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Quantifying rill bed incision provides fundamental information for process-based erosion modeling; while the morphodynamic and hydrodynamic mechanism in bed incision processes are still unclear. Thus, experiments were conducted to examine rill bed incision processes in upland concentrated flows. DEMs (2 mm × 2 mm resolution) obtained by photogrammetry were used for rill bed morphology analysis. Rill channel (2.0 m-long, 0.08 m-wide and 0.15 m-deep) with two slope gradients (15° and 20°) were subjected to four overland flow rates (1.0, 2.0, 3.0 and 4.0 L min −1 ). The results showed that sediment delivery, rill bed incision rate and average rill depth increased with inflow rate and bed slope. Sediment delivery increased from 0.060 to 0.226 kg min −1 per 1 L min −1 inflow increment and from 0.043 to 0.207 kg min −1 when bed slope increased from 15° to 20°. In a well-developed rill channel, rill bed incision could be divided into three phases: pre-headcut formation (dominated by rill flow shear stress), headcut incision (dominated by headcut advancing) and post-headcut incision (dominated by rill flow shear stress). Headcut incision phase, which only accounted for <15% of total experimental time, produced >65% of rill bed sediment. In the pre-headcut formation phase, rill flow velocity, shear stress and stream power increased with increases of inflow rate and slope gradient. Conversely, flow velocity showed no evident trend with increased inflow rate and bed slope during headcut incision phase. Initial headcut advancing rate could be predicted by a non-linear function based upon soil characteristics, rill flow shear stress and headcut height. Sediment delivery showed a power function with the product of inflow rate and squared bed slope. Because rill bed incision is dominated by headcut advancement and incision, practices for controlling headcut initiation should be implemented to decrease hillslope soil loss. [ABSTRACT FROM AUTHOR]

Mixing models are frequently used as part of sediment fingerprinting to quantify sediment source contributions. Much research effort has been devoted to improving these mixing models. The objective of this study was to develop and evaluate a new method using discriminant function analysis (DFA) to fingerprint sediment source contributions. It was hypothesized that the outcome of DFA, commonly used as a component of standard fingerprinting procedures, can potentially be used directly to quantify source contributions, avoiding mixing models altogether. This hypothesis was tested in the Bull Creek Watershed in Oklahoma State, USA. DFA results were compared with outcomes from the Collins mixing model and previous research. When conservative geochemical tracers were used, DFA results did not differ significantly from the mixing model results, indicating that DFA alone has the potential to accurately quantify sediment source contributions, while being simple to use. When using non-conservative tracers, however, the results from the two methods were significantly different. On the basis of a comparison with previous research, we suggest that DFA offers an intuitive method for characterizing sediment source contributions. [ABSTRACT FROM AUTHOR]

• Upslope sediment-laden flow caused additional erosion in ephemeral gully zone. • Transport-dominated sediment regime was found under filed and laboratory conditions. • An increase in sediment concentration of inflow decreased downslope soil erosion rates. • Laboratory simulations verified the field monitoring results for extreme rainfall events. Ephemeral gully (EG) is an important linear erosion feature, often occurring at mid-slope position, that can be greatly influenced by upslope inflow and its sediment transport capacity. How sediment concentration in upslope inflow influences downslope EG erosion is still unclear. Thus, field monitoring on a natural hillslope and indoor simulations on a dual-box system were conducted to better understand their relationship. The results showed that both field monitoring and indoor simulation displayed the additional sediment delivery caused by upslope sediment-laden flow, verifying the transport-dominated sediment regime in EG systems. In field observations, the sediment increment coefficient (SIC , ratio of net sediment delivery caused by upslope sediment-laden flow to the total sediment delivery) on an event scale varied from 4.6% to 88.6%. In indoor simulations, the SIC changed from 24.9% to 87.5%. Indoor simulation experiments with a range of 0–378.2 g L−1 sediment concentration supplied by a feeder box, showed that SIC linearly decreased as sediment concentration of upslope inflow increased. Field monitoring showed complicated phenomenon because of natural random variations. For rainfall events with PI 30 (the product of precipitation and maximum 30-minute rainfall intensity) larger than 400 mm2 h−1 (similar to the designed extreme rainfall in laboratory simulations), SIC also generally decreased as sediment concentration of upslope inflow increased, especially when sediment concentration of upslope inflow was smaller than 400 g L−1. Thus, laboratory simulations verified the field monitoring results of extreme rainfall events with large rainfall amounts and intensities. This study illustrated the necessity of in-situ field monitoring and indicated that design of indoor simulation experiments needs more comprehensive considerations on complicated hydrological and sediment conditions. [ABSTRACT FROM AUTHOR]

• Soil shear strength decreased as freeze-thaw cycles and soil moisture increased. • Soil disintegration rate increased with F-T cycle but decreased with soil moisture. • Ten F-T cycles are the threshold affecting mechanical behaviors of Mollisol. Freeze-thaw action is a common natural phenomenon in cold regions, which affects soil erodibility by changing soil structure and mechanical properties. However, quantifying the impacts of freeze-thaw cycles (FTCs) at different initial soil water contents (ISWCs) on soil erodibility is challenging due to the complex interactive soil mechanical responses. In this study, direct shear strength and soil disintegration tests were conducted to quantify soil erodibility indices. Seven FTCs (0, 1, 3, 5, 7, 10 and 13) and three ISWCs (16.5, 24.8 and 33.0%) were employed to investigate soil shear behavior and disintegration properties for two Mollisol soils. Results showed that repeated FTCs had cumulative impacts on soil mechanical properties, and the 10th FTCs might be the threshold number for influencing soil erodibility. The soil shear strengths decreased by 1.1–15.1% and 9.2–30.5% under four normal stresses at the first and 13th FTCs, respectively, for these two soils. Soil cohesion generally decreased with the increase of FTCs. Soil internal friction angle showed no trend with FTCs at the 16.5% and 24.8% water contents but a decreasing trend at the 33.0% water content. Soil disintegration rate increased with FTC and reached a maximum value at the 10th cycle but decreased with the increase of ISWC. Soil disintegration of the two Mollisols showed a sharp disintegration of about 60% in 133 s. This study provides a new understanding of the response of erodibility to freeze-thaw conditions for two Mollisols. [ABSTRACT FROM AUTHOR]

Channels (rill, ephemeral gully, gully and river channel) exhibit a continuum of sizes and flow magnitudes. Rill erosion, as the initial stage of channel erosion, accounts for >80% of total eroded sediments on sloping farming lands in many parts of the world. Previous researches often regard rill erosion as an entirety or focus on individual rill erosion process separately. Few attentions have been paid to the contributions of individual processes including rill headcut advance, bed incision, sidewall expansion and their interactions to the over-all rill erosion. Thus, simulated upslope inflow experiments were designed to investigate the impacts of individual processes involved and their interactions in rill erosion under four inflow rates (1.0, 2.0, 3.0 and 4.0 L min−1) and two slope gradients (15° and 20°). Photogrammetry and manual sampling were used to measure hillslope morphology variation and sediment delivery, repectively. The results show that headcut advance, bed incision and sidewall expansion interact with each other and exhibit both independent and dependent features across spatial and temporal scales. Headcut advance interacts with bed incision and sidewall expansion before rill head advancing to a critical slope length. Initial rill depth and width are determined by initial headcut morphology. Bed incision and sidewall expansion dominate rill erosion before and after the non-erodible layer is exposed to concentrated flow, respectively. Headcut advance contributed the largest amount of rill erosion (44%–68%), followed by bed incision (27%–44%) and sidewall expansion (3.8%–12%). Headcut advance contributed more (63%–83%) to total rill width increment while bed incision contributed larger percentage (51%–65%) to total rill depth increment. Prediction equations for length, width and depth of a single rill on a loessial hillslope and empirical equations of rill erosion for the individual erosion processes were fitted and validated. Quantification and understanding of the contributions of individual rill erosion processes provides the necessary scientific basis for the development of process-based rill erosion models, and then, for preventions of soil losses and land degradation. • Contributions of individual rill erosion processes to rill development were quantified. • Prediction equations for length, width and depth of a single rill were fitted. • Empirical equations of rill erosion for the individual erosion processes were fitted. [ABSTRACT FROM AUTHOR]