Quantification Studies on Ephemeral Gully Erosion

Ephemeral gullies (EGs) are major contributors to sediment loss and land degradation on cultivated lands. EG is an important linear erosion feature, often occurring at mid-slope position, that can be greatly influenced by upslope and lateral inflow. This study quantified the ephemeral gully erosion influenced by the upslope and lateral inflow, and the results showed that upslope and lateral inflow both contributed to the runoff connectivity of the EG channel and lateral rills in the EG system. For these simulated conditions, upslope inflow contributions to total runoff and soil loss were 62–78% and 65–81%, respectively, while lateral inflow only contributed around 10%. The contribution differences could be attributed to flow hydrodynamic characteristics in that shear stress and stream power in the EG channel were 4.9–8.6 times greater than those on the lateral slopes. From sheet flow to rill flow and EG channel flow, the flow regime gradually shifted from laminar and subcritical flow to turbulent and supercritical flow. The flow force, power, and energy correspondingly increased as the flow regime changed toward turbulent and supercritical. Both field monitoring and indoor simulation displayed the additional sediment delivery caused by upslope sediment-laden flow, verifying the transport-dominated sediment regime in EG systems. In field observations, the sediment increment coefficient (SIC, ratio of net sediment delivery caused by upslope sediment-laden flow to the total sediment delivery) on an event scale varied from 4.6% to 88.6%. In indoor simulations, the SIC changed from 24.9% to 87.5%. The SIC linearly decreased as the sediment concentration of upslope inflow increased. Field monitoring showed complicated phenomena because of natural random variations. SIC also generally decreased as the sediment concentration of upslope inflow increased. Laboratory simulations verified the field monitoring results of extreme rainfall events with large rainfall amounts and intensities. In future studies, multiple morphological criteria defining EG under various environments are urgently needed. Similar to the continuous sediment transport equations for rill erosion, continuous sediment transport equations for EG erosion need to be developed.