Geomorphic threshold conditions for gully erosion in Southwestern Iran (Boushehr-Samal watershed) Aliakbar Nazari Samani a, * , Hassan Ahmadi b , Mohammad Jafari b , Guy Boggs c , Jamal Ghoddousi d , Arash Malekian e a Faculty of Natural Resources, University of Tehran, Karaj, P.O. Box 31585-3314, Iran b Faculty of Natural Resources, University of Tehran, Karaj, Iran c School of Science and Primary Industries, Charles Darwin University, Darwin, NT 0909, Australia d Soil Conservation and Watershed Management Research Institute, Tehran, Iran e International Desert Research Center, University of Tehran, Tehran, Iran article info Article history: Received 31 December 2007 Received in revised form 25 January 2009 Accepted 2 February 2009 Keywords: Gullying processes Erosion Area–slope Landuse impact Threshold Topography Soil attributes Iran abstract 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 = aA b ) relationship. The data were collected through field and laboratory studies as well as Dig- ital 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 lithol- ogy 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 a and b coefficients varied, was further investigated based on the dominant gullying process and land use. Gullies occurring in the rangelands that were dom- inated 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 b 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 veg- etation 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. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Gully erosion is recognised as a major land degradation issue, causing both impacts on-site, through direct soil loss and off-site, through sediment deposition in downstream environments. Gul- lies are formed by water erosion and consist of several characteris- tics including a steep incised channel with an active headcut, unstable side wall and temporary water flow (Nordstrom, 1988; Poesen et al., 2003). A great deal of research has been undertaken on the contribu- tion of gully erosion to overall soil loss and sediment production in a wide range of environmental and climatic conditions and at a variety of temporal and spatial scales. Although the relative importance of gully erosion has been well documented in the liter- ature (Patton and Schumm, 1975; Vandaele et al., 1996; Poesen et al., 2003; Sidorchuk, 1999; Wasson et al., 2002), most soil loss equations and erosion models do not include the soil loss caused by gully erosion (Poesen et al., 2003). More than 60% of Iran’s area is located in arid and semi arid re- gions, with about 100 million ha at high risk of desertification (Ahmadi, 2004). The spatial and temporal variability of rainfall, 1367-9120/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2009.02.004 * Corresponding author. Tel.: +98 261 2249313; fax: +98 261 2227765. E-mail address: aknazari@ut.ac.ir (A. Nazari Samani). Journal of Asian Earth Sciences 35 (2009) 180–189 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jaes