A2. Effect of Rock Fragment Coverage on Soil Erosion

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Rain splash soil erosion in the presence of rock fragments was tested in laboratory flume experiments under controlled conditions. The surface area exposed to rain splash erosion was adjusted by placing rock fragments onto the surface of differently prepared soil in laboratory flumes (surface initially dry and hand cultivated or sealed due to the pre-wetting). The laboratory results showed that the eroded cumulative mass depended on the cumulative runoff, and that soil erosion was proportional to the soil surface area exposed to raindrops, in situations where an initially dry, hand cultivated and smoothed soil surface were ensured. The results showed that this relationship was controlled to a smaller extent by the soil’s initial moisture content, bulk density and soil surface characteristics. More in detail, it was observed that sediment concentrations during the first part of the erosion event were more sensitive to the initial state of the soil surface, whereas at steady state it was observed that the concentrations of eroded sediments were controlled mainly by the effective rainfall and area exposed to raindrops. Previously published field data on rain splash soil erosion were analyzed to ascertain whether the same behavior was evident under field conditions. It was found that rain splash erosion is in general not proportional to the area exposed. In contrast to the controlled laboratory experiments, the field experiments were characterized by non-uniform initial surface roughness, surface soil aging and heterogeneous rock fragment size and spatial distribution. An area based modification of the Hairsine and Rose (H-R) soil erosion model was employed to analyze the experimental data. The H-R model predictions agreed well with the measured sediment concentrations when high rainfall intensity and low rock fragment cover were used. Predictions were instead less accurate with low rainfall intensity and high rock fragment cover.

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