Sediment Sourcing of Gully Erosion and Factors Affecting it in Small Catchments

Soil erosion by water is one of the most important processes of land degradation in arid and semi-arid regions of Iran. In this regard, gully erosion is a well-known process degradation which being controlled by verity of environmental conditions. The main aim of this research consist of assessing multi-parameter fingerprinting method to discriminate the sediment contributions of gully erosion through the combination of tracer properties (organic carbon OC, Nitrogen N), Phosphorous P, C/N and 137Cs) in three catchments. Results indicate that gully erosion is responsible between 86 and 99% of the catchments sediment. Due to same conditions of soil, lithology and climate different achieved contribution of gully erosion could be addressed to geomorphic parameters (e.g. area, drainage density, hillslope length and gully distribution). Generally, at a given gully density when the catchment area increase the hillslope length, which reduce the transportation of soil eroded on the hillslopes by sheet and rill, will increase and consequently gullies would deliver more sediment than hillslope processes. Further more, radionuclide 137Cs with the amount of residual root mean square error (RRMSE) (0.12%) was the most informative tracer properties to separate top-soil and gully sediment source. Finally results indicate that a gully erosion rate becomes notable from an area about 5 ha. The large influence of gully erosion, channel and river banks on sediment yields of watersheds implies that estimation of erosion rate by models such as: USLE, MUSLE, WEEP, EUROSEM, GUEST and runoff plot, which mainly focused on sheet and rill erosion, is not reliable and should be taken account by land managers.