Hydrologic assessment of blind inlet performance in a drained closed depression

Abstract

Surface inlets installed in agricultural closed depressions can transport sediment- and nutrient-laden runoff directly to receiving streams or water bodies. Replacing tile risers with blind inlets has the potential to decrease these sediment and nutrient loads, but adoption of blind inlets has been met with hesitation from producers who fear that changes to the surface inlet may result in additional water ponding, crop damage, or crop loss. The objectives of this study were to assess hydrologic performance of blind inlets over time, and to determine whether blind inlets influenced the hydrology of a closed depression compared to a tile riser. Hydrograph characteristics including flow duration, time to peak flow, peak flow rate, and cumulative flow were evaluated for 549 storm events over a 12-year period (2006 to 2017) for a pair of closed depressions in northeastern Indiana. Surface water could be drained in both depressional areas with either a tile riser or blind inlet. Results showed that blind inlet infiltration rates declined approximately linearly over time (1.4 cm h⁻¹ y⁻¹). While changes in infiltration were largely controlled by annual tillage practices at the study sites, findings suggest that blind inlets have an effective service life of 8 to 10 years under the study conditions. Surface water management practices significantly influenced depression hydrology; however, the practices functioned differently between fields. Blind inlets did not influence the frequency of discharge, but they may increase or decrease the duration of flow and cumulative flow compared to a tile riser depending on the extent of subsurface tile drainage within the closed depression. Findings from this study underscore the inherent connections between surface and subsurface hydrologic processes in drained closed depressions and how these connections may influence both water quantity and water quality.