Aerial Mapping of Agricultural Subsurface Drainage Systems for Renewable Energy Projects

For solar and wind energy projects in agricultural areas knowing the locations of drain tiles is crucial to minimize the risk of pipe damage during construction activities (e.g., support columns). The usual methods of drainage mapping include the use of tile probes and trenching equipment. Tile probes are time-consuming, tedious, and hard to employ across large areas and the use of trenching equipment is extremely invasive and damages the drainage pipes which results in costly repairs. Advanced remote sensing-based tile-drainage mapping (e.g., brightness signatures) provides a potential nondestructive and cost-efficient solution. High-resolution aerial photographs can be used to create detailed maps showing the layout and distribution of drain tile networks. Identifying the location and extent of drain tile systems at large spatial scales is facilitated by visualizing the entire network from an aerial perspective.

Remote sensing approaches using aerial imagery collected with high-resolution cameras (e.g., visible-color, multispectral, and thermal infrared) are primarily dependent on soil reflectivity. Soil above the tile drainage tends to dry faster (higher reflectivity) compared to the soil between two tile drainage lines (low reflectivity). Since ground wetness level (i.e., moisture content) is a key factor, the ideal time to acquire aerial imagery for tile drainage mapping is within three days after a 1-inch or greater rainfall event¹. The degree of accuracy of a remote sensing approach to identify tile drainage over a large area mainly depends upon the time of acquiring aerial imagery after a rainfall event and the presence of vegetation cover². Given bare ground conditions, lighter-shaded dry soil surface features that are linear may be representative of drain lines as the dry soil surfaces reflect more visible and near-infrared (NIR) electromagnetic radiation than wet soil surfaces³. Consequently, visible-color, multispectral, and thermal infrared cameras are a suitable alternative for subsurface drainage mapping.

Aerial imagery can aid in the planning of development, construction activities, and repairs for drain tile systems. Furthermore, analyzing aerial imagery and interpreting drain tile systems accurately can be a complex task. Numerous factors including variations in color, shading, or image distortion, can make it challenging to distinguish drain tiles from other features on the ground. For improved analysis and to specify exact drain tile locations, aerial imagery can be combined with other non-destructive techniques commonly used to obtain soil and tile drainage information.

Ground penetrating radar (GPR) is a non-invasive solution for finding drainage pipes and is effective in many cases. GPR consists of transmitter and receiver antennas that transmit electromagnetic energy into the ground and a receiver antenna records the earth’s impulse response (i.e., the reflected and scattered energy)⁴. For drainage pipe mapping, the detectability arises because of the contrast in electrical properties between the material inside the drainage pipe (i.e., air, water) and the material surrounding the drainage pipe (i.e., soil). The material of the pipe (e.g., clay, ceramic, PVC) has no significant effect on the GPR drainage pipe response⁵. GPR for drainage pipe mapping can provide depth information and confirm that drain line signatures in the aerial imagery are actually caused by a drainage pipe. Still, complete coverage of a large field area with GPR can sometimes be expensive and impractical.

Given optimal conditions, both aerial imagery and GPR have proven useful for drainage mapping purposes. The complementary nature of aerial imagery-GPR and the information they provide concerning drain tile mapping shows exciting potential as an alternate solution. At SAM, a drain tile mapping project approach was designed using a combination of 4-Band aerial imagery (i.e., RGB & NIR) from manned aircraft and GPR. The aerial imagery was captured outside the growing season (i.e., bare ground conditions) two to three days after a significant rainfall event (1 inch or greater). The project sites, some covering several thousand acres, were in the Midwest region of the US and are known for their agricultural crop production. From a manned fixed-wing flight, 6-inch GSD NIR imagery was acquired and post-processed to determine the approximate location of the underground drain tiles. Client feedback for the remote sensing-based tile-drainage mapping from the 4-Band aerial imagery of the approximate drain tile locations has shown to be as accurate as +/-70%.

GPR was used to complement the 4-Band aerial imagery as a validation technique. GPR data along a limited spatial extent in combination with aerial imagery not only provided the depth information of the drainage pipes but was also helpful in setting apart the linear signatures caused by drain lines from those caused due to field operations. Data on the existing drain tile system was also obtained by coordinating with the landowner to support drain tile locating efforts. Data on the existing drain tile system was employed as an auxiliary tool to create a comprehensive mapping of known and suspected drain tile systems.

Several challenging factors were recognized when mapping the drain tile system based on aerial photography, GPR, and consultation with the landowner. Poor weather conditions, such as heavy cloud cover, rain, lack of rain, or strong winds, can limit or even prevent data collection. These limitations may introduce delays in the investigation process or require repeated survey attempts to obtain suitable data. Vegetation, especially tall crops, or dense foliage can obstruct the view of drain tile systems from the aerial platforms. The presence of vegetation and other factors such as image distortion can make accurately mapping the drain tile layout or identifying potential issues difficult. Expertise in image interpretation and the use of specialized software or tools may be necessary to overcome these challenges effectively. The expenses associated with acquiring, processing, and analyzing aerial data may be a limiting factor, particularly for smaller-scale drain tile investigations. Balancing the cost and benefits of aerial mapping is important when considering this approach.

While drain tile surveying is a specialty service and is constrained by weather and the availability of resources, it's a skill set that is invaluable when needed. In the last five years, employing this technique, SAM has saved our renewable energy developer clients millions in time spent searching for the drain tiles, repairing them when broken, and delays to the project while they are being fixed. If you need help locating drain tiles, or at any stage of your renewable project, we invite you to learn more about how we can ensure the success of your wind, solar, or battery storage project.

1. Northcott, W. J., Verma, A. K. & Cooke, R. A. Mapping subsurface drainage systems using remote sensing and GIS. ASAE Ann. Int. Meeting. 2625–2634 (2000). https://experts.illinois.edu/en/publications/mapping-subsurface-drainage-systems-using-remote-sensing-and-gis
2. Tetzlaff, B., Kuhr, P. & Wendland, F. A new method for creating maps of artificially drained areas in large river basins based on aerial photographs and geodata. Irrig. Drain. 58, 569–585 (2009).https://agris.fao.org/agris-search/search.do?recordID=US201301721525

3. Lobell, D.B.; Asner, G.P. Moisture effects on soil reflectance. Soil. Sci. Soc. Am. J. 2002, 66, 722–727. Moisture Effects on Soil Reflectance - Lobell - 2002 - Soil Science Society of America Journal - Wiley Online Library
4. Olhoeft, G.R. Electromagnetic field and material properties in ground penetrating radar. In Proceedings of the 2nd International Workshop on Advanced Ground Penetrating Radar, Delft, The Netherlands, 14–16 May 2003; pp. 144–147. https://www.researchgate.net/publication/4020503_Electromagnetic_field_and_material_properties_in_ground_penetrating_radar
5. Allred, B.J.; Daniels, J.J.; Fausey, N.R.; Chen, C.; Peters, L.; Youn, H. Important considerations for locating buried agricultural drainage pipe using ground penetrating radar. Appl. Eng. Agric. 2005, 21, 71–87. IMPORTANT CONSIDERATIONS FOR LOCATING BURIED AGRICULTURAL DRAINAGE PIPE USING GROUND PENETRATING RADAR (asabe.org)
   
   

Carlos Montoya, Ph.D.

Carlos Montoya, Ph.D. joined SAM in 2020 and has 13 years of engineering, aerial mapping, and project management experience. His expertise includes transportation and geotechnical engineering, LiDAR, photogrammetry, and mobile mapping.

Allen Siler, C.P., C.M.T.-L.

Allen Siler joined SAM in 2013. He has over 20 years of experience providing Aerial Mapping services, working across different areas in the Geospatial industry.

Chris Garza, R.P.L.S.

Chris Garza joined SAM in 2018. He has over 16 years of experience in the surveying industry, 7 of those years as an R.P.L.S. in Texas, and has vast experience working across many market sectors, and is currently focused on the Renewables market sector.