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Gully expansion and its temporal influence on catchment geomorphic characteristics and gully topographical thresholds in the semi-arid Ethiopian Rift Valley
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  • Shiro Mukai,
  • Paolo Billi,
  • Nigussie Haregeweyn,
  • Mesenbet Yibeltal,
  • Tilahun Hordofa
Shiro Mukai
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Paolo Billi
University of Ferrara
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Nigussie Haregeweyn
Tottori University
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Mesenbet Yibeltal
Bahir Dar University
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Tilahun Hordofa
Ethiopian Institute of Agricultural Research
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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.