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

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 agricultural subsurface 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). Some excellent UAV TIR drainage mapping results were obtained near sunrise/sunset due to TIR drain line responses that were more easily distinguished from those of farm field operations. However, difficulties were occasionally encountered processing this sunrise/sunset 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 alone, late morning through late afternoon are the best times for locating drainage pipes with UAV TIR surveys; however, in certain cases, UAV TIR surveys at sunrise/sunset can provide exceptional drainage pattern recognition. These results provide valuable guidance for those considering UAV TIR drainage mapping surveys.