Using nutrient transport data to characterize and identify the presence of surface inlets in regions with subsurface drainage

Abstract

Surface inlets route ponded surface water into subsurface drainage networks and are prevalent throughout North America. Despite serving as a nutrient loss pathway, contributing to downstream water quality degradation, surface inlets are thought to be underreported in drainage studies within the literature. Previous studies have demonstrated the footprint that surface inlets have on nutrient transport and drainage effluent but are site specific and focused on individual events. Moreover, although their ubiquitous presence is assumed, no regional surface inlet database exists. To this end, a structured review was undertaken with two goals. First, the MANAGE Drain Load database, consisting of nearly 1,500 site-years of drainage and nutrient data, was analyzed to determine distinctions between areas with and without surface inlets. The median annual total phosphorus (TP) load was greater at site-years with surface inlets (0.40 kg ha^-1 ) than site-years without (0.21 kg ha^-1 ). The opposite emerged for dissolved nitrogen (DN) loads as site-years with surface inlet had a smaller median annual load (3.3 kg ha^-1 ) than site-years without (23.0 kg ha^-1 ). This relationship is attributed to immobile TP being transported primarily through overland flow and routed to subsurface drains via surface inlets and to relatively more mobile DN being subsurface driven, bypassed in settings with surface inlets. No statistical differences were found in annual drainage or ratios of particulate P to TP between site-years with and without surface inlets. Second, a logistic regression model was developed that predicts the presence of surface inlets within MANAGE. Eighteen percent of site-years and 21% of sites were predicted to have surface inlets.