Predicting Sediment Transport by Coupling Sediment Connectivity With the Unit Sediment Graph: Method Development and Watershed-Scale Application

The unit sediment graph approach, analogous to the unit hydrograph method, was rarely applied in the past 50 years, presumably due to limitations from scaling the sediment kernel. We hypothesised that spatially explicit sediment connectivity modelling might be combined with unit sediment graph theory to estimate sediment source zones and time of mobilisation across the watershed and estimate sediment flux for hydrologic events. We formulated the model using the probability of sediment connectivity with log-normal parameterisation of the 1-h unit sediment graph. Simulations were carried out for a sediment transport application in a third-order watershed in Kentucky, USA, using a two-stage calibration procedure assisted by a high-performance computing cluster. Results showed sufficient evidence for the efficacy of the approach, including Nash-Sutcliffe Efficiency as high as 0.87 and 0.84 in Stages 1 and 2, respectively, of calibration and 0.88 for model validation. Results of the probability of connectivity showed variability across and within transport events, and 7.5% connectivity for the high flow isolated event. The log-normal distribution effectively estimated the rising limb and the falling limb of the sediment graphs. Post-processing of modelling results showed the importance of the probability of sediment connectivity, as simulations omitting it produced inadequate results. Post-processing with a shallow artificial neural network model showed that both sediment connectivity and surface runoff control sediment yield at the event scale. Results showed the ability of the hourly time step to capture the onset of sediment connectivity and peak connectivity across the ephemeral network.