Abstract
To analyse the driving forces of gully network expansion using a present
dataset of land use/cover involves limitations because past land
use/cover strongly regulates gully formation and evolution. The
vegetation cover in the gully catchment at the time of gully incision
may best explain the topographical threshold levels. 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 (i)
keep track of gully volume changes and (ii) analyse temporal transitions
in catchment geomorphology and topographical threshold of gully heads to
explain the difference in the gully volumes between two study sub-areas.
The topographic thresholds of the gully heads, expressed by the slope (=s ) and drainage area (= a ), formed (i) in each catchment
and (ii) in all the catchments in each sub-area during the same
individual period (before 1957, 1957–1972, and 1972–2005) were
approximated by power functions (s =ka-b ). Transitions in these threshold lines
showed clear temporal and spatial patterns: the threshold lines
maintained almost the same exponent b specific to each sub-area
while the threshold coefficient k decreased as time passed. The
expansion of the gully network induced by land use/cover changes lowered
the gully topographic threshold level in agroecology, which accelerated
further gully expansion and influenced the exponential increase in gully
volumes over time. Characteristics of temporal changes in catchment
geomorphology partly explained the difference in the area-specific gully
volumes between the sub-areas. (249 words)
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., 2019a). Most gully volumes showed
an exponential increase since gully incision except for 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). In semi-arid Ethiopian Rift Valley, the
area-specific gully erosion rate was 16.2 Mg ha-1y-1 (Mukai, 2017). The mean gully erosion rate of 1.93
Mg y-1 (1957–1972) was exponentially increased to
10.18 Mg y-1 (1972–2005). 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). 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 exponent b and threshold coefficients k , are
constants which depend on local climate, soil, and land use (Torri &
Poesen, 2014). Land use that reduces vegetation cover through increases
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; Yibeltal et al., 2019a).
An understanding of gully development from historical and current
perspectives is essential when addressing the causes and consequences of
land degradation (Frankl et al., 2013a) and to predict the future
behaviour of gullies (Li et al., 2017). Several studies recently
analysed quantitatively the factors controlling gully morphology and
gully network formation. Most of these studies used a statistical
approach, such as multivariable analysis to predict the values of
dependent variables, such as gully cross-sectional morphology (Frankl et
al., 2013b; Yibeltal et al., 2019b), gully erosion rate (Muňoz-Robles et
al., 2010) and land susceptibility index (Conoscenti et al., 2013), from
several independent variables, including catchment geomorphology, land
use/cover, rainfall, soil. These studies used present datasets to
determine the present factors of gully formation. However, these studies
involve some limitations because (i) the present land use/cover was not
a decisive factor of gully erosion (Kompani-Zare et al., 2011; Frankl et
al., 2013b; Mukai, 2017); (ii) the vegetation cover in the gully
catchments at the time of incision explained the difference in threshold
levels best (Vandekerckhove et al., 2000); or (iii) the rates of gully
erosion was strongly correlated with land use/cover when the indicators
of land use/cover were expressed in temporal variables (the rates of
area changes in land use/cover items in the catchments between two
periods; Mukai, 2017). The same goes for the topographic threshold
studies. The topographic threshold levels and conditions for gully head
development have been compared mainly between different land/use cover,
soil, land management, flow conditions in different environments (Torri
& Poesen, 2014). However, because most studies used present datasets,
temporal interaction between environmental changes and gully head
positions in the same catchments has not been assessed.
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) and to analyse the driving force of gully network
expansion back to when gullies were initiated (Mukai, 2017). Thus, the
combination of photogrammetric techniques and a simpleV –L relation may enable assessment of temporal
interactions between environmental changes and gully erosion/gully head
positions.
The objectives of this study carried out in semi-arid Ethiopian Rift
Valley were, (i) to keep track of gully volumes and area-specific gully
volumes in the catchments in two sub-areas; (ii) to analyse temporal
dynamics in catchment geomorphology and topographical threshold of gully
heads to explain the gully volumes and area-specific gully volumes
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