Effect of tillage on runoff and sediment yields and morphology development characteristic of ephemeral gully in loessial region

Abstract: In the hill-gully area of the Loess Plateau, serious man-made soil and water loss occurs in disturbed soils of sloping farmlands formed in the process of tillage operation. Frequent farming activities cause ephemeral gullies to develop continuously on sloping farmlands. Although an ephemeral gully may be refilled and rehabilitated, new ephemeral gully may develop in the original position in next rainy season. An indoor flow scouring experiment under artificially simulated rainfall was carried out in the State Key Laboratory of Soil Erosion and Dryland Agriculture on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, China in July 2014. The effects of tillage treatments on runoff and sediment yielding in ephemeral gully and its morphology were investigated at different rainfall intensities and slope degrees under the condition of upslope concentrated flow. Based on preliminary field investigations, 3 slope degrees of 15°, 20° and 25° were selected. Rainfall intensity was designed at 1.0, 1.5, and 2.0 mm/min. Flow discharges were 7.53 to 23.45 L/min. Plots of 8 m×1m were laid out for experiments and 3 flow sections were set up for runoff and sediment measurements. Before each test, rainfall intensity was calibrated repeatedly until the rainfall uniformity coefficient reached 85% or above. During each test, flow velocity was measured with dye tracing method and flow width and depth were measured with point gauge system. Runoff sample was taken once a minute within 3 minutes before runoff generation, and once 3 minutes after runoff generation. The results showed that: 1) Flow regimes for non-tilled and tilled ephemeral gullies were characterized by turbulent flow; Tillage could decrease Reynolds number and Froude number by 0.95%-30.77% and 3.41%-35.66%, and increase Darcy-Weisbach coefficient and Manning roughness coefficient by 4.01%-58.82% and 0.88%-32.79%, respectively; and 2) Compared to non-tilled ephemeral gully, the soil detachment rate for tilled ephemeral gully increased by 9.48%-37.87% under the experimental condition; The soil detachment rates for non-tillage and tillage ephemeral gullies were in a very significantly linear relationship with the interaction of slope and flow discharge, and the interaction of rainfall intensity and slope; It was also significantly linearly correlated with flow shear stress, stream power and unit stream power; Critical shear stress, stream power and unit stream power was 17.576 N/m2, 5.036 W/(m2•s) and 0.0381 m/s, respectively for non-tillage ephemeral gully, and 10.585 N/m2, 3.544 W/(m2•s) and 0.0277 m/s, respectively for tillage ephemeral gully; 3) Compared to non-tillage ephemeral gully, the eroded width and area of tilled ephemeral gully increased by 1.98%-31.79% and 0.87%-31.69%, respectively, but the incised depth was reduced by 2.84%-26.66%; 4) The soil erosion mass for ephemeral gully increased from 0.91% to 22.80% due to tillage treatment; The ratios of soil erosion mass for tillage and non-tillage ephemeral gullies to the total soil erosion mass were 44.09%-74.16% and 42.44%-56.44%, respectively. The erosion masses for non-tillage and tillage ephemeral gullies were a significantly linear function of the interaction of rainfall intensity and flow discharge. This study may provide valuble information for the construction of an ephemeral gully erosion model and the security and protection of agricultural eco-environment.