Effect of Topographic Factors on Gully Initiation (Case study: Dareshahr Area-Ilam Province)

Gully erosion is one of important erosion forms in Dareshahr، the Ilam province. The present study therefore aimed to recognize the watershed characteristics affecting gully development and dominant processes in gully initiation. Towards this attempt، 18 gullies with three types distributed in different topographic and soil characteristics were selected. Their associated relationships were then established. The results of topographic characteristics showed that the relation between slope and drainage area of linear gullies in rangeland areas was negative. The critical threshold area for gully development in slope of 12% was 6ha، highlighting the important role of surface runoff in the formation and development of gullies. However، no meaningful relationship could be established between slope and drainage area of frontal and digitated gullies. This confirms the significant effect of other factors compared to surface runoff influence. The study of gullies in different geologic formations also revealed that the sub-surface runoff plays an important role in Gsachsaran and Quaternary formations in the formation and development of gullies. In contrast، the Hortonian runoff was responsible for gully development in Pabadeh geologic formation. Soil sodium، silt and sand contents were also factors affecting gully topography with a reverse relationship with drainage area of the gullies.