MAPPING GROUNDWATER DISCHARGE SEEPS WITH THERMAL UAV ORTHOMOSAICS ON A CRANBERRY BOG

The identification and measurement of groundwater discharge sites are vital for understanding groundwater dependent ecosystems. In wetland ecosystems for example, groundwater provides habitat for plants and animals, maintains soil saturation, and thermally buffers the environment against extreme temperature. Locating groundwater discharge sites provides information for conservation, restoration, and further research about these environments. Using a thermal camera mounted on a drone, we mapped a 1-hectare wetland and are able to identify the locations of small (~20 to 100cm) groundwater seeps and thermal refugia throughout the bog.

The test site is a retired cranberry bog located in Plymouth MA. A straightened main channel connects numerous irrigation ditches and a large retention pond. Approximately half of the total water leaving the site is gained from groundwater seeps across the surface of the bog and channel beds.

The groundwater map is be captured when the difference between the surface water and inflowing groundwater is at its maximum. The temperature difference creates a strong distinguishable thermal signature, which can be clearly captured by a thermal camera. In Massachusetts, this can be accomplished in the summer, when the groundwater is colder, and during the winter, when the groundwater is warmer than the surface water. As thermal cameras record only the surface, or skin temperature, the thermal signature of upwelling groundwater is best captured when it is buoyant and rises to the surface during the winter in Massachusetts. Using a thermal camera attached to Matrice 210 unmanned aerial system (UAS), imagery of the entire cranberry bog was captured with an overlap of 80%, sufficient to produce an orthomosaic. This thermal orthomosaic provides locations of groundwater seeps as small as 30cm, and accurate to within 20cm.

The ability to produce a comprehensive groundwater discharge map of a large area can provide a basis for further study of individual seeps. But it also has various applications on its own merit: In this example it is used to aid in the restoration of this site to a freshwater wetland.