مریم محمدابراهیمی

Gully erosion in one of the different types of soil erosion by water that initiation and advancement of it can tend to the great changes on landscape and degrades the lands. The objective of the current study is to analyze the effects of soil-related factors on the occurrence of Gully erosion in the Age Imam watershed. This watershed, with an area of 5442.45 hectares, is located in the east of Golestan province, and experiences many substantial difficulties due to Gully erosion. In this study, ditches in the region were identified through field study. Then five ditches were selected and 10 soil samples from the within the ditch and 10 from outside the ditch were taken (a region in the adjacent area above the ditches), from two depths (0-30 cm, and 30-60 cm) and were subjected to laboratory analysis. The comparison of the averages of the two statistical populations was used to analyze the physical and chemical characteristics of soil taken from two depths, Results showed that significant differences existed (on a 5% level) among some soil characteristics between surface soil and the soil below that, as: Percentage of Silt (at both depths, taken from the ditch), percentage of saturation humidity (at both depths, taken from the ditch), electric conductivity (at the lower depth, taken from the ditch), neutralizing substances (at both depths, taken from outside the ditches), cation exchange percentage (lower depth, taken from outside the ditch), sodium absorption ratio (lower depth, taken from the ditches), and the percentage of exchange sodium (at the lower depth, taken from the ditches), had an effective role in Gully erosion, due to the higher amounts and statistical significances.

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 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.
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.
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.

Severe lack of data on soil erosion and sedimentation in many national watersheds requires the application of appropriate empirical methods for their estimation. This study is aimed to evaluate the efficiencies of Universal Soil Loss Equation (USLE) and some of its variations including MUSLT, MUSLE-E, MUSLE-S and AUSLE in semi-arid climates. This study was undertaken on standard plots in the ranges of Margsar, range and livestock management research station, Jashlobar, Mahdishahr, Semnan province. For this, all of required variables and input data for the respective models have been calculated for the respective plots. Finally, the estimations from both models and measured sediments have been compared during 15 showers. The results of t student’s t-test showed that there is no significant difference between MUSLE-E, MUSLT and MUSLE-S models and the measured sediment. Also, the results from efficiencies of the models by Nash-Sutcliffe method as well as relative root mean square error (RRMSE) showed that MUSLE-E, MUSLT and MUSLE-S models have higher efficiencies than other variations.

Agh Emam(2) Watershed is one of the watersheds in the Golestan Province with an area of 2595 hectares and has considerable problems with soil erosion, particularly gully erosion. In order to carry out this study, at first, the total number of linear gullies in the study area (eleven gullies) was determined. Then, different physical soil properties (sand, clay, silt, and moisture saturation percentage) and chemical soil properties (electrical conductivity, bio- carbon, lime, cation exchange capacity, sodium adsorption ratio, and exchangeable sodium percentage) were investigated by soil sampling (eleven samples) inside the gullies and in the watershed upstream of the gullies at two depths (0-20 centimeters and 20-50 cm). Non- soil effective factors in development of linear gullies including slope classes, altitude classes, land use, and lithology were also evaluated. The results showed that, among soil characteristics, clay percentage (surface), silt percentage (surface), saturated moisture percentage (sub- surface), electrical conductivity (sub- surface), bio- carbon (surface and subsurface), lime (surface and sub- surface) and also lithology from non- soil factors had significant role in development of linear gully erosion in the study area.