Determination of relationship between Physical characteristics and volume and length of Jahan Abad gullies located in North East of Buinzahra

Gully erosion is a destructive form of water erosion that cuse lost a significant amount of valuable soil and its control requires understanding the relationships between the characteristics that affect this phenomenon. The purpose of this study was determining the most effective variables on the gully volume and length.
Gullied area identifed with Google earth in the Jahan Abad, Buin zahra (Qazvin province), 33 gullies were selected, recorded by GPS and their morphologic characteristics including length, width and depth were measured. Surface soil samples were taken from headcut and were analyzed for texture, EC, pH, organic matter and some Anions and Cations. Indices related to the form of watershed were extracted from the digital elevation model which derived from UAV image processing. Correlation between factors, PCA and multivariate regression were performed to determine important and effective factors on gullies volume and length in MINITAB.
 Results indicated that gullies have vertical headcuts, trapezoidal cross-sections and linear general plan. The average depth of headcuts, depth at 50% of the gully length and the average gully length were 0.35, 0.47 and 13.46 m, respectively. Soil texture was loam and average soil texture components including sand, silt and clay were 38, 38 and 24%, respectively. The mean EC and pH of the samples was 11.2 dSm-1 and 8.7 respectively. Linear correlation between the total volume of soil volume and length and other characteristics showed that silt have significant correlation coefficients at 1% percent level 0.465 and 0.510, respectively. PCA with soil characteristics and watershed characteristics showed that the first and second components are close to 41% and 8 other dimensions with egen value more than one, with 84.2% are responsible for changes. Chlorine, electrical conductivity and sodium with negative effect and aggregate stability, canopy and amount of sand with positive charge in the first component have the most weight, while in the second component, sand, canopy and amount of gravel with positive effect and TNV, organic matter and saturation percentage play a more important role. The results of multivariate regression also showed that carbonate, magnesium, potassium, organic matter, saturation percentage, amount of sand, slope of gully floor, slope of inlet waterway to headcut, litter, basin primeter, respectively, effective properties on soil loss by gully in the region. Form coefficient had the greatest effect on soil loss, respectively, and were included in the prediction equation of soil loss with R2 of 0.837. Stepwise regression with the dependent variable of gully length showed that the relationship has R2 of 72.98% and is significant at the percentage level.
Factors affecting the length of the gully included the percentage of saturation, the amount of sand, specific gravity, the slope of the gully floor, the slope of the waterway entering the upstream, geravel, basin area and slope.