An Improved Express Fraction for Modeling Macropore/Subsurface Drain Interconnectivity

Abstract

The rapid transport of contaminants through macropores and into subsurface drains is a concern. Recent research has proposed methods for incorporating this direct connectivity into contaminant transport models. For example, the one-dimensional pesticide fate and transport model, Root Zone Water Quality Model (RZWQM), was modified to include an express fraction parameter based on the percentage of macropores in direct hydraulic connection to subsurface drains. When macropore flow first reached the top of the water table (point midway between the drains), a macropore express fraction of water and chemical was routed directly into the subsurface drain, which improved predictions of concentration peaks. The remaining water and chemical was allowed to fill and mix with the water table, resulting in a concentration bulge at the water table. This research proposes an updated express fraction for RZWQM, which distributes water across all saturated layers between the drain and water table. Implicitly assumed is a uniform spatial distribution of macropores. This updated express fraction is evaluated using data from two isoxaflutole/metabolite field experiments in Allen County and Owen County IN (2000), where concentrations of parent and metabolite were measured in the drain flow. The results showed a slight improvement in the prediction of chemical concentrations on the recession limbs of drainage hydrographs.