Determining hydraulic threshold conditions for gully initiation based on flow simulation

Gully erosion is one of the developed features of accelerated soil erosion which is known as a geomorphic threshold phenomenon and controlled by different environment factors. The main terms of these thresholds consist of: rainfall, hydraulic, pedology, landuse, topography and lithology. Most of researchers have used different criteria such as: shear stress, shear velocity, stream power and critical Froude number for investigation hydraulic threshold of rill and gully initiation. The most process based models for soil erosion prediction have used the 3.5 Pa for critical shear stress and consequently to assess their results it is necessary to having knowledge about hydraulic thresholds and effective factors on it in order to prevent over estimation. In this research an experimental flume was used to simulation flow in three landuse, dryfarming, range and abounded area, to investigate effects of land surface conditions on erosion and gully initiation. Results indicate that critical shear stress for soil erosion in rangeland; dryfarming and abandoned land are 83, 11 and 74 dyne/Cm2 respectively. In comparison to dryfarming in rangeland short vegetation cover causes to decrease the effect of ground cover on flow regime. However because of submergence condition during high discharge this effect was diminished in former landuse and incr4ease up to 25000. In addition in mean discharge the turbulence of flow in rangeland is less than other landuse which can be linked to effect of microtopography features in dryfarming area. Mean threshold shear stress for step initiation was calculated 192, 43 and 72 for range, dryfarming and abandoned area respectively. The main reason to explain this dramatic (3-4 times) variations is vegetation cover of soil surface. In fact this remarked decrease in dry farming land is related to effect of tillage practices on soil susceptibility and aggregate strength. Based on obtained results it can be drawn that using value of 35 dyne/Cm2 as critical shear stress in some process based models of erosion prediction is acceptable for sheet and rill erosion while for gully erosion these models are subject to serious uncertainty and unreliable estimation.