Rethinking percentage‐based approaches for edge‐of‐field practice design flow rates

Additional flexibility in the design of edge‐of‐field conservation practices may help scale their implementation. Subsurface drainage flow data from fields ranging from 6.9 to 29 ha in Illinois (25 site‐years) were used to assess design paradigms for denitrifying bioreactors. Implementing bioreactors at larger areas (e.g., > 20 ha vs. < 10 ha) could facilitate treatment of relatively greater flow volumes, and thus greater nutrient loads, even if a notable portion of the annual flow bypasses treatment. For example, a hypothetical bioreactor design flow rate of approximately 1.8 L/s resulted in mean treatment of 40,600 versus 15,300 m 3 /year at large versus small sites assessed here. This flow rate corresponded to 5% versus 50% non‐exceedance probabilities, respectively, meaning the design for the large drainage area would not meet current design expectations of 15%. Treatment of greater flow volumes at relatively larger sites challenges the idea that treating the majority of annual flow is inherently necessary, especially where watershed goals are based on overall load reduction. Core Ideas Subsurface drainage flow data were used to assess edge‐of‐field conservation practice design procedures. Flow volume and flow rate trends differed between small versus large drainage areas. Using similar design flow rates, more flow could be treated at larger versus smaller sites, despite much bypass flow. A design proxy flow rate showed bioreactors on larger systems better utilized the full bioreactor capacity. Practices need to be as effective as practical at the site level while also removing as much load as possible.