114 IV International Symposium on Gully Erosion. J. Casalí and R. Giménez (Eds.) © 2007 Universidad Pública de Navarra, Spain. ISBN 978-84–9769-198-7 WATERFALL EROSION AS A MAIN FACTOR IN EPHEMERAL GULLY INITIATION IN A PART OF NORTHEASTERN IRAN Sadeghi, S.H.R. 1* , Rangavar, A.S. 2 , Bashari, M. 1 , Abbasi, A.A. 2 1 Department of Watershed Management Engineering, College of Natural Resources and Marine Sciences, Tarbiat Modares University, Noor46414-356, Mazandaran, Iran. * sadeghi@modares.ac.ir 2 Razavi Khorasan Research Centre for Agriculture and Natural Resources, Iran. 1. Introduction Gully erosion is defined as the erosion process whereby runoff water accumulates and is concentrated in narrow channels and, over short periods of time, removes the soil from this narrow area to considerable depths. In some areas, gully erosion is also a main source of sediment yield (Poesen et al., 2003; Nagasaka et al., 2005). Classical gullies can be described for agricultural land as permanent channels too deep to easily ameliorate with ordinary farm tillage equipment, typically ranging from 0.5 to as much as 25-30 m depth (Soil Science Society of America, 2001). Ephemeral gullies result from concentrated flow erosion larger than rill erosion but smaller than classical gully erosion (Poesen et al., 2003). Gully erosion consists of four stages: formation, development, healing and stabilization. As reported by Sidorchuk (2005), the gully initiation stage comprises only about 5% of the entire gully lifetime, but in that stage more than 90% of gully length, 60% of its area and 35% of the gully volume may be formed. In the remaining gully lifetime the morphologic conditions are relatively stable. Thus is important to understand the controlling factors in the formation stage. Sidorchuk (2005) also mentioned that in humid conditions the linear water erosion at the gully bed and rapid shallow mass movement at the gully sides of major importance during the first stage of gully evolution. The present study assesses the factors controlling gully formation in a study area located in northeastern Iran. Gully erosion in northeastern Iran is very high because of the high rate of human encroachment and the resulting impacts on soil erosion. 2. Materials and Methods The study took place in the rolling hills region of the Sanganeh Plateau at almost 600 m absl in the vicinity of the Turkmenistan border (Fig. 1). This area is covered by highly erosion susceptible brown soils (Iranian Planning and Budget Organization, 1994). The area receives some 200- 300 mm of precipitation per annum (Jafari, 1997) and is primarily covered by annual grasses. These grasses grow during the time of the main precipitation, i.e. during December to March, and then diminish with the onset of rising temperatures. The main land use of the area is natural rangelands that are intensively and untimely grazed by sheep. The rate of soil erosion is very high and vegetation pedestals have been well developed in the area. The study was based on long-term field investigations and detailed monitoring of gully stages. The intermediate stages of soil erosion (i.e. sheet and rill erosion) were also considered to determine the dominant processes in gully formation and development. The study cases were then depicted pictorially. Fig. 1. General view and location of the Sanganeh study watershed, Iran.