Human movement and gully erosion: Investigating feedback mechanisms using Frequency Ratio and Least Cost Path analysis in Tigray, Ethiopia

Abstract

The cost of human movement, whether expressed in time, effort, or distance, is a function of natural and human related variables. At the same time, human movement itself, whether on land, air or sea, causes environmental cost. We are looking into the long-term environmental relationship of this interplay. Gullies—linear landforms, which dissect the landscape—are considered to be a cost for human movement, as they can form unpassable barriers destroying present path networks. On the other hand, human movement creates pathways, which flatten the surface and decrease the water permeability potential. This process results in runoff generation and possibly gully erosion. Accordingly, the spatial relationship between pathways and gullies is investigated. In the Tigray region of the Northern Ethiopian Highlands, gullies and pathways were mapped using remote sensing data. Frequency Ratio was used for assessing pathways as a variable affecting the location of gullies while Least Cost Paths were tested to evaluate the possible constraining impact gullies have on mobility. Based on these results, it is concluded that a positive feedback exists between the cost of human movement and gully erosion. We further discuss possible effects gullies may have had on trade, territory, and political affairs in Tigray. Consequently, we suggest that movement cost and gullying may not only hold strictly environmental or movement-related implications, but also socio-cultural ones.

Modeling Gully Erosion Susceptibility to Evaluate Human Impact on a Local Landscape System in Tigray, Ethiopia

“…In order to reproduce the terrain-forming process of gullying, various models aim to predict the influences on the erodibility threshold. Conditioning influences are approached mainly by topographic, climatic, pedogenic, and geological factors [21,[31][32][33][34]. Additionally, Valentin et al [5] emphasize the importance of land use change on the formation of gully erosion.…”

“…Following similar principles, Schütt et al [14] observe that a vast number of gullies formed along paths and cattle trails as well as in the vicinity of settlements. The compaction of sediment underneath paths and residential areas results in a reduction of infiltration capacity, in turn generating surface runoff [14,16,21]. Given these observations, it is evident that there is need for a model describing the spatial relation between gully erosion, pathways, and settlements.…”

“…Machine learning algorithms such as Support Vector Machines (SVM), AdaBoost, Artificial Neuronal Networks (ANN), Boosted Regression Tree (BRT), or Random Forest (RF) are frequently used for gully erosion modeling [7,8,31,45]. Statistical methods applied include Frequency Ratio (FR), Multivariate Adaptive Regression Splines (MARS), Weights-of-Evidence (WoE), and Maximum Entropy (ME) [7,8,10,21].…”

“…Three factors were selected to reflect possible human drivers of gully erosion: distance to settlements, distance to pathways, and land use land cover (LULC). These three factors are associated with the occurrence of gullies in the literature [5,14,16,21,48]. The number of variables is not equal when comparing environmental and human-related factors.…”

“…Gully erosion, in particular, has been shown to be related to different aspects of land use such as overgrazing, topsoil crusting, and unadapted irrigation techniques [5,16]. Furthermore, it is suggested that the occurrence of pathways, i.e., unpaved roads and footpaths, trigger the generation of concentrated runoff [14,[16][17][18]; hollow ways are a typical indication of past gullying processes along pathways [19][20][21].…”