Overland flow in an ephemeral gully (EG) system includes the EG channel, rill and interrill sheet flows, which comprise an interconnected drainage network. There are abundant researches on the hydraulics of rill flow and interrill sheet flow, as well as their relationships with soil detachment rates and sediment loads. Few studies have quantified the flow hydraulics of EG system which is highly related to the EG erosion and the development of better EG management strategies. Thus, two erosive rainfall intensities (50 and 100 mm h−1) and two slope gradients (15° and 20°) under different upslope and lateral inflow boundary conditions were employed to study the flow hydraulics of an EG system using simulated rainfall and inflow experiments. The results showed that flow velocity, shear stress, stream power and cross-sectional energy in the EG channel generally increased from the top to bottom of the slope with different trends and fluctuations under various inflow conditions. The flow hydraulic parameters of EG channel, rill, and interrill sheet flows all increased as rainfall intensity and slope increased. Flow regimes in the EG system were divided based upon the Reynolds number and Froude number. EG channel flow was classified as transitional to turbulent flow, rill flow was transitional, while sheet flows on interrill areas was laminar. 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 increase as flow regime changed toward turbulent and supercritical. Good relationships between sediment load and flow hydraulic parameters were observed and flow shear stress was found to be best correlated with soil erosion of the EG system (R2 = 0.92). This study quantitively mapped the flow hydraulics in the EG system and correlated these with erosion features, contributing to the development of process-based EG prediction models and better EG management strategies.

Flow hydraulics in an ephemeral gully system under different slope gradients, rainfall intensities and inflow conditions

Nonlinear response of hydrodynamic and soil erosive behaviors to biocrust coverage in drylands

Soil water conditions play an important role in the formation of a collapsing gully, but we are still at the early stages of understanding how the soil water changes on the slope after different rainfall events due to a lack of high-frequency continuous field observations. This study aimed to reveal the response of soil water dynamics to rainfall events for different slope aspects and positions based on continuous multi-depth observations of soil water on a typical collapsing gully slope from 2017 to 2019 in Wuhua County, Guangdong Province, China. The vegetation characteristics and soil properties were investigated, and the storage of soil water was also calculated. The results showed that the dynamics and storage of soil water varied with the slope aspect, slope position and vegetation cover. The response time of the soil water to intensive rainfall events on the sunny slope was shorter than that on the shady slope, while soil water storage in the sunny slope was significantly lower than in the shady slope (p < 0.01). For the different slope positions, the soil water response time to the intensive rainfall events on the upper slope was shorter than that in the middle slope, while the soil water storage in the middle slope was significantly higher than on the upper slope. This was mainly due to the redistribution runoff from the upper slope to middle slope, delaying the process by which rainwater infiltrated into the soil layers. Moreover, vegetation significantly allayed the response of soil water dynamics to an intensive rainfall event but increased the storage of soil water, owing to the protection of soil surface from rain and conservation of high soil clay content. The bare area in the middle position of the sunny slope was speculated to be the potential source of the collapsing gully because it lacked the cover of vegetation. Our findings highlight the importance of soil water dynamics on the formation of a collapsing gully and provided valuable insights for the optimization of soil conservation and management practices for collapsing erosion.

https://www.mdpi.com/2073-4441/12/8/2272/pdf

Bin He

Papers

Flow hydraulic responses to near-soil surface components on vegetated steep red soil colluvial deposits