The effect of rill network planforms on hillslope rainfall-runoff and soil erosion processes is usually neglected in modeling practices, although they can markedly alter the hydrologic and geomorphic processes. Based on the CeRIRM model and WEPP erosion theory, a simple approach is developed to account for these effects. In the framework, several characteristic parameters including the average rill width, rill orientation angle, the number of rills, and number of discontinuous rills and their variations along the hillslope are introduced to represent the planar characteristics of the rill network. The model is tested against experimental erosion data from a hillslope subjected to three successive rainfall events, resulting in continuous rill network evolution. The results show that rill network planforms alter the partitioning of interrill and rill flows, thereby modifying the hydraulics, erosion, and sedimentation in the rills. Rill characteristics are found to significantly affect the amount of rill erosion. The new approach is compared to WEPP, which ignores rill network features. The WEPP approach of simulating rill erosion on hillslopes using one set of model parameters leads to errors that are up to 30% larger than the new approach that is able to account for spatially and temporally varying rill characteristics. The differences in cumulative rill erosion amounts between the two models vary significantly with slope length, slope angle, rill orientation angle, number of rills, and discontinuous rills. The results are valuable for the development of general rainfall-runoff and soil erosion models on hillslopes with complex geomorphic features. Plain Language Summary Rill erosion is one of the main forms of soil erosion and is associatedwith fast flowing water in networks of small channels, that is, rills. Modeling rill erosion is an important yet challenging task informing soil conservation. Although actual rill networks are usually composed of irregular, tortuous, and even discontinuous rills, using parallel and straight rills to simplify the actual rill network is still the prevailing method in current models. This study demonstrates the impact of the irregular planform of rill network on soil erosion and proposes a simple and effective conceptualization to represent complex rill networks using only a few parameters, including rill orientation angle, number of rills, and number of discontinuous rills. The approach can better represent the spatially intricate partitioning of interrill and rill flow and improve the modeling of rill hydraulics, erosion, and sedimentation prediction. Owing to its improved physical basis, the proposed approach simulates both the total amount and the spatial distribution of erosion with improved accuracy. The results of this study are expected to help improve general rainfall-runoff and soil erosion models for hillslopes with complex geomorphic features, and benefit other scientific fields involving transport through complex rill networks.

The slope effect on flow erosivity and soil erosion still remains a controversial issue. This theoretical framework explained and quantified the direct slope effect by coupling the modified Green‐Ampt equation accounting for slope effect on infiltration, 1‐D kinematic wave overland flow routing model, and WEPP soil erosion model. The flow velocity, runoff rate, shear stress, interrill, and rill erosion were calculated on 0°–60° isotropic slopes with equal horizontal projective length. The results show that, for short‐duration rainfall events, the flow erosivity and erosion amounts exhibit a bell‐shaped trend which first increase with slope gradient, and then decrease after a critical slope angle. The critical slope angles increase significantly or even vanish with increasing rainfall duration but are nearly independent of the slope projective length. The soil critical shear stress, rainfall intensity, and temporal patterns have great influences on the slope effect trend, while the other soil erosion parameters, soil type, hydraulic conductivity, and antecedent soil moisture have minor impacts. Neglecting the slope effect on infiltration would generate smaller erosion and reduce critical slope angles. The relative slope effect on soil erosion in physically based model WEPP was compared to those in the empirical models USLE and RUSLE. The trends of relative slope effect were found quite different, but the difference may diminish with increasing rainfall duration. Finally, relatively smaller critical slope angles could be obtained with the equal slope length and the range of variation provides a possible explanation for the different critical slope angles reported in previous studies.

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We have developed a highly efficient, one-step approach for the synthesis of core cross-linked star (CCS) polymers using commercial macromonomers (polyethylene glycol methyl ether methacrylates) as the arms via RAFT-mediated emulsion polymerization in aqueous media. This approach employs a small molecular chain transfer agent (CTA), commercial macromonomer, hydrophobic cross-linker, and optional hydrophobic spacing monomer as the polymerization recipe to synthesize CCS polymers via direct onestep polymerization in aqueous buffer solution. Various polymerization parameters, including buffer concentration and molar ratio of macromonomer/cross-linker/spacing monomer relative to CTA, were investigated. CCS polymers of high yield and low dispersity were obtained within 4 h under a wide range of conditions. Analysis of polymerization kinetics and macromolecular parameters of the generated polymeric species during the polymerization process led to insights into the mechanistic aspects of the CCS formation process, which was proposed to involve three stages, i.e., polymer chain growth, crosslinking to form CCS, and CCS growth. Finally, synthesis of CCS polymers using macromonomers of different molecular weights pointed to the necessity for optimization of the polymerization conditions for each macromonomer, possibly due to different polymerization rates and steric hindrance.

Whereas current erosion models are successful in quantitative estimates of soil erosion by water flow, modeling the coevolution of geomorphological features, particularly rill network properties and soil erosion on hillslopes, is still a major challenge. In this study, we propose a rill evolution modeling approach and combine it with a rainfall-runoff and soil erosion model to simulate the feedback loop of hillslope geomorphic development and soil erosion processes. Rill evolution is mainly characterized by three rill network attributes, rill density, orientation angle, and rill width, all modeled with physical equations. The entire rainfall-runoff-erosion and rill evolution model is tested against a set of rill network evolution and soil erosion data from an experimental hillslope subjected to successive rainfall events. The simulated spatial and temporal variations of rill network characteristics and soil erosion agree well with the measured data. The results demonstrate that the three rill network characteristics continually alter the partitioning of interrill and rill flows and affect the interrill and rill flow erosivity and soil erosion, which in turn modify the rill geometry and rill network planform. Comparatively, existing approaches such as Water Erosion Prediction Project (WEPP) that ignore the rill evolution processes largely underestimate the hillslope soil erosion when using time independent model parameters. Moreover, a sensitivity analysis indicates that both the rill evolution and soil erosion processes are sensitive to the rill evolution parameters, rainfall intensity, and slope angle. These results can inform the development of general geomorphic evolution and soil erosion models on evolving rilled hillslopes.