3.1 Results and discussion
3.1.1
Causes of gully formation and development
In the study area, construction of gravel and asphalt roads, footpaths,
concentrated surface runoff from farmlands, overgrazing and improper
design of soil and water conservation practices were found to be the
causes of gully erosion (formation and development). However, the main
asphalt and gravel roads passing through the study watershed were found
to be the major causes for the formation and development of gullies
(Figure 3), which is in line with many research findings, for instance:
Montgomery et al. (1994); Wemple et al . (1996); et
al . (1996) and Moyerson et al. (2000). In the highlands
of Ethiopia, Birhane and Mekonen (2009); Rijkee et al . (2015) and
Mekonnen et al. (2015) also reported that construction of roads
and footpaths have a major role on the formation and development of
gullies. Hence, an appropriate runoff discharge mechanism following
roadsides that means stone-pave drainage lines along the roads should be
constructed to arrest the problem.
Concentrated surface runoff from farmlands and inappropriately designed
soil conservation practices (SWCPs), were found to be the second
important causes for gully formation and development, which agreed well
with different research results conducted in the northwest highlands of
Ethiopia, for example; at Debre Mewi watershed (Tibebu et al.,2010; Mekonnen and Melesse, 2011; Zegeye et al. 2014) and Minizr
catchment (Mekonnen et al., 2017). Concentrated surface runoff
from farmlands was also an important cause of gully formation and
development in the southern part of Ethiopia (Gebreslassie et
al., 2014). After their formation gullies serve as soil/sediment
transfer pathways and increase runoff connectivity between upstream and
downstream parts of the watershed (Mekonnen et al., 2017).
Therefore, implementing properly designed SWCPs within fields that can
reduce the energy of concentrated surface runoff and can increase water
infiltration in the run-on area of gully head watershed is highly
important. Constructing check dams that might reduce runoff
concentration and disconnect the runoff transfer role of gullies will
also be another important solution as recommended by Mekonnen et
al (2017) in the northwest highlands of Ethiopia. Moreover,
institutional integration is vital that means the road construction
authority and ministry of agriculture have to work together.
Principally, the road construction authority should construct
stone-faced runoff discharging waterways along roads during road
construction to discharge the excess runoff collected following
roadsides, and the ministry of agriculture will construct check dams
within the developed gullies and implement SWCPs within fields, which
will enhance water infiltration and reduce surface runoff concentration.
3.1.2.
Land use and gully characteristics
Table 1 shows the gully characteristics of different land use/cover
types. From the twenty-two gullies formed within the study watershed,
three (13.6%) were formed on grazing land, fourteen (63.6%) were
formed on cultivated lands and five (22.7%) were formed on both
cultivated and grazing lands. Longer and deeper gullies were found on
the cultivated lands. The most probable reason will be the nature of the
soil that soils in farmlands were deep and can be easily eroded with
runoff water at greater depth. From the total length of the investigated
gullies, 70,523.89 m (74%) was found on the cultivated lands while
30,224.6 m (26%) was on the grazing lands. The average width of gullies
in cultivated and grazing lands was 7.4 m and 9.4 m, respectively. Gully
density was found to be 45.3 m ha-1 on grazing lands
and 15.2 m ha-1 on cultivated lands. Higher gully
density was found on grazing lands than on cultivated land, which was
because of the lower area coverage of grazing lands than cultivated
lands. Similarly, Belay & Bewket (2013) reported that a large
percentage of gullies were located on cultivated and grazing lands.
Almost all of the gullies in grazing land were discontinuous (not
actively expanding) because of treatments like area closure and grass
plantation, whereas ~70% of the gullies found in
cultivated lands were continuous which means gully depth, width and
length were increasing in every direction. One important cause for the
continuous development of gullies in cultivated lands was that gullies
were serving as runoff discharging drainage channels or runoff transfer
pathways coming from cultivated land. Farmers’ are leading the runoff
from their cultivated land into gullies considering gullies as
waterways. Therefore, properly designed waterways within the cultivated
lands are essential that will carry and transfer the runoff from
cultivated lands to appropriate drainage channels like permanent rivers.
3.1.3. Gully surface area and
gully to area ratio
The total length of the 22 gullies was 9,093 m. The longest gully length
was 1,299 m and the shortest was 76 m with the mean length of 413 m,
which was very long compared with previous findings in Ethiopia. For
example, the longest and shortest gullies reported by Osore & Moges
(2014) in Alalicha watershed, southern Ethiopia was 427.4 m and 108 m,
respectively.
The total surface area occupied by gullies was 100,748.5
m2 (~ 10 ha). This means gully erosion
in the area is competing for productive agricultural lands by reducing
the farmers’ land size quantitatively. Different studies found that
gully erosion is reducing the size of agricultural lands significantly
in different parts of Ethiopia. For example, in the northwest highlands
of Ethiopia Tibebu et al ( 2010) found ~17.4 ha
land loss at Debre Mewi watershed. In the southern part of Ethiopia,
Belay and Bewket (2012) reported ~4.7 ha land loss at
Bora watershed, Osore & Moges (2014) reported ~2.6 ha
land loss in Alalicha watershed. Gully to watershed area ratio was found
to be 0.02, which means for every 1000 units of land ~20
units of land was damaged by gully erosion and becoming out of
production in the watershed. In the Kilie catchment central highlands of
Ethiopia, Woldegiorgis et al. (2007) found 0.136.
3.1.4. Gully density and category
Gully density, total gully length divided by watershed area, was found
to be 17.2 m ha-1, which implies that the watershed
was severely degraded because gully density between 10 and 25 m
ha-1 was categorized as severely degraded (Valentin et
al., 2005). Gully density reported by (Gebreslassie et al., 2014) in
Huluka watershed, central rift valley of Ethiopia, was 16.1 m
ha-1, which agreed well with the finding of this
study. In the northern highlands of Ethiopia, different studies found
different results, for example; Osore & Moges (2014) found 8.9 m
ha-1 in Alalicha watershed, and Belay and Bewket
(2013) found 6.7 m ha-1 in Bonda watershed, and
Tarekegn et al. (2010) found 67 m ha-1 in Kilie
catchment, central highlands of Ethiopia.
Pathak et al. (2006) and Sargeant et al . (1984) classified
gullies as small (< 1 m), medium (1-5 m) and large
(> 5m) based on depth, and small (<5 m), medium
(5-10 m) and large (>10 m) based on length. In this study,
in terms of depth 91% of the investigated gullies were medium and 9%
were large, and in terms of length, all gullies were under the large
category.
3.1.5.
Rate of gully development
Based on the measurement of eight gullies in the year 2017 rainy season
(June to September 2017), average gully depth was 3.26 m and 3.59 m
before and after the rainy season respectively, and gully width was
12.18 m and 12.68 m before and after the rainy season, respectively
(Table 2). Within a rainy season, gully depth increased by 0.33 m and
gully width increased by 0.5 m.
Figure 4 shows gully depth and
width before and after the rainy season and Figure 5 shows the
relationship between gully depth and width expansions. Gully depth
increment and gully width expansion showed a positive correlation using
a linear model. That means as gully depth increases, and gully width
also increases (Figure 4) with a correlation coefficient of 0.88 (Figure
5). Gully length, surface area, and volume showed, respectively, 64 m,
2727 m2 and 23,553 m3 increment in a rainy season
(Table 5).
3.1.6. Soil loss and land competition
In four decades (~40 years) the volume of soil loss from
22 gullies was 235,532 m3 or 340,956.7 t with surface
area coverage of ~100,748.5 m2 or
~10 ha. Within a rainy season from the investigated
eight gullies, ~23,553 m3 or 34,387.4
t soil was lost with an average bulk density of 1.46 g
cm-3 that means bulk density was found to be 1.34 g
cm-3 (upper), 1.46 g cm-3 (middle)
and 1.57 g cm-3 (lower) parts of the gully depth. The
result shows that ~10 ha of productive land was lost.
Loss of land and reduction of crop production were found as the major
impacts of gully erosion ( Desta et al., 2012; Yitbarek et al., 2012).
The rate of gully erosion was found to be ~62 t
ha-1 yr-1. Different studies found
different results, for example, in the semi-arid rift valley of
Ethiopia, (Mukai, 2017) found 16·2 t ha -1yr-1; in Debre Mewi watershed, northwest highlands of
Ethiopia Tibebu et al. ( 2010) found 530 t ha-1yr-1, and Nyssen et al. (2006) reported 6.2 t
ha-1 yr-1. According to Zegeye et
al. ( 2014), in Debre Mewi watershed, northwest highlands of
Ethiopia, the rate of gully erosion was 127 t ha -1yr-1. In Alalicha watershed, southern Ethiopia Osore
and Moges ( 2014) reported ~2.12 t
ha-1 yr-1. All the above study
results show that the rate of gully erosion was different, which will be
due to the difference in land use/cover, rainfall, soil type, etc.
3.1.7.
Impact of gully erosion on crop production
The main crop grown in the study watershed was the native crop Teff
(Eragrostis teff, E. abyssinica ) with a productivity of 2400 kg
ha-1. Taking into account this productivity and the
area lost (10 ha) due to gully erosion, on average
~24,000 kg of Teff grain yield was being lost annually.
The total grazing land lost was 30,225 m2 and
~14 t animals feed (grass) was lost annually with an
average animal forage productivity of 4.5 t ha-1. In
the Dangila district, northwestern highlands of Ethiopia, Belay, and
Bewket (2012) reported that 46,265 m2 lands were
damaged due to gullies, from this, 1,401 m2 was
cropland showing 502 kg of crop yield reduction annually. In Alalicha
watershed, southern Ethiopia ~25,761
m2 land was a loss due to gullies (Osore and Moges
2014). Such research results indicated that gully erosion is
significantly reducing the limited resource that is land and farmers’
income in Ethiopia.
3.1.8. Treatment practices
Gully erosion is becoming a priority agenda in Ethiopia. It is damaging
all resources available on land and hence affecting human existence.
Based on this study, the investigators recommended possible preventive
and remedial measures as follows: (i) Institutional integration-ministry
of road construction and agriculture should work together. During
constructing roads the ministry of road construction is not bothering
about soil erosion/gully erosion occurring because of road construction
and it is pushing the responsibility to the ministry of agriculture.
Hence their integration is vital; (ii) technical standards of soil and
water conservation practices (SWCP) - experts/ professionals should
properly put the design/ layout of SWCP since inappropriate designs are
leading to surface runoff concentration and hence formation and
development of gullies; (iii) immediate action – immediate actions like
removing the young gully at its rill stage through plowing is a best
remedial measure; (iv) constructing check dams - for large and deep
gullies, constructing check dams combining with vegetative practices
will help to rehabilitate the gully or at least it helps to stop its
development (Mekonnen et al. 2015).
4.1.
Conclusion
In this study, even though construction of gravel and asphalt roads,
footpaths, concentrated surface runoff, overgrazing and inappropriate
design of soil and water conservation practices were the causes of gully
formation and development, road construction (both gravel and asphalt)
and concentrated surface runoff from farmlands were found to be the main
causes. Most of the gullies located in farmlands were active (expanding
in all directions/dimensions) compared with the gullies located in
grazing lands. Gully formation and development was different for
different land use/cover type, which was serious on farmlands than
grazing lands.
Gully erosion also played an important role in soil and land losses.
About 340, 956.7 t of soil was lost and ~10 ha of land
damaged (became out of production). The annual rate of soil loss due to
gully erosion was found to be 62 t ha-1 with an
average gully density of 16.4 m ha-1. Gully erosion
also greatly reduced crop production (Teff grain yield) and livestock
forage due to land competition. Farmers are losing
~24,000 kg Teff grain yield and ~14 t
animals’ forage annually.
Gully erosion is competing for the agricultural land, reducing farmers’
income affecting crop production and animals forge in the study area.
Therefore, properly designed biological and physical soil and water
conservation practices, maintenance of roads and properly diverting
runoff generated along the roads to nearby natural waterways were found
to be possible solutions to rehabilitate the developed gullies and
protect new gully formation.
5.
References
Avni, Y. (2005). Gully incision as a key factor in desertification in an
arid environment, the Negev Highlands, Israel. Catena .
https://doi.org/10.1016/j.catena.2005.06.004
Belay, M., & Bewket, W. (2012). Assessment of gully erosion and
practices for its control in north-western highlands of Ethiopia.International Journal of Environmental Studies , 69 (5),
714–728. https://doi.org/10.1080/00207233.2012.702478
Belay, M., & Bewket, W. (2013). Farmers’ livelihood assets and adoption
of sustainable land management practices in north-western highlands of
Ethiopia. International Journal of Environmental Studies .
https://doi.org/10.1080/00207233.2013.774773
Bewket, W., & Teferi, E. (2009). Assessment of soil erosion hazard and
prioritization for treatment at the watershed level: Case study in the
Chemoga watershed, Blue Nile basin, Ethiopia. Land Degradation and
Development . https://doi.org/10.1002/ldr.944
Brhane, G., & Mekonen, K. (2009). Estimating soil loss using Universal
Soil Loss Equation (USLE) for soil conservation planning at Medego
watershed, Northern Ethiopia. Journal of American
Science , 5 (1), 58-69.
Daba, S., Rieger, W., & Strauss, P. (2003). Assessment of gully erosion
in eastern Ethiopia using photogrammetric techniques. Catena .
https://doi.org/10.1016/S0341-8162(02)00135-2
Desta, L., & Adugna, B. (2012). Hamid Reza Pourghasemi Mauro
Rossi .
EMA Ethiopian Mapping Agency (1987). Field survey by Ethiopian Mapping
Agency. Air photography 1:50 000 scales by SWEDSURVEY. Addis Ababa,
Ethiopia.
Frankl, A., Poesen, J., Deckers, J., Haile, M., & Nyssen, J. (2012).
Gully head retreat rates in the semi-arid highlands of Northern
Ethiopia. Geomorphology .
https://doi.org/10.1016/j.geomorph.2012.06.011
Gebrselassie, H., Dessie, G., & Moges, A. (2014). Rate of Gully
Expansion on Major Land Uses, the Case of Huluka Watershed, Central Rift
Valley, Ethiopia. Journal of Biology .
Hurni, H., Abate, S., Bantider, A., Debele, B., Ludi, E., Portner, B.,
… Zeleke, G. (2010). Land Degradation and Sustainable Land
Management in the Highlands of Ethiopia. In Global Change and
Sustainable Development : A Synthesis of Regional Experiences from
Research. Perspectives of the Swiss National Centre of Competence in
Research (NCCR) North-South .
Mekonnen, M., Keesstra, S. D., Baartman, J. E., Ritsema, C. J., &
Melesse, A. M. (2015). Evaluating sediment storage dams: structural
off-site sediment trapping measures in northwest Ethiopia.Cuadernos de Investigación Geográfica .
https://doi.org/10.18172/cig.2643
Mekonnen, Mulatie, Keesstra, S. D., Baartman, J. E. M., Stroosnijder,
L., & Maroulis, J. (2017). Reducing Sediment Connectivity Through
man-Made and Natural Sediment Sinks in the Minizr Catchment, Northwest
Ethiopia. Land Degradation and Development .
https://doi.org/10.1002/ldr.2629
Mekonnen, Mulatie, & Melesse, A. M. (2011). Soil Erosion Mapping and
Hotspot Area Identification Using GIS and Remote Sensing in Northwest
Ethiopian Highlands, Near Lake Tana. In Nile River Basin .
https://doi.org/10.1007/978-94-007-0689-7_10
MNREP. (1995). Ministry of Natural Resource and Environmental
protection, water Resource Development Authority, Addis Ababa, Ethiopia
Moeyersons, J. (2000). Desertification and man in Africa. Bulletin
des Séances, Académie Royale des Sciences d’Outre-Mer , 46 (2),
151-170.
Moges, Awde, & Holden, N. M. (2008). Estimating the rate and
consequences of gully development, a case study of umbulo catchment in
Southern Ethiopia. Land Degradation and Development .
https://doi.org/10.1002/ldr.871
Moges, Awdenegest, & Holden, N. M. (2007). Farmers’ perceptions of soil
erosion and soil fertility loss in southern Ethiopia. Land
Degradation and Development . https://doi.org/10.1002/ldr.795
Montgomery, D. R. (1994). Road surface drainage, channel initiation, and
slope instability. Water Resources Research .
https://doi.org/10.1029/94WR00538
Mukai, S. (2017). Gully Erosion Rates and Analysis of Determining
Factors: A Case Study from the Semi-arid Main Ethiopian Rift Valley.Land Degradation and Development .
https://doi.org/10.1002/ldr.2532
Nyssen, J., Veyret-Picot, M., Poesen, J., Moeyersons, J., Haile, M.,
Deckers, J., & Govers, G. (2006). The effectiveness of loose rock check
dams for gully control in Tigray, northern Ethiopia. Soil Use and
Management . https://doi.org/10.1111/j.1475-2743.2004.tb00337.x
Osore, A., Moges, A., & Engineering, U. (2014). Extent of Gully
Erosion and Farmer ’ s Perception of Soil Erosion in Alalicha Watershed
, Southern Ethiopia . 4 (15), 74–82.
Pathak, P., Wani, S. P., & Sudi, R. (2006). Gully control in SAT
watersheds. Journal of SAT Agricultural Research .
Poesen, J., Nachtergaele, J., Verstraeten, G., & Valentin, C. (2003).
Gully erosion and environmental change: Importance and research needs.Catena . https://doi.org/10.1016/S0341-8162(02)00143-1
Rijkee, P., Keesstra, S., & Gethahun, M. M. (2015). Low-land
Gully Formation in the Amhara Region , Ethiopia . 17 (2011),
15827.
Sargeant, I. J. (1984, May). Gully erosion in Vectoria.
In Proceeding of the Natural Soils Conference, May (pp. 13-18).
Tarekegn, T. H., Haile, A. T., Rientjes, T., Reggiani, P., & Alkema, D.
(2010). Assessment of an ASTER-generated DEM for 2D hydrodynamic flood
modeling. International Journal of Applied Earth Observation and
Geoinformation . https://doi.org/10.1016/j.jag.2010.05.007
Tebebu, T. Y., Abiy, A. Z., Zegeye, A. D., Dahlke, H. E., Easton, Z. M.,
Tilahun, S. A., … Steenhuis, T. S. (2010). Surface and subsurface
flow effect on permanent gully formation and upland erosion near Lake
Tana in the northern highlands of Ethiopia. Hydrology and Earth
System Sciences . https://doi.org/10.5194/hess-14-2207-2010
Valentin, C., Poesen, J., & Li, Y. (2005). Gully erosion: Impacts,
factors and control. Catena .
https://doi.org/10.1016/j.catena.2005.06.001
Wemple, B. C., Jones, J. A., & Grant, G. E. (1996). Channel network
extension by logging roads in two basins, western cascades, oregon.Journal of the American Water Resources Association .
https://doi.org/10.1111/j.1752-1688.1996.tb03490.x
Woldegiorgis, G. (2007). Assessment of Soil Quality and Erosion
Rate in Kilie Catchment, Lume Woreda, East Shoa, Ethiopia (Doctoral
dissertation, Addis Ababa University).
YDARDO. (2015). Yilmana Densa District Agriculture and Rural Development
Office field survey, Adet, Ethiopia
Yitbarek, T. W., Belliethathan, S., & Stringer, L. C. (2012). The
onsite cost of gully erosion and cost-benefit of gully rehabilitation: A
case study in Ethiopia. Land Degradation and Development .
https://doi.org/10.1002/ldr.1065
Zegeye, A. D., Damtew, S., Tilahun, A. S., Langendoen, E., Dagnew, D.,
Guzman, C., … & Steenhuis, T. S. (2014, May). Gully development
processes in the Ethiopian Highlands. In Proceedings of the second
International Conference on the Advancements in Science and Technology
(ICAST), Bahir Dar University (pp. 220-229).
Table1:
Shows the characteristics of the investigated 22 gullies on different
land use/cover types at Genbo Wonz watershed, north-west highlands of
Ethiopia from 1977 to 2017