Assessment of Macropore Component of RZWQM2 in Simulating Hourly Subsurface Drainage and Peaks

Abstract

<b><sc>Abstract.</sc></b> An understanding of preferential flow through soil macropores is critical to effectively manage subsurface drainage water quantity and quality. The objective of this study is to assess the macropore component of Root Zone Water Quality Model (RZWQM2) in simulating subsurface tile flow with a high time resolution. Observed hourly tile flow rate from two experimental sites in Ontario, Canada (2008-2011) and Iowa, USA (2007-2008) were used to evaluate the importance of including a macropore flow component in subsurface drainage simulation. Activating the macropore component in the model improved the simulation of drainage peaks, especially peak height, but it did not improve simulation of total drainage amount for each rainfall event. Simulation of the drainage peak recession varied from peak to peak, suggesting that further studies are warranted for drainage flow in the model. Sensitivity analyses showed that predicted total macropore flow and drainage peaks were insensitive to macroporosity and pore radius in the model, with a 50% change in the latter resulting in a <5% change in macropore flow. Activating the macropore component in RZWQM2 produced better simulation of drainage peaks during excess surface flow events but did not provide sufficient accuracy in representing the actual macropore flow process, which warrants further investigation and model modifications targeted towards improving RZWQM2‘s hydrologic simulation of macropore flow and subsurface drainage.