determination of effective factors on the occurrence of digitated gully erosion in the aghemam(2) watershed

introduction: soil erosion by water is one of the most important processes of land degradation, especially in semi-arid areas. among different types of the soil erosion, gully erosion accounts as one of the most critical processes which can cause soil destruction, changes in landscape and water resources filling of reservoir of dams,decrease in water transport capacity of rivers and agricultural lands destruction in the lowland areas. based on the posen definition, gully is a river with high slope walls and an erosive high slope and active head which occurs by erosion resulted from the surface flow (usually during or after high density rainfalls). different factors play role in occurrence and development of gullies and sediment production resulted of this kind of erosion which includes slope, amount and distribution of rainfall intensity, construction operations, vegetable cover destruction, land-use change, unsuitable utilization, and susceptibility of bed materials to the erosion and flood. considering the importance of gully erosion and the way of its occurrence and development and its control, more comprehensive studies are needed to be done. although, some studies have done in this subject that some of them are depicted below: rinkez et al showed that gully erosion is further occurred in soils with high exchangeable sodium percentage and sodium absorption ratio and they depicted that these two factors are important indices for soil diffusion amount in gullies. buma and imson investigated factors such as electricity conductivity, calcium carbonate percentage, and the type of clay mineral in white, brown, and gray samples of marl in the peter area, spain. according to their results, factors such as electrical conductivity and sodium absorption ratio had high correlation with erosivity of the bad lands. material and methods: agh emam(2) watershed is located between 55º 42´ 53˝ to 55º 45´ 43˝ eastern longitudes and 37º 41´ 01˝ to 37º 46´ 19˝ northern latitudes in the east parts of the golestan province with an area of 2,595 hectares. arab gharehaji rural is considered as the most important area which is located at the outlet of the basin. in current study, at first, frame work and total surface of the area were investigated by field observations and 9 digitated gullies were observed during field observations. then, spatial position of each gully was determined by using global positioning system and map of gully distribution was obtained for the study area. then, by overlaying this map on lithology, slope degree and altitude maps of the study area, the mentioned characteristics were determined for the gullies. after sampling from the soil, they were carried to the lab and clay percentage, silt percentage, sand percentage, saturation humid percentage, electrical conductivity, acidity, organic matter percentage, exchangeable sodium percentage, sodium absorption ratio, percentage of neutrophil materials for two considered depths were measured. then, two statistical tests were applied in order to determine the significance and or non- significance of different physical and chemical characteristics of the soil which are compare means of two statistical populations and t-student tests. also, a non- parametric (chi-square) test was used in order to determine the statistical analysis of the effect of non-soil factors of occurrence of digitated gullies including slope classes, altitude classes, lithology and land use. results: initial investigations of the field observations, aerial photos and satellite images showed that different factors play role in the occurrence of digitated gully in the study area. so, physical and chemical characteristics of the soil, slope classes, altitude classes, lithology and land use were investigated in current study. a: results of the effective physical and chemical characteristics of the surface and sub- surface soil on the occurrence of digitated gully erosion results showed that, there wasn’t statistically significant difference between the percentages of sand, electrical conductivity, organic matter and lime, out of the digitated gully erosion (upland of the gully and beholder area) and their amounts in surface soils in the digitated gully, but, there was significant difference for silt, clay, saturated humidity percentage, cation- exchange capacity (cec), exchangeable sodium percentage and sodium absorption ratio. also, the results showed that there wasn’t statistically significant difference between the sand percentage, silt percentage, clay percentage, acidity, organic material and lime in sub- surface soils out of the digitated gully erosion (upland of the gully and beholder area) and their amounts in sub- surface soils in the digitated gully, but, there was significant difference for saturated humidity percentage, electrical conductivity, cation- exchange capacity, sodium absorption ratio and exchangeable sodium percentage. b: results of the role of non- soil effective factors on the occurrence of the digitated gully erosion the results of the non- parametric chi- square test of the effect of slope classes, altitude classes, lithology and landuse on the occurrence of digitated gullies showed that only lithology affected the occurrence of the digitated gullies. the results of current study are compatible with the results of ghoddusi and davoodirad, zaremehrjerdi et al, servati et al, shadfar et al, khuje et al, rahi et al, bayati khatibi et al, sayadi and tashakkori. conclusion: totally, it can be said that saturated humidity percentage, cation- exchange capacity, sodium absorption ratio and exchangeable sodium percentage as soil factors had effective role on gully erosion occurrence in the study area, while, lithology was effective among the non- soil factors. therefore, considering these factors is necessary in order to control the erosion.