Synchronized mapping of water quantity and quality of a reservoir through an unmanned surface vehicle: A case study of the Daljeon reservoir, South Korea

This study developed a synchronized mapping technique for water quantity and quality via an unmanned surface vehicle (USV). The USV with the acoustic doppler current profiler (ADCP) and the multiparameter sonde of water quality sensors (YSI EXO2) was used for identifying spatial and seasonal patterns of the Daljeon reservoir in South Korea. With this technique, we measured bathymetry and nitrate concentration from August 2021 through July 2022 at the high resolution spatial resolution and tested the sensitivity of estimated nitrate loads to spatial variations of input variables (water volumes and nitrate concentrations). Results showed that measured bathymetry and nitrate concentration varies over the water surface of the reservoir and time, which are associated with seasonal variations of temperature and precipitation. Despite weak spatial variations of the nitrate concentration, the water level of the reservoirs showed strong spatiotemporal variations depending on the topography of the reservoir and the  rainfall occurrence. Furthermore, we figured out using the mean for nitrate load was underestimated by -20% of the nitrate load estimates by considering spatial variation. High-resolution bathymetry measurement play a role in estimating nitrate loads with a minor impact of spatial variations of measured nitrate concentrations. We found that rainfall occurrences more likely increase estimated nitrate loads when it accounts for spatially variations of input variables, particularly water volumes. This study proved the potential utility of USV in simultaneously monitoring water quantity and quality for integrative water resource management for sustainably development of our communities.