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 (VL 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 simpleVL 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 VL relation and field measurements is feasible to assess the interactions between environmental changes and gully erosion/gully head positions.
MATERIALS AND METHODS