Geomorphic threshold conditions for gully erosion in Southwestern Iran (Boushehr-Samal watershed)

Globally, a large amount of research has been dedicated to furthering our understanding of the factors and mechanisms affecting gully erosion. However, despite the importance of gully erosion in arid and semi arid regions of Iran there has been no comprehensive study of the geomorphic threshold conditions and factors influencing gully initiation. The aim of this article is to investigate the gullying processes and threshold conditions of permanent gullies in an arid region of Iran based upon examination of the slope–area (S = αA−β) relationship. The data were collected through field and laboratory studies as well as Digital Elevation Model (DEM) analyses. In total, 97 active headcuts were identified across the three study sites and classified based on dominant initiation process including piping, landsliding and overland flow. Soil properties, including EC, SAR and soil texture, as well as landuse practices were found to be the major factors initiating piping and bank gullies. All gullies initiated by landsliding and seepage processes were found to be located in steep areas (28–40% slope) with their distribution further influenced by the lithology and presence of a cohesionless sand layer within the soil profile. An inverse relationship between upslope area (A) and local slope (S), in which the α and β coefficients varied, was further investigated based on the dominant gullying process and land use. Gullies occurring in the rangelands that were dominated by overland flow had the strongest relationship while landsliding dominated gullies did not have a statistically significant S–A relationship. In comparison to theoretical and literature based relationships for gully initiation, relatively low values for β were obtained (−0.182 to −0.266), possibly influenced by the presence of seepage and subsurface processes in many gullies. However, this is consistent with other studies in arid regions and may reflect greater potential for gullying in arid zones due to low vegetation cover and high variation in rainfall. In addition, the soil attributes together with land use practices influenced gully initiation thresholds. Application of the solved S–A relation for predicting vulnerable areas to gullying indicates that it is possible to predict the location of gullies with an acceptable level of accuracy; however other environmental factors should be integrated with the S–A relationship to more accurately identify the location of permanent gullies in arid regions.