Abstract
To analyse the driving forces of gully erosion using a present dataset
of geomorphic parameters and land use/cover involves limitations because
past datasets at the time of gully incision may best explain the gully
formation and evolution at that time. The recent development of
photogrammetric techniques enabled to estimate temporal gully volume
changes. This study conducted in semi-arid Ethiopian Rift Valley used
field measurements and gully volume–length relation to analyse spatial
and temporal dynamics of catchment geomorphology and topographical
threshold of gully heads to explain the difference in the gully volumes
and area-specific gully volumes between two study sub-areas. The
topographic thresholds of the gully heads, expressed by the slope (=s ) and drainage area (= a ), (i) formed in each catchment
and (ii) that had the same land use/cover items (forest, grassland, and
farmland) in all the catchments of each sub-area were approximated by
power functions (s = ka-b ). Analysis of
covariance found that these threshold lines had clear spatial and
temporal patterns: the threshold lines maintained almost the same
exponent b specific to each sub-area while the threshold
coefficient k significantly decreased in the order of forest,
grassland, and farmland. The spatial variability and its temporal
changes in relief aspect of the catchment morphology can responsible for
the difference in the area-specific volumes of gullies between the
sub-areas, while the continuous reduction in vegetation cover over time
can be the main driving force of the similar scale and changing patterns
of the gully volumes between the sub-areas.
KEYWORDS: gully evolution, area-specific gully volume, gully
volume–length relation, catchment geomorphology, topographic threshold
INTRODUCTION
In East and South African countries, large-scale gullies can be seen
almost everywhere (Katsurada et al., 2007; Ndomba, Mtalo, &
Killingtveit, 2009; Boardman, 2014). In semi-arid Ethiopian highlands
(Tigray), the area-specific gully erosion rates (gully erosion rate per
unit area) since gully incision to 2001 were 6.2–17.6 Mg
ha-1 y-1 (Nyssen et al., 2006;
Frankl et al., 2013a). In sub-humid Ethiopian highlands (Amhara), the
area-specific gully erosion rates were 8.7–155 Mg
ha-1 y-1 (Tebebu et al., 2010;
Zegeye et al., 2016; Yibeltal et al., 2019). In semi-arid Ethiopian Rift
Valley, a lowland part of Ethiopia, the area-specific gully erosion rate
was 16.2 Mg ha-1 y-1 (Mukai, 2017).
Most gully volumes showed an exponential increase since gully incision
in Ethiopia except the ones in the areas where gully rehabilitation or
soil and water conservation programmes at watershed scale were
implemented (Nyssen et al., 2006; Frankl et al., 2013a). The
contribution of gullying to total soil loss from the area ranges from
28% in semi-arid highlands (Nyssen et al., 2008) to 64 to more than
90% in sub-humid highlands (Tebebu et al., 2010; Zegeye et al., 2016)
of Ethiopia.
Gully formation and its evolution are regulated by various factors, such
as several geomorphic properties of catchments, slope gradient, land
use, vegetation, and rainfall characteristics (Poesen et al., 2003;
Valentin, Poesen, & Li, 2005). Land use and vegetation cover are a
major controlling factor of gully initiation (Parkner et al., 2006;
Gómez Gutiérrez, Schnabel, & Lavado, 2009) and its evolution
(Martinez-Casasnovas, Ramos, & Garcia-Hernandez, 2009). Many studies
that used a statistical approach concluded that the present land
use/cover was not a decisive factor of gully erosion (Muňoz-Robles et
al., 2010; Kompani-Zare et al., 2011; Frankl et al., 2013b; Mukai,
2017). Using sequential historical aerial photographs and
photogrammetric technique, Gómez Gutiérrez, Schnabel, & Lavado (2009)
determined temporal changes in gullied areas over ~60
years in southwest Spain, which were explained by temporal changes in
land use and vegetation cover in respective years determined by the
combination of aerophoto-interpretation and field observation.
Martinez-Casasnovas, Ramos, & Garcia-Hernandez (2009) used similar
techniques to explain the impact of land use/cover changes on gully
sidewall erosion in northeast Spain. Mukai (2017) found that the gully
erosion rates of the study gully networks during the respective study
periods in the present and the past had significantly strong
correlations with the rates of area changes in land use/cover items
during the respective study periods. These pieces of evidence show the
importance of an analysis that analysed the relationship between gully
erosion and past land use/cover when gullies were initiated.
It is well known that gully initiation and gully head positions are
related to some critical conditions, e.g., the topographic threshold, a
combination of the slope at the gully head (s ) and upslope
drainage area (a ), which is expressed in equation (1):
s = ka-b (1),
where the threshold coefficient k , is a constant that varies with
local climate, soils, and vegetation covers, and the exponent b ,
is a constant which is related with the dominant process of the flow
conditions in a gully channel, e.g., surface or sub-surface flows (Torri
& Poesen, 2014). Land use that reduces vegetation cover, such as an
increase in cultivated area and transformation of forest to grassland,
tends to reduce the topographic threshold levels and increase the risk
of gully erosion on sites (Parkner et al., 2006; Gómez Gutiérrez,
Schnabel, & Lavado, 2009). Using sequential historical aerial
photographs and digital elevation models (DEM), Parkner et al. (2006)
determined the s -a relationships (topographic threshold)
of the gully heads formed in the 24 active gullies and gully complexes
in New Zealand back to the time when gully heads were incised. Land use
and vegetation cover in individual years was determined by mainly
aerophoto-interpretation. They found the s -a relationships
were related to land use/cover at the time of gully incision. When the
land use of a catchment was an indigenous forest, the topographical
threshold value was very high. As the land use changed to pasture, to
invaded scrub, and to reforestation, the value decreased rapidly, then
slowly increased, and finally returned to a similar level to that under
indigenous forest. Gómez Gutiérrez, Schnabel, & Lavadzo (2009) took a
similar approach and proposed a topographical threshold evolution model
based on land-use changes in respective study years in southwest Spain.
Thus, some studies have indicated that land use and vegetation cover in
the catchments at the time of gully incision explained the spatial
variability in gully topographical threshold values well. However, the
studies that quantitatively assessed the relationships between land
use/cover and a difference in topographical threshold values were rare.
Some gully morphological characteristics have recently been used to
determine temporal gully volume changes. Several studies have explored
the relationship between the gully volume (V ) and length
(L ) using a power equation V =
aL b (V –L relation; e.g., Frankl
et al., 2013b). Li et al. (2017) proposed a relation between the gully
volume (V ) and gully area (Ag ) using a power equationV = aAg b. These models have advantages
that the length and area of a gully can be easily determined from aerial
photographs and high-resolution satellite images. These photogrammetric
techniques were utilised to assess long-term changes in gully volumes
(Frankl et al., 2013a; Mukai, 2017).
Concerning other controlling factors of gully erosion that show temporal
variation, such as catchment geomorphic characteristics, many studies
analysed the present parameters and indices (Singh, Sarangi, & Sharma,
2008; Chandrashekar et al., 2015). Using a statistical approach, Tamene
et al. (2006) and Haregeweyn et al. (2008) quantitatively explained the
spatial variability of sediment yield and area-specific sediment yield
by various controlling factors that included the present catchment
geomorphic parameters in semi-arid Ethiopian highlands. However, the
relationships between the temporal changes in the catchment geomorphic
parameters and gully evolution (e.g., gully volume changes) have rarely
been quantitatively analysed except the V –L relation.
Mukai (2017) quantitatively analysed the relationship between the
temporal changes in the catchment geomorphic parameters and gully
erosion; however, how that relationship had implications to the spatial
variability of gully evolution in agroecology had not been analysed.
The objectives of this study carried out in semi-arid Ethiopian Rift
Valley were three-fold. It was, first, to analyse spatial and temporal
dynamics in catchment geomorphology quantitatively and explain the
difference in gully evolution (i.e., changes in the gully volumes and
area-specific gully volumes) specific to each of the two study
sub-areas. Secondly, it is to quantitatively assess the relationships
between temporal changes in land use/cover items and topographical
threshold of gully heads in each sub-area, and to explain the difference
in gully evolution specific to the sub-area; and (iii) to confirm that
the combination of the V –L relation and field
measurements is feasible to assess the interactions between
environmental changes and gully erosion/gully head positions.
MATERIALS AND METHODS