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

Environmental risk assessment of impacts due to agricultural subsurface drainage practices can benefit from information on drainage system outlet locations along 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. Consequently, the objective of this research was to provide a preliminary feasibility assessment on the applicability of UAV TIR imagery for locating these drainage system outlets. Drainage outlet detection with UAV TIR surveys was tested at five sites in Ohio, U.S.A. with a range of waterway sizes from drainage ditch to small and larger streams. There were four key findings from this preliminary study. (1) The UAV TIR surveys detected 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 change in apparent radiant temperature (Tᵣₐd) from upgradient of the outlet to downgradient of the outlet. (2) As shown by one example in this study, the outlet thermal response in cold winter months, when detectable, is likely the reverse of that obtained during warmer periods of the year. (3) The detected thermal response is rather subtle, exhibiting around a 1 °C ditch/stream water Tᵣₐd 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 larger sized streams. Since other factors in drainage ditches and streams may produce water Tᵣₐd variations that are of the same magnitude of a drainage system pipe outlet thermal response, the use of UAV TIR imagery as a means to map drainage system outlet locations might not be completely practical; however, more investigation is certainly warranted.