Effects of Geological and Topographical Factors on Determining Gully Erosion Thresholds

Due to considerable effects of gully erosion on land degradation, it is the one of the serious kind of water erosion and thereby more costly control. This objective of this research was to determine some environmental thresholds of gully erosion which conducted in the Kermanshah province, Iran. The results explored that, there are significant differences of gully morphometery and thresholds between Sahneh (SA) and Ghasreshirin (GH) regions due to different geologic, topographic and vegetation properties. However, GH is located on Aghajari formation comprising marl deposits promoting more piping and head-cut formation than SA area. The mean gully length in fair rangeland (FR) and weak rangeland (WR) of SH region was 296 and 312 m, respectively that significantly were lower in GH region (985 and 1110 m for FR and WR, respectively). In contract, gully depth and wide characteristics were reverse for these regions.  The respective gully catchment area for FR and WR was 1917 and 1236 m2 in SH region, while there was 5503 and 3668 m2 for GH region indicating significant higher value for GH due to gentle slope. However, relationship between slope and catchment area showed that the minimum slope and catchment areas for triggering gully erosion in the SH region was 11% and 280 m2, respectively, hence; in GH these respective levels were 3% and 1450 m2 also due to gentle slope of marl formation. It is concluded that gully properties and threshold are affected by geology and topography condition, while different gully characteristics within region is mainly controlled by landuse practices (converting FR to WR through improper grazing and subsequently decreasing plant cover and increasing bare soil). Thus there is more severe in GH because of considerable land use change and animal grazing, particularly during recent years.