Comprehensive modelling of overland flow requires models for both rill and interrill area overland flow. Evaluation of a physically based mathematical model for simulating overland flow generated on rill and interrill areas of hillslope was done using a data set gathered from a laboratory experimental setup. A rainfall simulator has been constructed together with a 6.50 m × 1.36 m erosion flume that can be given adjustable slopes in both longitudinal and lateral directions. The model was calibrated and validated using the experimental results from the setup of the flume having 5% lateral and 10% longitudinal slopes where rainfall intensities of 105 and 45 mm/hr were induced with the use of nozzles. Results show that for the given slope combination, the model was capable of simulating the flow coming from the rill and interrill areas for the two different rainfall intensities. It was found that significantly more of the flow occurred in the form of the rill flow. The model studied here can be used for the better prediction of overland flow and can also be used as a building block for an associated erosion and sediment transport model.

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Climate change can escalate rainfall intensity and cause further increase in sediment transport in arid lands which in turn can adversely affect water quality. Hence, there is a strong need to predict the fate of sediments in order to provide measures for sound erosion control and water quality management. The presence of microtopography on hillslopes influences processes of runoff generation and erosion, which should be taken into account to achieve more accurate modelling results. This study presents a physically based mathematical model for erosion and sediment transport coupled to one-dimensional overland flow equations that simulate rainfall-runoff generation on the rill and interrill areas of a bare hillslope. Modelling effort at such a fine resolution considering the flow connection between interrill areas and rills is rarely verified. The developed model was applied on a set of data gathered from an experimental setup where a 650 cm×136 cm erosion flume was pre-formed with a longitudinal rill and interrill having a plane geometry and was equipped with a rainfall simulator that reproduces natural rainfall characteristics. The flume can be given both longitudinal and lateral slope directions. For calibration and validation, the model was applied on the experimental results obtained from the setup of the flume having 5% lateral and 10% longitudinal slope directions under rainfall intensities of 105 and 45 mm/h, respectively. Calibration showed that the model was able to produce good results based on the R 2 (0.84) and NSE (0.80) values. The model performance was further tested through validation which also produced good statistics (R 2 =0.83, NSE=0.72). Results in terms of the sedigraphs, cumulative mass curves and performance statistics suggest that the model can be a useful and an important step towards verifying and improving mathematical models of erosion and sediment transport.

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