Time of day impact on mapping agricultural subsurface drainage systems with UAV thermal infrared imagery

Authors: Allred, Barry; Martinez, Luis; FESSEHAZION, MELAKE - Volunteer; ROUSE, GREG - Ross County Soil & Water Conservation District; KOGANTI, TRIVEN - Aarhus University; FREELAND, ROBERT - University Of Tennessee; EASH, NEAL - University Of Tennessee; WISHART, DEBONNE - Central State University; FEATHERINGILL, ROBERT - Retired Non ARS Employee

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2021
Publication Date: 7/20/2021
Citation: Allred, B.J., Martinez, L.R., Fessehazion, M., Rouse, G., Koganti, T., Freeland, R., Eash, N., Wishart, D., Featheringill, R. 2021. Time of day impact on mapping agricultural subsurface drainage systems with UAV thermal infrared imagery. Agricultural Water Management. 256: Article 107071. https://doi.org/10.1016/j.agwat.2021.107071.
DOI: https://doi.org/10.1016/j.agwat.2021.107071

Interpretive Summary: There are both economic and environmental reasons for finding viable, effective, efficient, and nondestructive means for mapping agricultural subsurface drainage systems. Previous research indicate that unmanned aerial vehicle (UAV) thermal infrared (TIR) imagery is a potential drainage mapping solution. Knowing the best times of day to conduct UAV TIR surveys for drainage mapping is important for the successful employment of this technology within agricultural settings. In order to determine the best times for carrying out a UAV TIR drainage mapping survey, a set of sunrise to sunset UAV TIR surveys were carried out at each of four different farm fields in Ohio, USA, with one of the fields visited twice. The key findings of this research are listed as follows. 1) Even under surface conditions ranging from extremely dry to extremely wet, UAV TIR surveys were able to measure small apparent radiant temperature differences of less than 1.0 degrees C between the soil surface over drain lines versus the soil surface between drain lines, thereby allowing subsurface drainage pipe network patterns to be mapped. 2) High relative humidity (>60%), in some cases, can adversely affect UAV TIR image quality, causing inability to produce a complete orthomosaic map that can be used for drainage mapping. 3) The best time of time day to conduct a UAV TIR survey for drainage mapping, based strictly on consistency of success, is late morning through late afternoon. For UAV TIR surveys carried out from late morning through late afternoon, drainage pipe locations are indicated by lighter shaded linear features within the orthomosaic maps generated from survey imagery. These lighter shaded linear features are indicative of the soil surface apparent radiant temperature over the drain line being greater than between drain lines. 4) Drain line locations were indicated by darker shaded linear features in some of the orthomosaic maps generated from UAV TIR surveys conducted near sunrise or sunset. This darker shaded linear feature TIR response is caused by apparent radiant temperature in the soil surface over drain lines being less than the soil surface apparent radiant temperature between drain lines. These sunrise/sunset orthomosaic maps, with drain lines represented by darker shaded linear features, are excellent for determining subsurface drainage network patterns. However, a major disadvantage to sunrise/sunset UAV TIR surveys is the high relative humidity occurring during these times of the day, which can degrade the quality of the TIR images collected and prevent construction of an orthomosaic map. 5) For site cases in which the TIR drain line response changes back and forth between lighter shaded linear features and darker shaded linear features, there are transition periods where no TIR drain line response is observed (i.e. apparent radiant temperature is the same for the soil surface over drain lines as between drain lines). Early morning and early evening are the times of day that these transitions are most likely to happen and should be avoided for UAV TIR drainage mapping surveys. Future research should focus on developing site condition guidelines for UAV TIR drainage mapping; test plot experiments to quantify the ground surface TIR response to soil type, dryness/wetness, crop residue/stubble cover, etc.; evaluation of advanced image processing techniques that better highlight TIR drain line responses; and development of machine learning procedures to automatically differentiate TIR drain line responses from those due to farm field operations.

Technical Abstract: Due to economic and environmental considerations, there exists a need for effective, efficient, and nondestructive methods for locating buried agricultural drainage pipes. Previous research indicates that thermal infrared (TIR) imagery obtained with an unmanned aerial vehicle (UAV) has potential for mapping subsurface agricultural drainage systems, thereby warranting further investigation to determine the best time of day to conduct these UAV TIR surveys. Accordingly, a set of sunrise to sunset UAV TIR surveys were carried out at four different farm field sites in Ohio, U.S.A. Late morning through late afternoon UAV TIR surveys were generally found to work well for determining drainage system patterns. During late morning through late afternoon, the apparent radiant temperature of the soil surface over the drain lines was higher than between the drain lines (i.e. emitted TIR radiation from the soil surface over a drain line was greater than between the drain lines). Conversely, near sunrise or sunset, the UAV surveys often showed the apparent radiant temperature of the soil surface over the drain lines to be lower than between the drain lines (i.e. less emitted TIR radiation over the drain lines than between drain lines). Even though some excellent UAV TIR drainage pipe mapping results were obtained near sunrise/sunset, difficulties were occasionally encountered processing this TIR imagery, likely due to the impact on image quality from high relative humidity during these times of the day. Consequently, strictly on a consistency of success basis, late morning through late afternoon are the best times for locating drainage pipes with UAV TIR surveys, but in certain cases, UAV TIR surveys at sunrise/sunset can provide exceptional drainage pattern mapping results.