Location: National Soil Erosion ResearchTitle: Thresholds for runoff generation in a drained closed depression
Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2019
Publication Date: 4/30/2019
Citation: Williams, M.R., Livingston, S.J., Heathman, G.C., McAfee, S.J. 2019. Thresholds for runoff generation in a drained closed depression. Hydrological Processes. https://doi.org/10.1002/hyp.13477.
Interpretive Summary: Many closed depressions or ‘potholes’ across the U.S. Midwest have been drained and converted to highly productive agricultural land. While depressions are a common landscape feature, little information exists on how rainfall and soil moisture conditions influence surface runoff and subsurface tile drain flow. We examined 289 rainfall events greater than 0.25 inches over a 9-year period (2006-2014) and surface runoff and tile flow from two closed depressions in northeastern Indiana. Results showed that surface runoff and tile drain flow did not occur unless rainfall amount exceeded the capacity of the soil to store water. For all rainfall events, tile drain flow began before surface runoff, which was likely due to the formation of perched groundwater tables on the sideslopes of the depression. Study findings highlight the impact of tile drainage on water transport and indicate that water management strategies will be important for decreasing nutrient loss from closed depressions. Results also suggest that the potential risk of nutrient loss after fertilizer application would be greatest during the late-winter through spring (March through May), as only a small amount of rainfall is required to trigger flow.
Technical Abstract: Hydrological threshold behavior has been observed across hillslopes and catchments with varying characteristics. Few studies, however, have evaluated rainfall-runoff response in areas dominated by agricultural land use and artificial subsurface drainage. Hydrograph analysis was used to identify distinct hydrological events over a nine-year period and examine rainfall characteristics, dynamic water storage, and surface and subsurface runoff in a drained and farmed closed depression in northeastern Indiana, USA. Results showed that both surface flow and subsurface tile flow displayed a threshold relationship with the sum of rainfall amount and soil moisture deficit (SMD). Neither surface flow nor subsurface tile flow were observed unless rainfall amount exceeded the SMD. Timing of subsurface tile flow relative to soil moisture response on the shoulder slope of the depression indicated that the formation and drainage of perched water tables on depression hillslopes was likely the main mechanism that produced subsurface connectivity. Surface flow generation was delayed compared to subsurface tile flow during rainfall events due to differences in soil water storage along depression hillslopes and runoff generation mechanisms. These findings highlight the substantial impact of subsurface tile drainage on the hydrology of closed depressions; the bottom of the depression, the wettest area prior to drainage installation, becomes the driest part of the depression after installation of subsurface drainage. Rapid connectivity of localized subsurface saturation zones during rainfall events is also greatly enhanced because of subsurface drainage. Thus, less fill is required to generate substantial spill. Understanding hydrologic processes in drained and farmed closed depressions is a critical first step in developing improved water and nutrient management strategies in this landscape.