Erosion morphology and runoff generation and sediment yield on ephemeral gully in loess hilly region in field scouring experiment

Abstract: Ephemeral gully is a kind of erosion channel formed due to alternative action of runoff scouring and tillage. It can be easily filled by normal tillage and reform again in the same location by erosive rainfall events. Ephemeral gully is not only the main channel of sediment transport, but also is the main source of eroded sediments. A field scouring experiment was conducted on a typical sloping farmland in Yan'er watershed of Baota region (36°11′-37°09′N, 109°21′-110°03′E), Yan'an, Shannxi to investigate runoff characteristics and sediment yield process and morphology parameters on ephemeral gullies with different gradients under different flow discharges. Soil of the farmland was composed of 7.8% clay and 66.5% silt and 25.7% sand. With consideration of physical condition of the farmland, the gradients of ephemeral gullies were selected as 14°-29° according to preliminary field investigations. The range of flow discharge was decided as 5-25 L/min through a theoretical equation, in which rainfall intensity and runoff coefficient and some of topographic factors were given. The size of ephemeral gully plot for the experiment was designed with the length of 5 m and width of 1 m, constructed as "imbricated land-form" with 10 cm of elevation from the bottom to the edge of the channel. A set of equipment was laid out to provide evenly and stable water flow with a valve to obtain a desired and constant flow discharge. Before each test, flow discharge was calibrated repeatedly to guarantee uniformity and veracity. During the experiment, flow velocity was measured with dye tracing method and flow width and depth were measured with a thin steel rule and point gauge every 5min. Runoff sample was firstly taken once a minute within 3 minutes after runoff generation, then once 3 minutes within the rest time. After an event, morphology parameters such as width and depth of erosion gully were measured for every 5 cm of distance for transverse sections from up-slope to down-slope. The results showed that: 1) Runoff generation time ranged from 7.00 s to 68.02 s and a significant power function with a negative index was found between the runoff generation time and the interaction of gradient and flow discharge. Stable runoff rate, with a range of 3.06-23.71 L/min, increased linearly with flow discharge increasing and showed an increasing tendency when the gradient increased. A very significant power function was established between the stable flow velocity and interaction of gradient and flow discharge, and the stable flow velocity changed with the maximum value of 55.59 cm/s and the minimum of 27.35 cm/s; 2) Sediment yield rate varied with time as 2 types of tendency of "decrease with fluctuation - stable" and "increase - decrease with fluctuation - stable". Average sediment yield rate increased as an exponential function with increasing flow discharge and increased firstly and then reduced with increase of gradient with a critical value of 26°; 3) The ranges of average width and depth of erosion gully were 4.45-17.09 cm and 1.88-10.15 cm, respectively. Average width-depth ratio varied with a range of 1.45-2.39. Average cross-section area, ranged from 11.40 to 197.91 cm2, was the best one of all the morphology parameters to describe the sediment yield with a super-significant linear function. The results obtained may provide support for the establishment of empirical and process-based model of soil and water loss on ephemeral gullies in Loess Plateau.