ENHANCED MODELS AND CONSERVATION PRACTICES FOR WATERSHED RESOURCE MANAGEMENT AND ASSESSMENT
Location: Grassland, Soil and Water Research Laboratory
Title: The impact of asynchronicity on event flow estimation in basin scale hydrologic model calibration
Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 20, 2012
Publication Date: N/A
Interpretive Summary: Hydrologic models are commonly used by USDA and EPA to determine the environmental impacts of conservation practices and changing climate. Considerable errors in model results can occur due to synchronization errors in precipitation when a model continuously proceeds from one time step to the next (typically one day). The amount of runoff and pollutant loads tend to be underestimated due to these precipitation errors. In this study, a method was developed to better accommodate rainfall-runoff synchronization errors. Output from the SWAT (Soil and Water Assessment Tool) model compared to measured flows at the ARS Little River Experimental Watersheds in Tifton, Georgia, showed signifiant improvement using the method. The method improved model accuracy thus improving our confidence in conservation planning.
The calibration of basin-scale hydrologic models consists of adjusting parameters such that simulated values closely match observed values. However, due to inevitable inaccuracies in models and model inputs, simulated response hydrographs for multi-year calibrations will not be perfectly synchronized with observed response hydrographs at the daily time step. An analytically derived formula suggests that when timing errors are significant, traditional calibration approaches may generally underestimate the total event flow volume. An event-adaptive time series is developed and incorporated into the Nash-Sutcliffe Efficiency (NSE) objective function to diagnose the potential impact of event flow synchronization errors. Test sites are the 50 km**2 subwatershed l of the Little River Experimental Watershed (LREWswl) in southeastern Georgia, USA, and the 610 km**2 Little Washita River Experimental Watershed (LWREW) in southwestern Oklahoma, USA, with the Soil and Water Assessment Tool (SWAT) used as the hydrologic model. Results suggest that simulated surface runoff generation is 55% less for LREWswl when the daily time series is used compared to when the event-adaptive technique is used. Event flow generation may also be underestimated for LWREW, but to a lesser extent than it may be for LREWswl, due to a larger portion of the event flow being lateral flow.