Assessment of land use impact on hydraulic threshold conditions for gully head cut initiation

A gully as an accelerated erosion process is responsible for land degradation under various environmental conditions and has been known as a threshold phenomenon. Although the effects of gullying processes have been well documented, few soil erosion models have taken into account the threshold condition necessary for gully development. This research was devoted to determining the effects of land use change on hydraulic threshold condition and stream power of water flow through an in situ experimental flume (15m x 0.4 m). Results indicated that head cut initiation and detachment rates showed a better correlation to stream power indices than shear stress (tau(cr)). The threshold unit stream power value (omega(u)) for head cut initiation in range-land, abandoned land, and dry farming land was 0.0276, 0.0149, and 4.5 x 10(-5) ms(-1), respectively. Moreover, the micro-relief condition of soil surface and surface vegetation affected the flow regime of discharge and velocity. It is seen that the composite hydraulic criteria of Froude number (Fr) and discharge (Q) can clearly discriminate the land uses' threshold. In fact, the remarkable decrease of tau(cr) in dry farming was related to the effect of tillage practice on soil susceptibility and aggregate strength. The findings indicated that using the unit steam power index instead of critical shear stress could increase the models' precision for prediction of head cut development. Compared to the Ephemeral Gully Erosion Model (EGEM) equation for critical shear stress, it is important to point out that for modelling of gully erosion, using single soil attributes can lead to an inaccurate estimation for tau(cr). In addition, based on the findings of this research, the use of threshold values of tau(cr) = 35 dyne cm(-2) and omega(u) = 0.4 cm s(-1) in physically based soil erosion models is susceptible to high uncertainty when assessing gully erosion.