Effects of variabl rainfall intensity events on interrill erosion

Soil erosion, as a complicated natural phenomenon, is a global challenge threatening soil and water resources. Therefore, accurate understanding of soil erosion and sediment transport processes and their interactions is necessary for prediction of soil erosion.To distinguish between the dominant processes involved in soil loss, soil erosion has been classified into rill and interrill erosion. Rainfall-induced detachment and transport of sediment have a fundamental contribution to interrill and sheet erosion. Instead of the observation of natural rain showers, simulation of rainfall is widely used for better understanding of processes involved in soil erosion and their interactions. Rainfall intensity is variable during a natural rain showers, such that peak rainfall intensity in an event can exceed the mean event intensity by an order of magnitude. Variations of rainfall intensity during a rainfall event is called ‘event profile’. However, the available information is inadequate to understand its effects on runoff and soil loss processes. Thus, this study was aimed to quantify the effect of event profile on runoff and soil loss in rainfall-induced erosion. The experiments were based on the use of simulated rainfall on disturbed soils in a small laboratory detachment tray. Since kinetic energy controls runoff and soil loss processes, the effects on the soil surface of rainfall events with the same average intensity may be different. Hence to study only the effects of event profile, we used four simulated rainfall events, each with a different profile but all delivering the same total kinetic energy to the soil surface. The study was conducted in the rainfall simulation laboratory, Department of Hydraulic, Faculty of Civil and Environmental Engineering, Tarbiat ModaresUniversity, Iran. For this study, four events with different temporal profiles of rainfall intensity, each with the same kinetic energy, were chosen; (a) constant intensity, (b) increasing intensity, (c) decreasing intensity and (d) increasing - decreasing intensity. The soil samples used in the experiments were a sandy soil (soil A) and a sandy loam soil (soil B). Disturbed soil samples were collected, air dried, crushed to pass a 4.75-mm sieve, and thoroughly mixed. For each experiment soil was packed into a drainable detachment tray to a specific bulk density. The test area of this tray was 15cmwide by 30cmlong and 15cmdeep.Eliminating the effects of soil moisture on results, soil was saturated from the bottom using drainage outlet tube connected to a water reservoir 24hr before each experiment. The tray was setup at a slope of 0.5% and exposed to simulated rainfall events. For each simulation, runoff and sediment were sampled at regular intervals. Our results showed that while the peak runoff was affected by event profile for the two soil types, there was no significant difference in total runoff among rainfall events examined for both soil types. However, our experiments showed that soil type significantly affected the soil loss. In addition our results showed that models that derive interrill soil erosion directly from rainfall intensity can, therefore, be expected to perform poorly in predicting soil erosion from varying intensity rainfall events. It may imply that various and complicated mechanisms might be activated for runoff and erosion under natural rainfall conditions. Our results may indicate that negligence of event profile may lead to inaccurate understanding of mechanisms involved in runoff and soil erosion.