Morphological characteristics and topographic thresholds of gullies in different agro-ecological environments

Gully erosion is an important sediment source and causes severe environmental degradation, particularly in the drought-prone regions of Ethiopia. We investigated the morphological characteristics of gullies, the topographic thresholds of gully formation, and estimated headcut retreat rates in three agro-ecologies of the Upper Blue Nile basin, Ethiopia: highland (Guder), midland (Aba Gerima), and lowland (Dibatie). Gully morphological characteristics were analyzed using data measured in the field, whereas the topographic thresholds were estimated using slope and upslope drainage area. Average annual linear (R-l) and volumetric (V-e) headcut retreat rates were estimated by visual interpretation of very high resolution (0.5-1.5 m) satellite images (QuickBird, IKONOS, SPOT-7, and Pleiades) in a GIS environment. A significant power relationship, fitted between gully volume (V) and length (L) for the three sites, is V = 8.097 L-1.032 (R-2 = 0.902, n = 94). The average annual R-l, V-e, and soil loss rates were estimated at 0.76 m yr(-1), 6.77 m(3) yr(-1), and 8.73 t ha(-1) yr(-1) in Guder, 2.09 m yr(-1), 19.58 m(3) yr(-1), and 20.76 t ha(-)1 yr(-1) in Aba Gerima, and 3.42 m yr(-1), 42.16 m(3) yr(-1), and 49.33 t ha(-1) yr(-1) in Dibatie. The higher gully headcut retreat rate in Dibatie is mainly related to the highly erodible nature of Vertisols. The coefficients of the slope (S)-drainage area (A) relationship S = aA(-b) were a = 0.219 and b = -0.139 in Guder, a = 0.133 and b = -0.234 in Aba Gerima, and a = 0.113 and b = -0.216 in Dibatie, indicating that topographic thresholds for gully initiation varied among the agro-ecologies. The results of this study can be used to estimate gully erosion rates and identify areas for gully initiation, thereby supporting the planning of appropriate gully control measures in the study sites and other areas with similar environmental settings. (C) 2019 The Authors. Published by Elsevier B.V.