Surface‐to‐tile drain connectivity and phosphorus transport: Effect of antecedent soil moisture

Abstract

Macropores connecting surface soils to tile drains can alter water and nutrient transport through the subsurface. In this study, laboratory rainfall simulations with artificial macropores combined with edge‐of‐field monitoring were used to evaluate surface‐to‐tile drain connectivity and phosphorus (P) transport as a function of antecedent moisture conditions. Laboratory rainfall simulations using repacked soil boxes with different macropore layouts (i.e., no macropore, surface‐connected macropores, and disconnected macropores) were used to examine changes in water sources and flow pathways to tile drains with varying degrees of connectivity and antecedent wetness. Water, tracer, and P fluxes from a tile‐drained field were also monitored to quantify linkages among water flow pathways, antecedent wetness, and P delivery to tile drains. Both laboratory and field results showed that surface‐to‐tile drain connectivity was important for water transport through the subsurface under both dry and wet antecedent conditions. When soil conditions were dry, discharge was minimal and primarily comprised of event water that bypassed the soil matrix. Increasing wetness resulted in similar event water transport, but greater mobilization of stored pre‐event water and greater discharge; thus, the dominant source of tile water and the magnitude of tile discharge were substantially altered with changing antecedent moisture. Field data revealed that changes in drainage water source and discharge with increasing wetness impacted dissolved P transport. Dissolved P concentration decreased and loading increased with increasing wetness. Findings indicate that greater mobilization of pre‐event water under wet antecedent conditions acted as both a hydrologic and chemical buffer for subsurface dissolved P transport. Comparison of study results to water quality data from a larger edge‐of‐field network suggest that relationships between antecedent moisture conditions, water flow pathways, and P transport from the current study are broadly applicable across tile‐drained fields. Understanding processes controlling P delivery to tile drains has direct applicability for conservation practice implementation and improving process representation in models.

Preferential flow in the shallow vadose zone: Effect of rainfall intensity, soil moisture, connectivity, and agricultural management

“…Event water fractions increased from 3%–7% during rainfall simulations 1–3 when leachate from these lysimeters was slow to negligible to 92% following the installation of the macropores in rainfall simulation 5. Previous studies using artificial macropores have also noted their strong influence on soil hydrology (Akay & Fox, 2007) and event water transport (Williams, Penn, et al, 2023). Only lysimeter 10 was similar to the magnitude of event water fraction observed in the artificial macropore lysimeters (Figure 4).…”

“…Rapid mobilization of pre‐event water has been reported in tile‐drained fields because of matrix‐macropore interaction (i.e., preferential flow of old water; Klaus et al, 2013). Laboratory soil column studies with artificial macropores have also found that 9% of water transported through the macropore was pre‐event water (Williams, Penn, et al, 2023). Results from the current study therefore indicate that soil physical properties such as soil texture and infiltration rate may be more important than rainfall intensity for determining the mixing (and subsequent transport) of water in the shallow vadose zone.…”

“…An artificial macropore was added to these three lysimeters so we could continue to use them as part of the study and provided an opportunity to examine the effect of direct surface‐to‐subsurface connectivity on water fluxes through the shallow vadose zone. Following the methods of Williams, Penn, et al (2023), artificial macropores were created with a drill and extended from the soil surface to the perforated plate at the bottom of the lysimeter. A small rainfall simulation (15 mm) was conducted on all 10 lysimeters after an artificial macropore was added to lysimeters 2, 5, and 8 to help stabilize the artificial macropore walls and flush any loose sediment prior to proceeding to rainfall simulation 4.…”

“…Tillage of agricultural soils, for example, may result in smaller macropores and less‐developed pore networks in surface soil horizons compared to no‐tillage practices (Budhathoki et al, 2022; Hellner et al, 2018). Laboratory experiments have demonstrated the effect of this connectivity on subsurface flows, with surface‐connected macropores substantially influencing flow through the subsurface compared to homogeneous soil (Akay & Fox, 2007; Williams, Penn, et al, 2023). Disrupted surface macropore networks resulting from tillage may therefore lead to decreased preferential flow (Cullum, 2009; Williams et al, 2016).…”

Tile drainage as a driver of streamflow flashiness in agricultural areas of the Midwest, USA

“…The variability of the magnitude, intensity, and location of precipitation are all lost, especially in larger watersheds (Singh, 1997), since precipitation is a basin‐wide and multi‐year seasonal average in this study. Moreover, the amount of runoff generated by precipitation can also be influenced by antecedent soil moisture, which can increase the variability of an area's runoff response (Tarasova et al, 2018; Williams et al, 2023). The multivariate regression found that the strength of the relationship between ETₒ and RBI was weak and insignificant.…”