The Impact of Soil Water Repellency and Slope upon Runoff and Erosion

Soil water repellency (SWR) increases the amount of runoff and erosion from soils. Previous experiments on water-repellent soils had shown intricate runoff movement and high rates of water and soil losses; our aim was to observe these processes, noting their influences and quantifying impacts. The experiment used a 60 × 60 cm rainfall simulation plot setup to represent an agricultural plot with packed water repellent soil and no interference of plants. We measured inputs, and monitored surface flows and erosion on 3°, 6° and 9° slopes. A surfactant treated plot was used as a control. Eroded sediment samples were collected and measured for particle size, and together with runoff analysed for organic carbon, nitrogen and macronutrients. Runoff coefficients were high (0.53 to 0.78) for untreated soil with erosion decreasing over the course of the experiment (1.41 t ha^-1 to 0.74 t ha^-1 over five 2 mm rainfall events, of 2 min duration, on the 9° slope). This resulted in cumulative erosion of up to 5.35 t ha^-1 after a total of 10 mm of rainfall (5 × 2 mm). Importantly, silt and clay were preferentially eroded (up to 12 × higher in eroded soil than the baseline composition) and this correlated with the loss of organic carbon, nitrogen, and macronutrients. As expected, the steepest slope (9°) caused greater runoff and erosion than the lowest (3°), however, there was no significant difference in runoff and erosion between the 6° and 9° slope. Surfactant significantly decreased runoff coefficients by two orders of magnitude compared to the untreated soil. Surface flow was initially characterised by beading and rivulet formation, minimising soil contact. Over time, however, a perched, protective water layer formed over the surface, allowing subsequent water flow with reduced soil interaction. The runoff mechanisms for these small rain events were only observed on the water-repellent soils and likely reduced the degree of erosion given the high runoff coefficients. Initial water flow also promoted armouring of the soil by the attraction of smaller particles, leading to increased loss of organic carbon and nitrogen as well as macronutrients.