Particle size distribution of sediment detached from rills under raindrop impact in semi-arid soils.

• Temporal decrease in the contribution of raindrop impact (RI) in rill detachment. • More contribution of RI in rill detachment in soils with higher stable aggregates. • The use of sediment's PSD for detemining the sediment size selectivity in rills. • More dependency of sediment enrichment ratio on soil texture than slope gradient. • Quantifying the sediment homogeneity using d g and δ g measures. Limited information is available on the role of raindrop impact (RI) in the particle size distribution of sediment (PSD s) and the homogeneous of sediment particles eroded in rills under slope gradients particularly in semi-arid soils. Toward this, a laboratory experiment was designed with three dominant semi-arid soils consist of loamy sand, clay and sandy clay, at four slope gradients (5, 10, 15, and 20%) under two soil surface conditions (SSC) i.e, with raindrop impact (RI) and without RI using a rainfall simulator with 90 mm h−1 in intensity for 30 min. Forty eight rainfall events were setted up to soil samples packed into the flumes with 0.4 m width by 4 m length. Runoff and sediment ratio for the two SSC was computed using the ratio of values with RI to their values without RI in each time interval from starting runoff. Based on the results, the soils appeared different behavior in runoff and sediment ratio during runoff generation time. In clay and sandy clay, contribution of RI in runoff and sediment increased during runoff generation in the rills which was associated to higher percentage of water-stable aggregates. The two soils produced higher runoff and sediment under RI with increasing slope gradient. The sediment selectivity by concentrated flow was determined using the comparison between PSD s in steady-state runoff and the PSD of original soil (PSD o). The PSD s curves of loamy sand were similar for the two SSC at slope gradients and overlapped almost with the PSD o , indicating lower importance of RI in the sediment transport selectivity in the soil with lower water-stable aggregates. Enrichment ratio of particles in sediment (ER s) varied with both soil texture and slope gradient. The contribution of RI in variation of ER s was more obvious in sand, indicating lower selectivity of sand to detachment by concentrated flow. The geometric mean diameter (d g) and standard deviation (δ g) of the sediment particles were defined as a measure to determination of the homogeneity of sediment. Values of these indices increased under RI, indicating the contribution of RI in the transportability of all soil particles especially coarse particles. This study revealed that the RI is the major factor controlling runoff, sediment, the selectivity of soil particle by concentrated flow and sediment homogeneity in semi-arid soils. These variables can be quantified by PSD s , ER s , d g and δ g for the two soil surface conditions and compared with the original soil. [ABSTRACT FROM AUTHOR]