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Soil erosion by water is mostly the result of rainfall-driven and runoff-driven processes taking place simultaneously during a storm event. However, the effect of interaction between these two erosion processes has received limited attention. Most laboratory experiments indicate that the rate of erosion in a rain-impacted flow is greater than for un-impacted flows of similar depth and velocity, however negative interaction between the two processes has also been reported. There is no provision for such interaction in any of the current erosion models. This paper reported on the results of a number of exact experiments carried out in the flume of Griffith Universitys large Rainfall simulator to study interaction between rain and runoff processes. The results show that the interaction between flow-driven and rain-driven processes is always positive when flow is in sheet form and no rill is allowed to form. No negative interaction was observed prior to rill formation. Further analysis of the eroded sediment showed that the increase in sediment concentration in the combined rain and runoff experiments is mostly due to a significant increase in the proportion of <0.02 mm and 0.5-2 mm particles. Two different mechanisms appear to be responsible for the preferential loss of these two size classes of particles. Peeling action of raindrop when impacting on large aggregates is responsible for the formation and transport of fine particles. Rolling action of medium-large particles on the soil surface by sheet flow or through stop/start push applied by the impacting raindrops is responsible for the loss of these particles. Detailed analyses of rain-driven and flow-driven processes and their interactions and preferential transport of certain size ranges of soil particle are discussed in this paper.