The basic primary and secondary processes of soil erosion by water are not well understood for Hawaii's soils despite substantial research and empirical information on overall factors. A series of laboratory and field rainfall simulation experiments were conducted to study the mechanisms and processes of soil erosion and sediment enrichment from a well-aggregated, uniformly-textured Oxisol from the island of Oahu, Hawaii. Interrill splash and wash were partitioned with a specially-designed laboratory device under varying rainfall intensities and slope gradients. Results indicated that both splash and wash can be predominant erosion processes, depending on slope and runoff conditions. The power function, E =al^b , where E is the erosion rate and I is the rainfall intensity, well described rainfall intensity effects on all the splash and wash components. However, b values for splash were close to 1 while these for wash were above 2. Slope controls on interrill splash and wash dynamics were modeled with linear or near-linear slope functions, showing a monotonic increase of erosion with increasing slope angle. Rill erosion was evaluated in a ridge-and-furrow system using a series of simulated storms and overland inflows. Results indicated that flow detachment in rills responded to the average flow shear stress differently under laminar and turbulent flow regimes but the relationship can be modeled with a linear function for both. The quality of detached sediment was determined in association with the above interrill splash and wash and rill erosion processes. With splash, sediment sizes varied with directional components and were generally coarser than with wash. Enrichment of the <0.063 mm fraction was observed in both splash and wash sediments. Sediment transported by rill flow became coarser as flow shear stress increased. Soil erosion did not display selective transport of primary clay particles from this "clay" textured soil. However, soil organic carbon (OC) and extractable phosphorus (Ext-P) were enriched due to selective transport of fine aggregates which were richer in OC and Ext-P than coarser one. This chemical enrichment was partially associated with the reduced accessibility of sorption sites inside large aggregates.

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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Agronomy and Soil Science; no. 3341

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