Effects of rainfall patterns on hillslope soil erosion process of cinnamon soil in contour ridge system

Abstract: Varying of rainfall intensity during rainfall events is a common phenomenon, and soil erosion processes are strongly affected by intra-storm variations in rainfall characteristics. In general, the combination of rainfall intensities in the rainfall process can be regarded as the storm pattern. In agricultural fields, the influence of storm patterns on erosion processes is largely related to tillage treatments. Contour ridging is an effective soil conservation practice and is used throughout the world. However, less information is available regarding the effect of storm pattern on soil erosion processes in a contour ridge system. In this study, the rainfall simulation experiment was conducted to determine the characteristics of runoff and sediment yielding during inter-rill and rill erosion stages under 4 storm patterns (the rising, falling, rising-falling, and falling-rising patterns) for cinnamon soil in a contour ridge system. And the ridge direction and field slope could be simultaneously changed in the experimental plot. Each storm pattern included 3 rainfall intensities, i.e. 30, 60 and 90 mm/h, which respectively lasted for 20 min during rainfall and comprised the same total rainfall amount and kinetic energy. Results showed that not only the total runoff but also sediment yield exhibited significant differences among storm patterns during both inter-rill erosion and rill erosion stages. Runoff for varied storm patterns was ranked in the following order: falling-rising > falling > rising-falling > rising pattern, but the difference in sediment yield showed a sequence of falling-rising > rising-falling > falling > rising pattern. Runoff from the falling-rising, falling, and rising-falling patterns increased by 1.49, 1.41, and 1.25 times, respectively, compared to that from the rising pattern, while the corresponding increase ratio of sediment yield was 43.03%, 3.30%, and 10.03%, respectively. However, the differences were more pronounced during the rill erosion stage. Compared with the rising pattern, runoff and sediment yield from the falling, falling-rising patterns and rising-falling during this stage increased by 8.89% and -27.05%, 77.60% and 92.59%, 193.62% and 238.89%, respectively. For a given rainfall intensity, runoff, runoff contribution rate, and sediment yield contribution rate gradually increased with the delay of rainfall intensity occurring sequence during the rainfall, while sediment yield by unit runoff presented the opposite tendency. Even at the same occurring stage of a given rainfall intensity, runoff, sediment yield, and their contribution rates were significantly different among different storm patterns, while the influence of occurring sequence for a given rainfall intensity on soil erosion process was the most obvious under 30 mm/h rainfall intensity. Power function fitted the relationship between runoff rate and sediment yield rate for both inter-rill erosion and rill erosion stages, but the exponent was lower than 2. However, there was obvious difference in the exponent of power function among storm patterns, especially in the rill erosion stage. The exponent of power function from the falling, rising-falling, and falling-rising patterns during the rill erosion stage increased by 2.46, 2.52 and 1.46 times, respectively, compared to that from the rising pattern. This indicated that storm pattern greatly affected runoff and sediment yield, especially during the rill erosion stage. This is mainly because soil erodibility and sediment production process varied with the change of storm pattern. So, the effect of storm pattern should be considered when developing soil erosion models. These findings are helpful to deeply understand erosion mechanism in a contour ridge system and supply guidance for implementing contour ridge systems.