Predicting Sediment Detachment and Channel Scour in the Process-Based Planning Model ANSWERS-2000

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Virginia Tech

ANSWERS-2000, a continuous simulation, distributed parameter nonpoint source model for simulating runoff, sediment, and nutrients from disturbed watersheds was updated to include a critical-shear rill detachment subroutine, an improved interrill detachment subroutine, and a channel scour subroutine. The existing version of ANSWERS-2000 did not simulate channel scour. The original detachment equations used in the model were developed through regression analysis of data from a northeast Indiana watershed, and were not directly applicable to other watersheds. The new detachment equations are based on process-oriented equations developed for the WEPP model and they can be applied to ungaged watersheds with a wide variety of soils and land use conditions.

The new model (ANSWERS-2000) was evaluated on three watersheds and its predictions were compared with the previous version of ANSWERS. On the largest watershed (2070 hectares), both models appeared to predict sediment loss adequately. On the second watershed (1053 hectares), ANSWERS-2000 improved sediment yield predictions compared to the original model. Neither model adequately described sediment loss from the smallest watershed (1.2 ha) used for validation. The sediment prediction errors were caused by errors in runoff prediction, despite an attempt to increase runoff prediction accuracy by calibration. The channel scour subroutine was evaluated by comparison with observed channel erosion data from the largest watershed. The new model appeared to consistently over predict scoured depth. However, more research is required to determine if the channel scour component is flawed or if the uncertainty in the observed channel scour data was large as suspected. Ultimately, the erosion prediction accuracy of the ANSWERS-2000 model was improved in two of the three upland evaluation data sets. The channel scour data set was not successfully validated. Model input parameter requirements increased significantly as a result of the new erosion and channel scour submodels.

NPS, sediment, erosion, nonpoint source, critical-shear, model