Soil Erosion Hazard in Errer Dembel Sub-Basin, in Shinille Zone of the Ethiopia Somali Regional State
Yohannes Gerezihier Gebremedhin*
Department of Natural Resources Management, Adigrat University, Ethiopia
Submission: January 10, 2019;Published: February 05, 2019
*Corresponding author: Yohannes Gerezihier Gebremedhin, Department of Natural Resources Management, College of Agriculture and Environmental Sciences, Adigrat University, p.o.x., 50 Adigart, Tigray, Ethiopia
How to cite this article: Yohannes G G. Soil Erosion Hazard in Errer Dembel Sub-Basin, in Shinille Zone of the Ethiopia Somali Regional State. Int J Environ Sci Nat Res. 2019; 17(1): 555951. DOI:10.19080/IJESNR.2019.17.555951.
Human activities disturb the land surface of the earth, and thereby alter natural erosion rates. Errer-dembel sub-basin is located within awash-basin in Shinille zone of the Ethiopia Somali region. Soil erosion hazard and land degradation were comprised from field measurements, survey and investigation of the area were made. Various forms of erosion and land degradation exist in the basin with varying level of severity and extents were found. The types of land degradation observed in the basin include water erosion, wind erosion, soil fertility decline, sedimentation/soil burial, de-vegetation and rock outcrop. the causes to these include poor farming practices, overgrazing in the range lands, clearing of forest for charcoal making and animal feed, forest and bush burning and lack of awareness. A maximum soil loss of 40.80tha-1yr-1 and minimum of 0-3tha-1yr-1 is estimated in the sub-basin. Similarly, the soil loss tolerance limit estimates, indicated high values 7tha-1yr-1 for the high rainfall areas with intensive cultivation, and low values of 1-4tha-1yr-1 in the low rainfall areas in the north of the basin, owing to the prevailing unfavorable conditions involved in soil formation processes. Soil formation rate is in the range of 0.5-4tonsha-1yr-1. This indicates that the soil loss is much beyond the tolerable level in the southern and south eastern part of the sub-basin with the resultant high index of erosion hazard. Thus, the soils of Errer-dembel sub-basin are found to be highly susceptible to soil erosion under any circumstances.
Soil erosion, land degradation and desertification are terminologies commonly employed in describing the condition of an environment that is in disequilibrium and mainly are the result of man’s interaction with his surroundings. Accelerated soil erosion is often a problem where unsystematic or inappropriate land use prevails and can simply be defined as the process of removal or displacement of soil particles from one place to another by the forces of water and/or wind and mass slide. The continues removal of soil particles from a given area often leads to the complete removal of the nutrient rich top soil, reduction of the soil depth and its water holding capacity and thus in turn lead to reduced productivity .
Varieties of human activities disturb the land surface of the earth, and thereby alter natural erosion rates . Land degradation, according to the United Nations’ Environmental Program’s (UNEP’s) definition of 1991 , implies reduction of resource potential by one or a combination of processes acting on the land. These processes include water erosion, wind erosion and sedimentation by those agents, long-term reduction in the amount or diversity of natural vegetation or decrease of crop yield where relevant and salinization and sodification of soils. Therefore, soil erosion is the major causes of land degradation in Ethiopian in general and Errer-dembel sub-basin area in particular.
Soil loss rates for different land unit classes (land cover types) were estimated using the Universal Soil Loss Equation (USLE) adapted for Ethiopian conditions. Hurni  noted that these results correlated well with test-plot measurements made by the Soil Conservation Research Project (SCRP), although, as noted previously, such estimates can greatly overstate the net impact of erosion on soil loss, because of re-deposition of soils elsewhere in the landscape.
Hurni reports  gross soil losses and hence the loss figures are not net of soil re-deposition and soil formation. Soil formation was separately reported so that gross losses could be compared to soil formation, as an indicator of sustainability. Estimation of soil formation was “based on assumptions of soil regeneration as a function of geology and soils (unit and depth), slope gradient, climate (temperature, rainfall, and length of growing period), and land use” . Hurni also noted that the model was not validated thoroughly with field data.
Water erosion is the dominant form of soil erosion in the Errer-dembel sub-basin. Slight wind erosion problem exists particularly on the central and north west of the sub basin around Aysha-Dewele watershed of the sub basin. Accelerated erosion is caused by the activities of man and is responsible for depleting soil productivity, destroying land and filling reservoirs with sediments.
This review was aimed at assessing the rate soil erosion
hazard in Errer dembel sub-basin, in Shinille zone of the Ethiopia
Somali regional state, for assessing constraints to development
in soil conservation and proposing strategies for effective
implementation of the development programme in sub-basin.
Eere-dembel Sub-Basin is located within Awash-Basin
bounded between 1029241-1227630UTM N and 686844-
985384UTM E. The Sub-Basin is found in Shinille Zone of the
Somali regional state covering a total area of 2,913,600ha. Based
on the land use dynamism and meet the required scale of 1:50,000
scales, Eere-dembel sub-basin is divided in to sub watersheds,
namely: Shinile-dembel, and Ayisha, sub watersheds. Shinille,
which is the administrative capital of Shinile zone, is 523km far
away from Addis Ababa towards the east direction and about
167km far away from Jigjiga which the capital is cite of Ethiopia
Soil erosion hazard and land degradation were comprised
from field measurements, survey and investigation of the area,
interviewing key informants and households, group discussions
with the communities in the basin, collection of relevant secondary
data, review of literature and documents and analysis of data.
The data and information’s collected in the field was compiled
and analyzed. Moreover, climate, soil, land use/land cover, slope,
farming practice and other factors were analyzed and used in soil
erosion and land degradation assessment. The Universal Soil Loss
Equation (USLE) is applied for estimating soil loss. Soil formation
rates and soil loss tolerance estimates are made using the methods
suggested by Hurni  (Figure 1).
Water erosion: Sheet erosion is the dominant form of erosion
occurring in all of the area in Errer-dembel sub-basin. However, it
is more serious where the vegetation cover is severely depleted
especially in overgrazed land. Erosion pedestals with grasses and
leaves perched on them, deposition in relatively lower points
within and at the boundaries of fields, and exposed roots of trees
in all areas of the Errer-dembel sub-basin indicate the severity
of the sheet erosion problem. Many areas experiencing heavy
livestock grazing pressure turn into bare lands before the rainy
seasons. Moreover, trampling by livestock throughout the dry
season further pulverizes the soil and both exposes the land in to
sheet wash during the rainy season.
Rill erosion: occurs in all areas of the Errer-dembel sub-basin
and, is also the important form of erosion. In fact, in almost all of the
areas where sheet erosion is occurring one would find symptoms
of rill formation. However, rill erosion is more conspicuous where
the slope is relatively higher and the run-off faster. These often
are areas where concentrated flow enters a natural water course
such as a depression along the slope, a previously formed gully, or
un-stabilized road side drain.
Gully erosion: Gully erosion is a serious problem in the Errerdembel
sub-basin. Gullies are created mainly as a result of either
of the following:
a) The concentration of the run-off in natural drainage
lines from the surrounding slopping lands which have very
little or no vegetation cover and that cuts into the sub soil;
b) Along footpaths, tracks, cattle roots that are regularly
used by men, motor and livestock movement particularly to
and from watering points, settlement areas and frequently
used grazing fields;
c) Inappropriate and un-stabilized road embankments and
road side drains and culverts disposing concentrated run-off onto unprotected and/or erosion prone un-stabilized sloping
areas were also observed in some parts of the Errer-dembel
Gully formations have devastated most areas in the Errerdembel
sub-basin. There is very severe gully cutting in Dembel
and Awbere Woredas’. Most of the gullies are already developed to
irreversible stages with over 10meter depth and about 20meter
width. In some areas several chains of gully cuttings have formed
gorges and hummocks.
Stream bank erosion: The stream bank erosion in many
parts of the Errer-dembel sub-basin is causing tremendous
damage to graze and croplands lying adjacent to rivers. There is
no economically feasible method for controlling bank erosion, and
occurrence of ox-bows and meanders in many valley floors, shows
that this process is very active. The most casual observation of the
braided channels in the valley floors is clear indication that these
mass movement processes are the source of the huge amounts of
coarse bed load material.
Stream bank erosion is not only disastrous in terms of
losing eroded land but also damage of downstream plain land
due to sedimentation coarse materials. Stream bank erosion is
severe throughout the Errer-dembel sub-basin. However, the
wadies and river courses and its tributaries in Shinile Woreda’s,
Derwenageand-Arabi are extremely eroding adjacent lands.
Some of the streams are cutting the banks up to 50meter width
and about 10meter depth and still expanding, thus have already
reached irreversible stage.
Since Errer-dembel sub-basin areas, is arid and semi-arid
land with scarce vegetation cover windy events prevailing in most
parts of the year with high speed. In spite, of the shortage of data
on wind speed for the other areas in the Errer-dembel sub-basin,
wind erosion is considered to be a problem in Aydora, Meto and
Hori Pas; and parts of Asbuli, Adigala and Aysha woreda’s of the
Sub-basin. However, it is more serious in areas of low vegetation
cover. According to local informants, the wind in these areas is not
only very erosive but also highly damaging to properties. They
said, the winds are so strong that sometimes trees are uprooted,
and roofs are blown. Moreover, when windy events occur in these
areas, it was indicated that the atmosphere suddenly fills with
dust, turns to dusty cloudy and even makes movement difficult.
Sometimes the cloudy situation is said to last for half an hour and
covers very large areas.
The interaction of several factors accelerates the process of soil
erosion, which in turn accelerates the process of land degradation.
The following are among the major causes for accelerated soil
erosion in Errer-dembel sub-basin.
Rainfall: in Errer-dembel sub-basin heavy storm characteristic
of short period rainfall is a major cause of erosion. Average and
maximum daily rainfall is 56.3mm and 115mm respectively, while
minimum daily rainfall is 23mm.
Overgrazing; as the grazing animal’s moves around the field,
it tends to exert a considerable pressure on the surface, which
affect both the soil and grass/vegetation. In Errer-dembel subbasin
the average weight of a mature sheep is about 60-80kg that
of a mature cow is about 500kg. Given the total hoof areas of 80-
100cm2 and 250-350cm2, respectively, indicates that the animals
may exert a force of up to 1600gcm2, In the case of the younger
animals, the body weight is low but because the ratio between
weight and hoof area remains fairly constant, the pressure
exerted on the soil is much the same. The effect of compaction and
deformation by trampling are wide ranging. Closure of the pore
spaces inhibits root development and reduces plant growth. It
increases moisture retention and causes a reduction in infiltration
capacity. It destroys soil organism and thereby interferes with
organic matter decomposition and nutrient cycling. Ultimately
it may lead to changes in grass composition and soil erosion.
There is about 50% loss of dry matter from trampling and soil
detachment. Overgrazing has resulted in soil erosion and land
degradation in all areas of the Errer-dembel sub-basin with more
pronounced effects in Shinile, Aysha Dewele.
Erodible nature of the soil; Erodible nature of the soil: the major soils in the Errerdembel
sub-basin area (Table 1) Leptosols, Fluvisols, Cambisol,
Regosols, Luvisol Solonchack sand few Andosol. The natural
characteristic; friable and loose/massive structure, low shear
strength, poor water holding capacity coupled with shallowness
and coarse texture (mainly sandy loam) makes the soils of the
area to be susceptible to erosion and land degradation.
Deforestation: the emergence of small towns mainly along
the roads has increased the demand for forest products for
purposes such as construction and fuel wood. Settlers sometimes
set fire to the forest surrounding their settlement areas in fear
of wild animals. In Errer-dembel sub-basin Charcoal making
has significantly contributed to deforestation. All pastoralist
interviewed were reported that they do not have alternative
income than charcoal making for their daily expenses.
Steep slopes, undulating and rugged terrain: some areas in
the Errer-dembel sub-basin have undulating and rugged terrain
with hills, hummocks, escarpments, mountains with steep slopes.
All these favors erosion and land degradation by different agents
including human interference.
Flooding and sedimentation: Flooding in the sub-basin
is a problem mainly along the banks of the wadies river and its
tributaries. These floods originate from areas within the Region
as well as from areas outside the sub-basin. Flooding and
sedimentation have damaged the good land by covering/with
water (water logging), buring the good and palatable vegetation,
encouraging growth of undesirable species, blocking accesses and
trekking routes and force rerouting. Land deterioration in Shinile,
Hari area due to overflow and sediments deposition which
transported from the upland’s lands where stones, boulders, silts,
sand etc. buried, the once productive grazing land.
Drought: during periods of drought, livestock have often
concentrated in areas where water is available, such as the plains
along the sides of perennial vegetation and perennial rivers and
streams. These areas are subjected to heavy grazing pressures
resulting in denudation, which in turn makes them vulnerable to
sever wind and water erosion.
Another effect of drought is its direct influence on vegetation.
In the absence of moisture most vegetation disappears on its own.
The fact that the livestock are desperately feeding on the remnant
vegetation (even poorly palatable species) during these times of
rainfall shortage forces most plant species to disappear quickly
and the land has no protection from the eroding forces of both
water and wind.
Change in climate: major climatic elements such as rainfall
(daily, mean monthly and mean annual rainfall), temperature
(min. max. and average), humidity, wind speed, sunshine hours,
of Hurso and Errer station and other nearby stations such as Dire
Dawa and Jigjiga have been reviewed. Review of the last 20years
rainfall data reveals that, the intensity of rainfall is decreasing from
time to time, whereas evapotranspiration is drastically increasing
as a result of increase in temperature and relative humidity. Water
balance calculation has shown moisture deficit throughout the
year thus aggravate the problem of erosion by decreasing surface
cover that obstruct erosion.
Poverty: of all poverty is the fundamental cause of natural
resource degradation. Poverty forces the semi nomadic people in
the sub-basin to extract as much as they can from the land and
it’s by products for short term survival without any choice against
long term interests.
Generally, in Ethiopia, as in many developing countries, most
of the data on the extent of soil degradation is based more on
gross estimates than on agro-climatic-ecology based site-specific
studies. According to one of these estimates, the erosion hazard
assessment of FAO/UNDIP , different types of degradation
significantly affect some 60% of the land surface of the country.
Of this, about 94% is due to soil erosion. Considering the relative
contribution of water and wind erosions, about 84% of the erosion
induced soil degradation is due to water erosion and about 16%
is a result of wind erosion. Errer-dembel sub-basin is not an
exception to this phenomenon, though the relative contribution
of water erosion to degradation is expected to be higher due its
Estimates by Hurni  show that, on grasslands, soil loss
rates exceed soil formation rates by a factor of 0.8 to 2.3. These
estimates also show that grass lands with altitudes between 500
and 1,000m.a.s.l and rainfall between 200 and 900mm loose
up to 0.7mm depth of soil each year, while the corresponding
soil formation rates are estimated to be 0.3mmyear-1. This
corresponds to soil formation and loss rates of 3 and 7tonsha-
Moreover, it should be noted that the consequence, particularly
on soil water holding capacity of an annual net decrease of 0.4mm
of soil depth, in arid and semi-arid environments such as Errerdembel
sub-basin, where rainfall is scanty and of very short
duration, and evaporation extremely high, is very detrimental.
If you leave the scientific aspects and continue discussing the
current erosion practice, most of the areas in the sub-basin are
suffered from 20-30cm depth of soil erosion per annum. This will
practically result in bare land.
The original soil erosion model called Universal Soil Loss
Equation (USLE) was empirically derived from more than 10,000
plots and years of runoff and soil loss data contributed from 49
locations in the United States . It is the most widely used erosion
model to predict soil loss . USLE was designed to provide a convenient tool for soil conservationists and can be used to any
geographic region with its modified factors. It has been used in
developing conservation plan and land use decisions.
Some recent researches led to a revision of USLE that provides
more accurate estimation of soil loss i.e. the Revised Universal Soil
Loss Equation (RUSLE). RUSLE has the same formula with USLE
but has several improvements in determining factors. Hurni 
has modified the USLE to fit the Ethiopian conditions. He has
modified some of the input factors used by the USLE model like,
rainfall erosivity (R), land cover (C) and management (P) factors
for the Ethiopian condition.
Therefore, in this review, to assess soil erosion the revised
universal soil loss model, which was developed by Wischmeier &
Smith  and modified by Hurni , to the Ethiopian conditions,
was used because of its less input requirement, computational
simplicity and wide applicability.
Since the aim of the erosion hazard assessment was to
assess relative erosion hazard for use in catchment selection for
watershed management interventions, the actual rate of soil loss
is not considered as important as indicating relative rates of soil
loss. It considered that the Revised Universal Soil Loss Equation
 would be suitable and that the inclusion of parameters
specifically identified for the Ethiopian situation would give as
good an assessment of the actual erosion rates as possible. The
RUSLE methodology is based on the Universal Soil Loss Equation
In the USLE, the ‘K’ factor is determined from a monograph
using values of: percent silt and very fine sand, percent sand,
organic matter content, topsoil structure grade; and Permeability
grade. In reality, especially at reconnaissance level, these data are
often difficult to obtain and may not be suitable for extrapolation
from one area to another. For this reason, Hurni  derived K
values based on easily observable soil colour.
The Abbay Master Plan  though calculated K values for
the soil units occurring within the basin and found that whilst
some correlated relatively well with SCRP values [11,12] red soils’ calculated K values were consistently lower that the SCRP K value
(0.07-0.1vs 0.25). With other SCRP research data indicating that
the USLE monograph methodology consistently underestimated
the soil erodibility K factor, that Luvisols do not correlate well
and that Leptosols, Cambisols, Regosols and Fluvisols also have
widely variable values, illustrating the inherent uncertainties
in calculating K values, it was decided to follow the most recent
erosion hazard assessment undertaken in the country and use the
values from the Woody Biomass Project . Using the K-values the
erodibility map of the sub basin is shown in Figure 3 & Table 2.
By using a DEM with 30m resolution it was possible to calculate
both slope length ‘L’ and gradient ‘S’ rather than having to use a
combined factor as has been the case in the past, resulting in far
greater accuracy than in previous assessments. Both Reynard et
al.  & Nyssen et al.  have indicated that above 22% slope the
curve of soil loss versus slope flattens therefore the parameters
used have taken this into account and have re-assigned the Hurni
slope parameters (Figure 4).
As Nyssen et al.  commented, the land cover factor ‘C’, is
of paramount importance in the determination of erosion hazard
assessment because of the large difference between its minimum
and maximum values and therefore slight mistakes in land cover
mapping can easily result in large over or under estimations of
soil loss. For this reason, it was imperative that as accurate and
up-to-date land cover data as possible is used and therefore
Woody Biomass Project TECSULT  data was used.
This data was compiled from satellite imagery mapping with a
great deal of field ground-trothing and can therefore be considered
to be appreciably more accurate than older data or more
contemporary global data. Land Cover parameters were obtained
from Halcrow  FAO/LUPRD , SCRP  and Hurni and the
WBISPP parameters were also derived using these historical data.
The land cover map was shown in Figure 5A. Except in some areas
of Lefeissa and Errer there is no land management practice (P) in
general. The land management map was shown in Figure 5B.
Soil loss tolerance has to be assessed with the soil formation
rates  under research plots. Soil loss tolerance may always
be as high as the accumulations on the soil surface and the
decomposition rate of organic materials. However, if intensive
weathering occurs within the soil profile, Soil loss tolerance may
even be higher.
The maximum soil loss tolerance for tropical regions is
25tonsha-1yr-1 . A commonly used soil loss tolerance rate
is 5-12tonsha-1yr-1 for shallow to deep soils . The tolerance
value for tropical soils has not yet been formulated at international
level, but Hurni  & Hudson  established annual soil loss
tolerance limits that vary between 0.2 and 11tonsha-1yr-1 .
The soil loss tolerance values are used for the determination of
critical area of soil erosion in the watershed by comparing soil loss
rates with soil loss tolerance. If the soil loss is less than or equal to
the soil loss tolerance, the soil loss is still accepted. However, if the
soil loss is more than soil loss tolerance, conservation measures
to reduce soil erosion should be taken into account until a level of
equal or less than the soil loss tolerance has been reached (Figure
An empirical model for the calculation of soil formation
rates has been developed for use in the Ethiopian Highlands
Reclamation Study in 1983 . The model developed is:
Where; F=mean annual soil formation rate, fm=maximum
soil formation rate, t=f(Ta), T=mean annual temperature, r=f(Ta),
Ta=mean annual Rainfall, l=f(l), L=Length of growing period,
u=f(U), U=soil unit, d=f(D), D=soil depth, s=f(S), S=slope gradient
and c=f(C), C=land cover and use
Mean annual soil formation rate (fm) has the value 24tonsha-
1yr-1 for tropical zone and all other factors receive values between
1 and 1.1 for optimum conditions and decreasing values for less
favorable conditions. Thus, optimum temperature of 17.5-22.5oC
obtain the factor 1, optimum rainfall over 1950mm per year has
a factor 1. Optimum length of growing period over 270days has 1.
Optimum soil units such as Fluvisols, Nitosols, vertisols, cambisols
for best sub units and phases have the value of 1.05. Optimum soil
depths more than 100cm has 1. Optimum slope gradient less than
10% is 1 and land cover, intensive cultivation has 1.1. The mean
annual soil formation rate as shown in the above formula is the
product of all 8 factors [20,21].
Using the model, soil formation rate of Errer-dembel subbasin
has been calculated having the value t=0.95, r=0.5, l=0.6,
u=0.5, d=0.4, s=0.5 and c=0.6. The soil formation rate is in the
range of 0.5-4tonsha-1yr-1 and, is far less than soil erosion rate
of 5-41tonsha-1yr-1. Thus, the soils of Errer-dembel sub-basin
are found to be highly susceptible to soil erosion under any
circumstances (Figure 7).
The authors acknowledge the Oromia water works design and
supervision enterprise/OWWDSE and Deridawa council for their
technical material support and encouragement in carrying out
his review work; thanks, are also to Jigjig University and Adigrat