Evaluating Hydrologic Response in Tile‐Drained Landscapes: Implications for Phosphorus Transport

Abstract

Phosphorus (P) loss in agricultural discharge has typically been associated with surface runoff; however, tile drains have been identified as a key P pathway due to preferential transport. Identifying when and where these pathways are active may establish high‐risk periods and regions that are vulnerable for P loss. A synthesis of high‐frequency, runoff data from eight cropped fields across the Great Lakes region of North America over a 3‐yr period showed that both surface and tile flow occurred year‐round, although tile flow occurred more frequently. The relative timing of surface and tile flow activation was classified into four response types to infer runoff‐generation processes. Response types were found to vary with season and soil texture. In most events across all sites, tile responses preceded surface flow, whereas the occurrence of surface flow prior to tile flow was uncommon. The simultaneous activation of pathways, indicating rapid connectivity through the vadose zone, was seldom observed at the loam sites but occurred at clay sites during spring and summer. Surface flow at the loam sites was often generated as saturation‐excess, a phenomenon rarely observed on the clay sites. Contrary to expectations, significant differences in P loads in tiles were not apparent under the different response types. This may be due to the frequency of the water quality sampling or may indicate that factors other than surface‐tile hydrologic connectivity drive tile P concentrations. This work provides new insight into spatial and temporal differences in runoff mechanisms in tile‐drained landscapes. Core Ideas Activation of surface runoff and tile flow differ with soil texture and season. Timing of flow path activation was used to infer hydrological processes. Connectivity between the surface and tiles exists on clay soil during growing season. Rapid connectivity between the surface and tiles occurs less frequently on loam.

Impacts of preferential flow and agroecosystem management on subsurface particulate phosphorus loadings in tile‐drained landscapes

“…Our results provide insight into how preferential flow contributions are affected by desiccation crack networks that bypass the drainage layer during the growing season. Surface runoff was second greatest in the summer, which agrees well with other recent studies that have shown that high‐intensity rainfall on clay soils can trigger infiltration‐excess surface flow with either simultaneous or zero tile response (e.g., Kokulan, Macrae, Lobb, & Ali, 2019; Macrae et al., 2019). We found that quickflow contributions constituted 80% of tile flow during summer months, suggesting simultaneous connectivity of surface and tile pathways, which occurs in spring and summer in clay soils (Macrae et al., 2019).…”

“…Field‐scale nutrient studies in tile‐drained agroecosystems have evolved over the past 50 yr and now focus on methodologies for continuous monitoring of flow‐weighted mean concentrations (FWMCs) of dissolved and total nutrient species using automated samplers on surface flumes and subsurface tile mains (Harmel et al., 2018; Williams et al., 2016). Temporal and economic constraints often limit analyses that can be performed on samples, and, as a result, most long‐term monitoring programs typically only analyze bioavailable nutrients (orthophosphate, nitrate, and ammonium) and total nutrient concentrations (Christianson et al., 2016; Macrae, English, Schiff, & Stone, 2007; Macrae et al., 2019; Reba et al., 2013; Williams et al., 2016). Regarding total nutrients, both alkaline and alkaline/acid persulfate digestions on unfiltered samples have been used for coupled measurements of total P (TP) and total N (TN), although the alkaline persulfate digestions are recognized to underpredict TP when suspended sediment concentrations are significant (Dayton, Whitacre, & Holloman, 2017; Koroleff, 1983; Patton & Kryskalla, 2003).…”

Contribution of preferential flow to tile drainage varies spatially and temporally

“…The observed events accounted for ∼58% of annual flow at the clay site, and 38% of annual flow at the loam site during the study period (Macrae et al., 2019). The events covered a wide range of event types (e.g., snowmelt without rain, convection storms, rain on snow), event sizes, and seasons and were therefore an accurate and strong representation of climate and processes in the Great Lakes region.…”

“…Two sites in the Lake Erie watershed that have been used for ongoing agricultural runoff research (Macrae et al, 2019;Plach et al, 2019;Van Esbroeck, Macrae, Brunke, & McKague, 2016) were selected for further monitoring of preferential flow (Figure 1). One site, located near Essex, ON, was characterized by mesic Typic Argiaquoll clay soil (hereafter referred to as clay of 900 mm (10% as snow) and 1,024 mm (30% as snow) (ECCC, 2019).…”

Phosphorus mirabilis: Illuminating the Past and Future of Phosphorus Stewardship

“…This leads Smith et al (2019) to highlight the challenges of applying science to agricultural management, and call into question “one‐size‐fits‐all” approaches. Macrae et al (2019), for example, examine the activation of surface runoff and tile flow as pathways of P transport and demonstrate that region‐specific soil type and seasonality need to be considered when designing management strategies to decrease P loading via tile drains.…”