Mapping agricultural subsurface drainage systems with GPR and UAVs: A research summary

Abstract

Due to economic and environmental considerations, there exists a need for effective, efficient, and nondestructive methods for locating buried agricultural drainage pipes. The traditional ways of locating buried drainage pipe involve the use steel rod tile probes or trenching machinery, which can be tedious, time consuming, and/or cause pipe damage. Under certain circumstances, ground penetrating radar (GPR) has proven to be a viable, nondestructive method of finding drainage pipes. The effectiveness of GPR drainage pipe detection is influenced by a number of factors including; soil type, shallow hydrologic conditions, antenna frequency, orientation of the drain line relative to the GPR measurement transect, drainage pipe depth, GPR equipment settings, and computer processing steps. By integrating GPR with Real-Time Kinematic (RTK) Global Navigation Satellite System (GNSS) technology, up to 20 ha of field area can be mapped for subsurface drainage in a single day. For field areas greater than 20 ha, a more efficient drainage mapping method is required, and imagery obtained by Unmanned Aerial Vehicles (UAVs) offer a potential solution. Fixed-wing UAVs can easily cover a 80 ha field area in a single one-hour flight. A fixed-wing UAV mounted with a visible color (VIS-C), multispectral (MS), or thermal infrared (TIR) camera was tested at agricultural field sites across the Midwest U.S. Overall results showed that UAV VISC, MS, TIR imagery detected at least some of the drainage pipe present at 48%, 59%, and 69% of the sites, respectively. To date, there have been five key findings. (1) Although TIR generally worked best, there were sites where either VIS-C or MS proved more effective than TIR for mapping subsurface drainage systems. (2) Timing of UAV surveys relative to recent rainfall can sometimes, but not always, have an important impact on drainage pipe detection. (3) Linear features representing drain lines and farm field operations can be confused with one another and are often both depicted on UAV imagery, but knowledge of subsurface drainage system installation and farm field operations can be employed to distinguish between the two. (4) The drain line response depicted by UAV TIR imagery can change during the day, and relative humidity, which also changes during the day, can impact TIR image quality. (5) Although UAV imagery obtained outside the growing season is generally better for drainage mapping, good results are sometimes achieved with crops in place. Although UAV surveys are more efficient than GPR for drainage mapping, GPR can still be useful for ground truth of drain line locations determined by UAV imagery. With GPR, drainage pipe depth information can be obtained, which is not possible with UAV imagery. Consequently, there are cases where the complementary employment of both GPR and UAV methods are needed for agricultural drainage mapping applications.