Determining topographic threshold and effect of soil properties on gully development in three region of Ardabil province

Background and objectives: Gully erosion is an important type of soil erosion worldwide with respect to sediment production and land degradation that also occurs in different parts of Iran (12, 19, 26). Topography and soil properties of gully's drainage catchment have important effects on gully's development (17). Various studies reported the effect of soil on gully formation (22, 25). The objective of this study is investigate hydrologic process of gully forming by topography index and determining of surface and subsurface soil properties effect on gully extension in 2 year and select the most important of this properties.
Materials and Methods: In order to determine the main hydrological process that shapes the gully's shapes three regions of Ardabil province were selected including Orta Dagh, Mola Ahmad and Sarcham, and topographic threshold of gully forming was determined as where S and A are the slope and area of gully's drainage catchment, respectively and a and b are the regional coefficients. Furthermore to investigate the effect of soil properties on gully's development in each region, a set of similar gullies were selected and the changes of volume and cross section area for gullies in four points along its length were measured as indicator of gully development during a two years time. 19 different soil properties at surface and subsurface layer of gullies and at drainage area were measured and their effects on gully development were investigated. At the end, stepwise regression models were introduced to predict the value of gully development and to determine the main factors of gully development in each catchment. Precision of regression models determined with R2 and RMSE.
Results: The results showed that in Sarcham catchment b power of topographic threshold was positive, therefore piping and tunnel erosion seems to be main reason for gully forming but in two other catchments the values of b power was negative therefore runoff may have caused gully formation. Correlation analysis between soil properties and gully's development revealed that in Sarcham catchment erodibility and presence of soluble materials in subsurface layers of soil had a strong relation with gully development. While in other two catchments, erodibility of surface and subsurface layers of soil had similar effects on gully development. In general, application of regression models that used total data set of surface as well as subsurface as independent variable for prediction of cross sectional area changes showed higher precision. For prediction of changes of gully volume, total properties of drainage area were used as independent variables. The precision of this model in Orta Dagh and Sarcham was high but in Mola Ahmad was low. The results of stepwise regression model demonstrated that in Orta Dagh catchment, MWD, very fine sand, clay contents in surface layer, gypsum, calcium carbonate, dispersible clay in subsurface layer, percentage of plant cover and CROSS index in drainage area of gully, in Mola Ahmad catchment, organic carbon and bulk density at surface layer, dispersible clay content and SAR in subsurface layer, and mean slope of gully drainage area and in Sarcham catchment, bulk density and dispersible clay content in surface layer, bulk density, silt and sand contents in subsurface layer, and curve number (CN) and Miller shape index of gully drainage area had more meaningful effect on gully development.
Conclusion: Used topography index had careful for determining of gully formation hydrological process. In Sarcham gully's extension related to soluble material in subsurface soil but in two other region gully's extension related to erodibility of surface and subsurface soil. Between properties of gully catchment properties that effected on runoff volume had main effect on gully's extension.