Topographic Threshold of Gully Erosion in Iran: A Case Study of Fars, Zanjan, Markazi and Golestan Provinces

Abstract

The relationship between the slope gradient (S ) and drainage area (A) upslope the gully head or point of gully incision represents the topographic conditions for development and positions of gully erosion in different environments. In this study, 300 gullies from 4 provinces including Fars, Markazi, Zanjan and Golestan were selected, and their physical characteristics such as watershed, soil, ground surface and their dimensions were measured. A digital elevation model (DEM) was produced with ArcGIS 9.3, using topographical maps with 1:25,000 scale. The location of gullies was recorded using GPS in the field and transferred on the DEM; also, the boundary of the gullies was depicted using Av. SWAT. Upslope drainage area and slope of the soil surface of gullies were measured using DEM and field survey. The gullies were classified into homogeneous groups, using Cluster analysis method, and effective factors for categorizing were determined using factor analysis. Values, coefficient of determination and significance of the relationships were determined using regression method in Minitab 16 and SPSS 23 software. The results of this research indicated a strong relationship between ground slope and drainage area in both cases of gully development and incision at 1% significance level. The relationship for gully development had an exponent (b) equal to −0.365 and R2 equal to 0.564, indicating the dominant impact of overland flow on the gully development. Although using values of ground slopes measured on DEM did not decrease the significance level, values of exponent (b), intercept (a) and R2 decreased. Value of intercept (a) indicates low resistance of landscape to gully erosion in the studied environments.