Towards unprecedented spatiotemporal observations in hydrological systems using Uncrewed Vehicles
Authors:   , Koutantou Kalliopi 
Published: January 2024

The improvement of water resources management has been a big challenge and the appropriate use of water is of high importance for the ecosystems and the human population globally. Climate change affects the water cycle and therefore water use and human activities around the world. There is therefore an increased need for better monitoring of the water resources and the understanding of the hydrogeological processes that take place in hydrological and hydrogeological systems. Remote sensing technology has been intensively used to monitor water bodies, but so far mainly via satellites and crewed aircraft. However, there are still limitations in terms of data resolution and lack of in situ data in isolated and difficultly accessible areas. Therefore, there is a need for more advanced technologies that can observe the water at finer spatial and temporal scales. In this context, uncrewed vehicles with their innovative autonomous technology and their high spatiotemporal resolutions open a new era in the study of hydrogeological systems. Uncrewed aerial vehicles (UAVs) have been used in hydrogeological studies due to their operational flexibilities such as their ability to fly at low altitudes, in challenging environments, and whenever the user needs them at low costs. These flexibilities overcome the low spatial and temporal resolutions of the satellite data and the high costs of crewed aircraft data acquisitions. Besides the UAVs, uncrewed surface vehicles (USVs) have also been used in ocean studies due to their autonomous and long-range navigation. Their high sampling frequencies and their ability to directly sample the ocean surface and not just a few meters underneath, make them more favorable than traditional in situ measurement platforms such as buoys or other crewed boats. The aim of this Ph.D. thesis is to improve the understanding of hydrogeological processes using this innovative technology of uncrewed vehicles. By combining various remote sensing platforms and data, we produced new datasets for the scientific community with unprecedented spatiotemporal resolutions. Our studies, using these uncrewed vehicle-borne data with their unprecedented spatiotemporal resolutions, prove the efficiency and potential of the uncrewed vehicle technology. We pursue four different studies related to different hydrogeological processes in different environments. The first study is a technical contribution to snow mapping techniques using UAVborne LiDAR technology. The second study, which is the first-ever study conducted in steep forested terrain using UAV-borne LiDAR demonstrates the effect of canopy structure and solar radiation in the formation of snow patterns within the steep forested slopes. We also extract valuable technical contributions for similar future campaigns. The third study focuses on sea surface temperature dynamics on the California Coast and proves that Saildrone, an uncrewed surface boat, is capable of validating satellite products with its one-minute sampling frequency. It is the first study to compare MODIS level-2 with Multi-scale Ultra-high Resolution (MUR) level-4 satellite products over the California Coast and the first one to assess the accuracy of MODIS level-2 quality levels over the California Coast. The last study aims at mapping topsoil organic matter at high resolution by exploiting the soil spectral properties along with traditional laboratory analysis.