Macdonald N, Reeves RR, Graham DJ, Brakenrig T. 2019. Obtaining water level data at Whakarewarewa Geothermal Valley using unmanned aerial vehicle photogrammetry. Lower Hutt (NZ): GNS Science. 20 p. (GNS Science report; 2019/14). doi:10.21420/HCDF-P412.
Unmanned aerial vehicles (UAVs) offer a novel means of studying dangerous and inaccessible areas. They have the advantage of having the ability to survey large study areas relatively quickly, while utilising a wide array of sensors. Monitoring the water level of geothermal pools is important for providing insight into how fluids may be flowing and interacting between geothermal surface features and shallow groundwater aquifers. Making manual measurements of water levels in geothermal areas can be either impossible or hazardous due to factors such as unstable ground, dangerous gas concentrations, hot water and hot ground. In this study, a method for measuring water levels of geothermal pools using an UAV was developed to produce a potentiometric map of the shallowest groundwater surface, across the Whakarewarewa Geothermal Valley, Rotorua. Methods for estimating the elevation of the water surface of geothermal pools from high-resolution aerial photography were evaluated. We found that the uncertainty of deriving water elevations of pools was primarily affected by non-random photogrammetric point errors that occurred across the central parts of pools, and by the physical characteristics of geothermal pools such as upwelling of source fluid, ebullition (boiling) and water clarity. These physical characteristics adversely affected the image processing. The accuracy of the water level elevations was greatly improved when using photogrammetric pixels located at the edges of the pools. Therefore, point data with DSM1 was used to obtain water surface elevations from a selection of geothermal pools and generate a potentiometric map. The resulting potentiometric map of Whakarewarewa Geothermal Valley shows a southwest/northeast orientated area of elevated water levels. This occurs at higher ground elevation on the southern side of the valley, in an area containing steam-fed, acid-sulphate pools. The potentiometric map provides some insights into possible perched groundwater aquifers, and probable groundwater flow characteristics, in the area. This preliminary study on using UAVs to measure water level elevations of geothermal pools looks promising, however, a clear understanding and control of uncertainty in the data is critical to develop a reliable interpretation of results. (auth)