Catchment scale analysis of the effect of topography, tillage direction and unpaved roads on ephemeral gully incision

High-resolution airphotographs and digital elevation models (DEMs) were used to study areas prone to gully incision in a small agricultural catchment in northern Israel. Data are analyzed to understand better the integrated effect of environmental and human factors on gully incision. The effect of flow accumulation, slope, unpaved roads density and tillage direction on gully width and length is found discontinuous and topography - in the form of the topographic threshold - was found to be the most dominant factor on gully incision in the catchment scale. The topographic threshold coefficients observed in Yehezkel catchment were lower than those found in previous studies, due to the effect of large rainfall events during the early winter season, when the soil in fields is still unprotected by crops. In addition, a survey of previous studies shows that, in cultivated areas, there is relationship between the topographic threshold coefficients a (multiplier) and b (exponent). The influence of unpaved roads and tillage direction on gully incision was also found significant and consistent during the two seasons observed, while the non- parametric test showed that it added to the prediction iñ of the gullies. Incision points density plot show that of the two parameters, the unpaved roads were found to have greater influence on gully-head location than tillage direction. Studying the phenomenon on the catchment scale allows better understanding of the effect of spatial variation on gully-head location.