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. 1 INTRODUCTION Subsurface tile drainage is a known pathway for contaminant loss to downstream surface water bodies. Although tile drains have been shown to transport sediment (Estrany,

Tile drainage causes flashy streamflow response in Ohio watersheds

“…The inlets create a direct connection between field surfaces and tile outlets and could contribute to a flashier downstream hydrologic response. However, Flores et al (2021) found no statistical difference in annual drainage volume between sites with and without surface inlets. Using the relationships between annual dissolved nitrogen and phosphorus loads, Flores et al (2021) developed a logistic regression model that can be used to predict the presence of surface inlets.…”

“…These inlets can cause ponded surface water to completely bypass the soil matrix and provide a direct pathway for water, sediment, nutrients, and agrochemicals to enter waterways. Using 1500 site‐years of drainage and nutrient data, Flores et al (2021) calculated that annual phosphorus loads were twice as great in sites containing surface inlets (0.40 kg ha −1 ) compared to sites without (0.21 kg ha −1 ), but an opposite pattern for dissolved nitrogen was identified where sites with surface inlets had smaller annual losses (3.3 kg ha −1 ) compared to sites without surface inlets (23.0 kg ha −1 ). The inlets create a direct connection between field surfaces and tile outlets and could contribute to a flashier downstream hydrologic response.…”

“…Despite the high accuracy of AgTile-U.S. with the USDA Census of Agriculture for estimates of tile drainage in Ohio, the dataset does not provide information on the location and extent of surface inlets to tile drainage used to drain closed depressions. The impacts of surface inlets are thought to be underreported in drainage studies, possibly due to the lack of available databases at regional, state, or county levels that provide the location of these structures (Flores et al, 2021). The number of surface inlets in the western Lake Erie basin for example has been estimated to be as high as 75 000 (Feyereisen et al, 2015).…”

Updates to the MANAGE database to facilitate regional analyses of nutrient runoff

“…Specifically, Harmel (2015a, 2015b), Christianson et al (2016), and Hertzberger et al (2019aHertzberger et al ( , 2019b added data from 79 studies of drainage N and P concentrations and loads mainly from the midwestern and eastern United States. In addition, the MANAGE database has been used for meta-type analyses of management impacts on drainage nutrient losses (Flores et al, 2021;Hertzberger et al, 2019a;Liu et al, 2021;Zhao et al, 2016) and on the impact of agricultural conservation practices on nutrient losses in surface runoff (Nummer et al, 2018;Qian & Harmel, 2016;Reckhow et al, 2009). The database has also been used to evaluate modeling tool predictions of best management practices effectiveness, regional water balances, and regional ranges of runoff, sediment, and nutrient loss (e.g., Merriman et al, 2009;White et al, 2010White et al, , 2015aWhite et al, , 2015b.…”