
It has been shown that the assessment and classification of surface soil erosion into sheet, rill,
gully, dry-anthropogenic, and winnowing of soil particles correspond very well to previous
studies, as comprehensibly reviewed by Roose (1996) and Usman (2011). The effects of these
types of soil erosion on the surfaces of agricultural soils are physical (soil removal, aggregate
disorientation), chemical (organic matter and important chemical disappearance) and biological
(microbial biomass decreased) (Seeger, 2007). The main causes were attributed to the lack of
proper surface soil protection against raindrop impact, decreased aggregate stability, long and
steep slopes, intense rainfall, decreased infiltration by compaction or other means (USDANRCS, 1996; Kimaro et al., 2008). These might have led to complete surface soil layer
disappearance and subsequent subsurface soil layers damage (e.g. Figure 1). Similarly, a
decrease in soil fertility and soil quality and crop yield decline, may result, because
disintegrated aggregates and high soil particle erodibility enables easy movement and transport
of soils by runoff (Bradford et al., 1987) and wind (Gomes et al., 2003) thus leading to surface
soil deterioration characterised by different surface soil erosion structural shapes (see Table 4).
Such deterioration (e.g. Figure 1) can result in a noticeable decline in organic matter on
agricultural fields and finally leads to partial or even complete destruction of soil productivity
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by damaging soil structure and aggregate stability and reducing porosity, water infiltration
rates, and storage and availability of nutrients (Su et al., 2003).
Figure 1:
Typical examples of surface soil erosion in Sudan Savannah: types, shapes and
impacts
Generally speaking, a more meaningful assessment and evaluation of surface soil erosion can
be made if the precise objective of such actions is known. For instance, if the main objective is
to assess only the surface rill/gully erosions the overall properties, the dynamic nature and
classes of soil affected by rills/gullies would be indentified in more detail. In the present
assessment, the different types of soil erosion have been only considered as preliminary.
Although the target is to evaluate the types and classes of soil erosion, the information may not
be sufficient for future sustainable soil management. However, the assessment has given an
overview of the physical surface soil conditions in term of soil quality and land suitability in
the erosion affected sites. It is believed that the soil particles removed from each shape of the
specified type of soil erosion (refer to Table 4) explained the level of soil quality and land
suitability for agricultural production. Thus, areas affected by rills (I, C, S, X shapes) have less
impact (Ls1, Ls2, Ls3) and better surface soil quality (Sq2, Sq3) than those affected by gullies
(U, V, Y shapes – Ls4, Ls5 and Sq4, Sq5). These classes tallied with the risk classes of soil
degradation defined by Lal et al. (2003). According to their definitions, Ls1, Sq1, Ls2 and Sq2
represent the slight and moderate impact classes because the original surface soil biotic
function is still intact but full restoration of the impacted areas is required. On the other hand,
the Ls3 and Sq3 classifications fall under the poor land class characterised by small-size
channels of rill, because most of the surface area has been degraded. Sq4, Ls4, Sq5 and Ls5
correspond to the extreme class, where re-establishment of the landscape components (soil,
vegetation) is needed for some level of agricultural productivity and biotic function. This leads
to the conclusion that the physical surface soils of some agricultural sites in the Sudan
Savannah of the State have been damaged by soil erosion and that the extent and severity level
of this damage could only be quantified by direct measurement or any other quantitative
analyses. Thus, this measurement, rather than just a visual assessment, is required in a situation
where surface soils have been significantly affected by rill and gully erosions.
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ACKNOWLEDGEMENT
I thank the Kebbi State Government for funding this work through a scholarship programme,
under the leadership of Alhaji Saidu Usman Nasamu Dakingari – The Executive Governor of
Kebbi State Nigeria.
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