Effects of topography on gully formation in Ilam Province

Gully erosion is one of the important forms of soil degradation especially in semi-arid areas. Gullies are occupying 13 percent of Ilam Province area. This research focused on studying the, topographic threshold of gullies, dominant process of gully initiation and development as well, in agricultural and rangeland land uses. Ilam province with 2 million hac area is located in western Iran. Ilam has 3 climatic zones with 425 mm annual average precipitation. Dominant land use in Ilam is dried forest with 640000 hac area. Topographic characteristics of 120 gullies including initiation and development slopes, initiation and development contributing areas and surface characteristics of gullies contributing area including vegetation, rock fragment and bare soil by 1 m2 plot and soil samples from top soil and gullies wall were collected, as well, in semi-arid part of Ilam province.Results show that Initiation and development topographic threshold value in croplands are 0.02 and 0.04 and increase to 0.1 and 0.2 in rangelands. Topographic threshold in rangelands is more 5 times than croplands. Dominant process in gully initiation and development is surface runoff. Effective factors on gully topographic threshold are organic matter, vegetation cover, rock fragments and bare soil that describe 31 percent of topographic threshold; also, organic matter with beta coefficient of 62 percent has the most contribution in topographic threshold variation. Gully frequency has negative exponential relationship with rock fragment and vegetation cover and decay rate for rock fragment and vegetation cover are 60 and 35 percent respectively.Topographic thresholds showed that topography has important role in gully format5ion but environmental characteristics as vegetation cover, rock fragment and land management are more important