Subsurface drainage outlet detection in ditches and streams with UAV thermal infrared imagery: preliminary research

Authors: Allred, Barry; Martinez, Luis; KHANAL, SAMI - The Ohio State University; SAWYER, AUDREY - The Ohio State University; ROUSE, GREG - Ross County Soil & Water Conservation District

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/17/2022
Publication Date: 6/7/2022
Citation: Allred, B.J., Martinez, L.R., Khanal, S., Sawyer, A., Rouse, G. 2022. Subsurface drainage outlet detection in ditches and streams with UAV thermal infrared imagery: preliminary research. Agricultural Water Management. Article 107737. https://doi.org/10.1016/j.agwat.2022.107737.
DOI: https://doi.org/10.1016/j.agwat.2022.107737

Interpretive Summary: Agricultural drainage practices release nitrate and phosphate from farm fields into adjacent waterways that in turn have unfavorable environmental consequences on local, regional, and national scales. Watershed scale risk assessment of these adverse impacts due to agricultural subsurface drainage practices could benefit from information on subsurface drainage system pipe outlet locations along drainage ditches and streams. Unmanned aerial vehicle (UAV) thermal infrared (TIR) surveys may provide a means, through thermal response recognition, to map these subsurface drainage system pipe outlets that in the Midwest U.S.A. are typically installed within the bank of a ditch or stream channel. Drainage system pipe outlet detection capabilites with UAV TIR surveys were tested at five sites in Ohio, U.S.A. with a range of waterway sizes from drainage ditch to small, medium and large streams. The key findings of this preliminary study are listed as follows. 1) the UAV surveys did detect the ditch/stream water thermal responses associated with drainage system pipe outlets at both a drainage ditch and a small stream. This thermal response is characterized by a ditch/stream water temperature change from upgradient of the outlet to downgradient of the outlet. 2) When detectable, this thermal response in cold winter months may be the reverse of what is obtained during warmer periods of the year. In cold winter months, the ditch/stream water upgradient of the outlet may be colder than downgradient, while during warmer periods of the year, the ditch/steam water upgradient of the outlet may be warmer than downgradient. 3) The thermal response, when detectable, is rather subtle, exhibiting around a 1 OC ditch/stream water temperature change from upgradient of the outlet to downgradient of the outlet. 4) The UAV TIR surveys did not detect thermal responses associated with drainage system pipe outlets at three sites with moderate and large sized streams. This finding for the larger streams in this study may be due to discharge from the drainage system pipe outlet being exceeding small by comparison to the flow in the adjacent stream channel, which in turn prevents the extent of development for a thermal response that is recognizable with UAV TIR imagery. Other factors in drainage ditches and streams (i.e. channel width/depth, bed roughness affecting turbulence, and water turbidity) may produce water temperature variations that are of the same magnitude of a subsurface drainage system pipe outlet thermal response. Consequently, the use of UAV TIR imagery as a means to map pipe outlet locations might not be totally feasible for situations typical of the Midwest U.S.A.; however, more research is certainly warranted before making any final conclusions. On the other hand, where pipe outlet locations are known to produce a detectable thermal response, UAV TIR surveys conducted over time could provide valuable insight regarding the overall hydrologic impact on a ditch/stream due to a subsurface drainage system pipe outlet. The potential use of UAV TIR imagery for quantification to some extent of the hydrologic impacts of drainage system outlets on ditches/streams is definitely a promising avenue for future research.

Technical Abstract: Watershed scale risk assessment of the adverse environmental impacts due to agricultural subsurface drainage practices could benefit from information on subsurface drainage system pipe outlet locations along drainage ditches and streams. Unmanned aerial vehicle (UAV) thermal infrared (TIR) surveys may provide a means, through thermal response recognition, to map these subsurface drainage system pipe outlets. Drainage system pipe outlet detection capabilites with UAV TIR surveys were therefore tested at five sites in Ohio, U.S.A. with a range of waterway sizes from drainage ditch to small, medium and large streams. There were four key findings from this preliminary study. (1) The UAV surveys did detect the ditch/stream water thermal responses associated with drainage system pipe outlets at both a drainage ditch and a small stream. This thermal response is characterized by a ditch/stream water temperature change from upgradient of the outlet to downgradient of the outlet. (2) When detectable, this thermal response in cold winter months may be the reverse of what is obtained during warmer periods of the year. (3) The thermal response, when detectable, is rather subtle, exhibiting around a 1 OC ditch/stream water temperature change from upgradient of the outlet to downgradient of the outlet. (4) The UAV TIR surveys did not detect thermal responses associated with drainage system pipe outlets at three sites with moderate and large sized streams. Since other factors in drainage ditches and streams may produce water temperature variations that are of the same magnitude of a subsurface drainage system pipe outlet thermal response, the use of UAV TIR imagery as a means to map pipe outlet locations might not completely practical. However, more investigation is certainly warranted not only on UAV TIR outlet mapping, but also UAV TIR qualitative/quantitative assessment of overall waterway hydrologic impacts due drainage outlet presence.