The topographic threshold of gully erosion and contributing factors

The topographic threshold is based on the power relationship between area and slope and is widely applied in gully-erosion research; however, this relationship requires further testing. Accordingly, the Alamarvdasht Lamerd and Fadagh Larestan regions in Fars Province, Iran, were selected as case studies to explore the topographic threshold for gullies. Thirty active gullies were identified in each study area during field surveys, and data describing land use and land cover, drainage areas, slope, and the physical and chemical properties of the soils were assembled. Multivariate analysis was conducted using SPSS to determine the effects of these factors. Using the power relationship between the catchment area and slope for each gully, the analyses explored critical controls for gully development. The results showed that surface runoff was the most significant effective factor for gullies in the study areas. Sparse ground cover, fine-textured soils, and inappropriate land use contribute to gully development. The results suggest that the relationship between slope and drainage area in the Fadagh Larestan case study is S = 0.0192 A(-0.159) for gully headcut areas and S = 0.0181 A(-0.258) at gully outlets. The corresponding values of the exponent beta at the gully headcuts and outlets at Fadagh were - 0.15, and -0.25, respectively. The corresponding relationships for gullies in the Alamarvdasht Lamerd area for the gully headcuts and outlets were S = 0.0143 A(- 0.061) and S = 0.0073 A(-0. 18), respectively, with beta values of - 0.06 and - 0.18. This study provides a basis for determining the thresholds for initiating gully development. Analyses of the effective factors provide clues to improve the management of bare lands to prevent the initiation of gully erosion.