Assessment of submarine groundwater discharge by handheld aerial infrared imagery: case study of Kaloko fishpond and bay, Hawai'i

Handheld aerial infrared imagery was used to infer submarine groundwater discharge (SGD) to a Hawaiian fishpond and adjacent bay at Kaloko, Hawai'i, using heat as a tracer for the relatively cooler groundwater. Use of a handheld infrared camera aboard readily available, unmodified aircraft is a convenient and less expensive alternative to use of a camera mounted in the belly of a customized aircraft, although it favors taking images with an oblique view instead of the preferable nadir view. Pond-wide patterns of SGD were readily apparent in oblique images and were typically more apparent in infrared imagery than in ground truth data, due to the formation of thin ( order of cm) surface strata of groundwater which could easily fail to be observed with conventional temperature probes. Absolute temperature measurement is affected by the variation of surface emissivity and reflectivity with angle of camera view; corrections based on use of Fresnel's equation were of the order of several degrees centigrade at convenient oblique aerial viewing angles. Other factors that may affect apparent water temperature include sky temperature and camera error. Surface waves may also account for variations in average surface emissivity and reflectance that were not accounted for by the aforementioned corrections. Under suitable conditions, handheld aerial infrared imagery revealed spatial patterns of groundwater inflow, detected differences in water temperature at the meter scale, and measured absolute water temperature with accuracy on the order of 2 to 3 degrees C.