Abstract
Agricultural subsurface drainage systems are commonly installed on farmland to remove the excess water from poorly drained soils. Conventional methods for drainage mapping such as tile probes and trenching equipment are laborious, cause pipe damage, and are often inefficient to apply at large spatial scales. Knowledge of locations of an existing drainage network is crucial to understand the increased leaching and offsite release of drainage discharge and to retrofit the new drain lines within the existing drainage system. Recent technological developments in non-destructive techniques might provide a potential alternative solution. The objective of this study was to determine the suitability of unmanned aerial vehicle (UAV) imagery collected using three different cameras (visible-color, multispectral, and thermal infrared) and ground penetrating radar (GPR) for subsurface drainage mapping. Both the techniques are complementary in terms of their usage, applicability, and the properties they measure and were applied at four different sites in the Midwest USA. At Site-1, both the UAV imagery and GPR were equally successful across the entire field, while at Site-2, the UAV imagery was successful in one section of the field, and GPR proved to be useful in the other section where the UAV imagery failed to capture the drainage pipes’ location. At Site-3, less to no success was observed in finding the drain lines using UAV imagery captured on bare ground conditions, whereas good success was achieved using GPR. Conversely, at Site-4, the UAV imagery was successful and GPR failed to capture the drainage pipes’ location. Although UAV imagery seems to be an attractive solution for mapping agricultural subsurface drainage systems as it is cost-effective and can cover large field areas, the results suggest the usefulness of GPR to complement the former as both a mapping and validation technique. Hence, this case study compares and contrasts the suitability of both the methods, provides guidance on the optimal survey timing, and recommends their combined usage given both the technologies are available to deploy for drainage mapping purposes.
Highlights
In all the orthomosaics, the drain lines showed up as lighter shaded linear features due to a higher reflectance in the VIS, NIR, and red bands and a higher emitted thermal infrared (TIR) radiation from the drier soil above the drain lines compared to the wetter soil in between the drain lines
In the north-central region depicted in the enlarged inset of the TIR imagery (Figure 5b), the drain lines trend in the east–west direction to the eastern part and north–south direction within the western part
A hyperbolic response is caused by a point size object, i.e., either when the ground penetrating radar (GPR) traverse is perpendicular or at a somewhat modest to large angle (i.e., 15◦ < x◦ < 90◦ ) to the drain line orientation
Summary
Subsurface drainage systems are installed in agricultural areas to remove excess water and convert poorly drained soils into productive cropland. Some of the most productive agricultural regions in the world are a result of subsurface drainage practices [1]. Subsurface drainage provides many agronomic, economic, and environmental benefits by lowering the water table, enhancing optimal conditions for proper aeration of the plant roots and improving trafficability for timeliness of field operations, thereby increasing crop yields [2,3]. Drain lines shorten pathways for solute transport, causing increased leaching and offsite release of nutrients, pathogens, and pesticides from the agricultural areas, in turn increasing the potential risk for eutrophication and contamination of surface water bodies [4,5,6,7]. Knowledge of subsurface drainage system locations is important for the understanding of the local hydrology and solute dynamics for consequent planning of mitigation strategies such as constructed wetlands, saturated buffers, denitrifying bioreactors, and phosphate filters [8,9,10,11,12,13]
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