High-Resolution Reservoir Monitoring Using Uncrewed Surface Vehicles: Spatiotemporal Dynamics of Nitrate Loads and Drought Impacts

Water quantity and quality monitoring systems have developed at high spatial resolution for water resources management using satellites and uncrewed aerial vehicles. In situ measurements at a high-resolution remain limited for validation. Here we conduct 100 surveys of the Daljeon Reservoir in South Korea from July 2021 through December 2025, using an uncrewed surface vehicle equipped with an acoustic Doppler current profiler and a multiparameter sonde. We generate 10-me resolution maps of water depth, surface water temperature, dissolved oxygen, pH, electrical conductivity, and nitrate concentration in the reservoir. Results show strong seasonality in water level and nitrate associated with the monsoon season. The 2022 drought year demonstrates higher nitrate concentrations than in the normal year. We find that a single-point approach overestimates nitrate loads by 30.8 ± 5.1% on average, increasing to 36.4 ± 7.8% during the 2022 severe drought. This study suggests the value of USV-based monitoring techniques for estimation of reservoir-scale nitrate loads in a reservoir and delivers high-resolution data sets for evaluating remotely sensed products for water resources management.