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Title: Soil pipe flow tracer experiments: 1. Connectivity and transport characteristics

Author
item Wilson, Glenn
item Rigby Jr, James
item Ursic, Michael - Mick
item Dabney, Seth

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/12/2015
Publication Date: 4/15/2016
Publication URL: http://handle.nal.usda.gov/10113/62652
Citation: Wilson, G.V., Rigby Jr, J.R., Ursic, M.E., Dabney, S.M. 2016. Soil pipe flow tracer experiments: 1. Connectivity and transport characteristics. Hydrological Processes. 30(8):1265-1279.

Interpretive Summary: Soil pipes, defined here as large flow paths generally parallel to the slope, play an important role in many soil erosion problems. Much debate has occurred in hydrology regarding the connection of subsurface flow paths from upslope areas to catchment outlets. This study was conducted in two catchments, one with three upper branches and one with a single branch and each of these having a soil pipe. The four soil pipes have eroded the subsurface to the point of collapse to form over 100 collapse features on the surface including gullies, sinkholes, and flute holes (small openings at the surface). These catchments were formed in a loess soil with a soil layer that fosters lateral flow. The study aimed to determine the connectivity of the pipe collapse features for the four soil pipe networks as well as determine pipe flow velocities. Dye was injected directly into each soil pipe at the upper most pipe collapse feature. The change in dye concentration with time, call a breakthrough curve (BTC), was determined by sampling multiple pipe collapse features downslope of the release. The BTCs were used to determine the average and maximum flow velocities. This study confirmed that these catchments contain four distinct soil pipes that are continuous for over 190 m and connect the upper most hillslopes areas with the catchment outlet. While the flow paths are continuous, the individual pipe networks consist of alternating sections of subsurface flow through soil pipes and sections of surface flow through gullies formed by pipe collapses. In addition, flow can be occurring both through the subsurface soil pipes simultaneous with surface flow generated by the soil pipes. The pipe flow velocities were as high as 0.3 meters per second which was in the range of streamflow velocities. These pipe flow velocities were also in the range of velocities observed in erosion tests suggesting that these large, mature soil pipes are still actively eroding.

Technical Abstract: Much debate has occurred in catchment hydrology regarding the connectivity of flow paths from upslope areas to catchment outlets. This study was conducted in two catchments, one with three upper branches, in a loess soil with a fragipan that fosters lateral flow and exhibits an extensive distribution of soil pipe collapse features. The study aimed to determine the connectivity of multiple soil pipe networks as well as determine pipe flow velocities during storm events. Fluorescein dye was injected directly into soil pipes at the upper most pipe collapse feature of four different hillslopes. Breakthrough curves (BTC) were determined by sampling multiple pipe collapse features downslope. The BTCs were used to determine the “average” (center of mass) and “maximum” (first arrival) flow velocities. This study confirmed that these catchments contain individual continuous soil pipe networks that extend over 190 m and connect the upper most hillslopes areas with the catchment outlet. While the flow paths are continuous, the individual pipe networks consist of alternating reaches of subsurface flow through soil pipes and reaches of surface flow through gullies formed by pipe collapses. In addition, flow can be occurring both through the subsurface soil pipes simultaneous with surface flow generated by artesian flow from the soil pipes. The pipe flow velocities were as high as 0.3 m/s which was in the range of streamflow velocities. These pipe flow velocities were also in the range of velocities observed in pinhole erosion tests suggesting that these large, mature soil pipes are still actively eroding.