Fig 1.
A: The Horn of Africa with the Tigray region of the Ethiopian Highlands marked in dash green line. B: The study area in central eastern Tigray. Numbered squares correspond to 10x10 km sample units: 1. Rama, 2. Yeha, 3. Daragà and 4. Wuqro. Sources: A: Natural Earth open source data available at naturalearthdata.com. B: color-coded hillshade map based on 30 m SRTM DEM provided by USGS earth explorer [81–83].
Fig 2.
Topographic overview of the four sample units: Rama (1), Yeha (2), Daragà (3) and Wuqro (4). Color-coded hillshade maps based on 30 m SRTM DEM provided by USGS earth explorer [81–83].
Fig 3.
A. Gully along pathway in the Rama sample unit (1), B. Initial gully head development perpendicular to a pathway in the Rama sample unit (1), C. Gully in the Yeha sample unit (2) with a small pathway descending into it, D. Gully in the Daragà sample unit (3).
Table 1.
Classes distribution of the different natural, human and movement related variables tested for FR calculation of relative gully occurrences.
Fig 4.
Gullies (blue lines) and randomly distributed Least Cost Paths (red lines) within the four sample units: Rama (1), Yeha (2), Daragà (3) and Wuqro (4). Figures are showing the north-central section of each unit for comprehensible view of LCPs’ starting points as well as the gullies’ distribution in the landscape. Color-coded hillshade maps based on 30 m SRTM DEM provided by USGS earth explorer [81–83].
Table 2.
Distribution of gullies and holloways in the study area.
Fig 5.
Frequency Ratio (FR) box plot diagrams of all classes within a given variable.
Boxes represent 50% of FR distribution. Median is marked by a horizontal line while arithmetic mean marked by X. Circles represent the FR values whenever they were not used to generate the box plots borders itself. LULC—Land Use Land Cover, NDVI—Normalized Difference Vegetation Index, TWI—Topographic Wetness Index, distance from pathways I–based on Soviet topographic maps, distance from pathways II–based on CORONA satellite images. Black squares are due to irrelevance of the variable for a given sample unit, i.e., one soil type at Yeha sample unite (2) or no officially mapped rivers in the vicinity of the sample units (2) and (3). Random pixels tested for their relative frequency in the ‘distance from pathway’ variable are marked in light grey at the right edge of the plot. For TWI in the Wuqro sample unit (4), highest FR is 10.4 and is not presented due to graphic limitations (see S1 Table for full results).
Fig 6.
Changes in length of Least Cost Paths (LCPs) following the introduction of barriers along gully locations (‘modified DEM’).
Change is expressed by differences in the length of Least Cost Paths generated between two random points across the four sample units. The length difference (LCP without gullies as barriers / LCP with gullies as barriers) is expressed in percentage along the Y axis. Box plot representing 50% of changed LCPs (median). Mean marked by X while circles represent each changed LCP (n = 2000 for each sample unit; See S2 Table for full results).
Fig 7.
Comparison of slope values along Least Cost Paths (LCPs) based on the unmodified and modified DEM. Unmodified (original) LCPs appear as grey lines while modified LCPs appear as red lines. Selected for display were only the LCPs with the largest observed length reduction. Notice the black fine-dashed rectangles around the higher slope angles values of the LCP with gullies as barriers, as well as the dashed blue rectangles around the longer segments on the path-distance axis in the same modified LCP.
Fig 8.
Spatial distribution of the Least Cost Paths (LCPs) with the largest observed length reduction presented in Fig 7.
LCPs presented from north to south points. Unmodified LCPs appear as grey lines while modified LCPs appear as red lines. Numbers correspond to sample units. Notice areas with relatively higher slope angles. Slope maps based on 30 m SRTM DEM provided by USGS earth explorer [81–83].