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

Authors: FLORES, LUKE - COLORADO STATE UNIVERSITY; BAILEY, RYAN - COLORADO STATE UNIVERSITY; Harmel, Daren

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2020
Publication Date: 1/20/2021
Citation: Flores, L., Bailey, R.T., Harmel, R.D. 2021. Using nutrient transport data to characterize and identify the presence of surface inlets in regions with subsurface drainage. Journal of Environmental Quality. 50(2):396-404. https://doi.org/10.1002/jeq2.20188.
DOI: https://doi.org/10.1002/jeq2.20188

Interpretive Summary: Surface inlets route ponded surface water into subsurface drainage pipe networks and are prevalent throughout North America. Though imperative for agro-economic sustainably, surface inlets serve as a nutrient loss pathway from agricultural landscapes, contributing to the degradation of downstream water quality. Despite this impact, 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 waste but are often site-specific and focused on individual precipitation events. Moreover, while their presence is commonly assumed, no regional surface inlets location database exists. To this end, a meta-analysis was undertaken with two goals. First, the MANAGE Drain Load database, consisting of nearly 1500 site-years of drainage and nutrient data, was analyzed to determine trends and distinctions between areas with and without surface inlets. The median annual total phosphorous load was greater at site-years with surface inlets (0.40 kg/ha) than site-years without surface inlets (0.21 kg/ha). The opposite trend emerged for dissolved nitrogen loads as site-years with surface inlets had a smaller median annual load (3.3 kg/ha) than site-years without surface inlets (34.1 kg/ha). These relationships can be attributed to immobile total phosphorous being transported primarily through overland flow, routed to subsurface drains via surface inlets, and relatively more mobile dissolved nitrogen being subsurface matrix-driven, bypassed in a setting with surface inlets. Second, based on these nutrient trends, a multiple logistic regression model was developed which predicts the presence of surface inlets within the MANAGE Drain Load database.

Technical Abstract: Surface inlets route ponded surface water into subsurface drainage pipe networks and are prevalent throughout the North America. Though imperative for agro-economic sustainably, surface inlets serve as a nutrient loss pathway from agricultural landscapes, contributing to the degradation of downstream water quality. Despite this impact, 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 often site-specific and focused on individual precipitation events. Moreover, while their ubiquitous presence is assumed, no regional surface inlets location database exists. To this end, a meta-analysis was undertaken with two goals. First, the MANAGE Drain Load database, consisting of nearly 1500 site-years of drainage and nutrient data, was analyzed to determine trends and distinctions between areas with and without surface inlets. The median annual total phosphorous (TP) load was greater at site-years with surface inlets (0.40 kg/ha) than site-years without surface inlets (0.21 kg/ha). The opposite trend emerged for dissolved nitrogen (DN) loads as site-years with surface inlets had a smaller median annual load (3.3 kg/ha) than site-years without surface inlets (34.1 kg/ha). This asynchronous relationship can be attributed to immobile TP being transported primarily through overland flow, routed to subsurface drains via surface inlets, and relatively more mobile DN being subsurface matrix-driven, bypassed in a setting with surface inlets. Second, based on these nutrient trends, a multiple logistic regression model was developed which predicts the presence of surface inlets within the MANAGE Drain Load database. Approximately 44% of site-years and 31% of study locations were predicted to have surface inlets.