Search

Remote sensing helps foster our understanding of inland water processes allowing a synoptic view of water quality parameters. In the context of global monitoring of inland waters, we demonstrate the benefit of combining in-situ water analysis, hydrodynamic modelling and remote sensing for investigating biogeochemical processes. This methodology has the potential to be used at global scales. We take the example of four Landsat-8 scenes acquired by the OLI sensor and MODIS-Aqua imagery over Lake Geneva (France—Switzerland) from spring to early summer 2014. Remotely sensed data suggest a strong temporal and spatial variability during this period. We show that combining the complementary spatial, spectral and temporal resolutions of these sensors allows for a comprehensive characterization of estuarine, littoral and pelagic near-surface features. Moreover, by combining in-situ measurements, biogeochemical analysis and hydrodynamic modelling with remote sensing data, we can link these features to river intrusion and calcite precipitation processes, which regularly occur in late spring or early summer. In this context, we propose a procedure that can be used to monitor whiting events in temperate lakes worldwide.

Freshwater resources play a central role in social and economic development of modern civilisations, yet their value is often underestimated and neglected in developed countries. In fact, freshwater habitats are facing unprecedented threat because of human activities, and it is necessary to provide reliable water quality indicators to monitor the response of aquatic systems. In this context, remote sensing has a great potential to provide a complementary source of data for monitoring and under-standing the processes involved in inland waters around the world at fine temporal and spatial reso-lutions. The scientific approach adopted in this thesis is based on the integration of complementary sources of information provided by state-of-the-art monitoring methods to foster our understanding of freshwater habitats. Specifically, we demonstrate the additional value provided by combining com-plementary sensors with bio-geochemical measurements and hydrodynamic models, using a rare event in Lake Geneva which got a wide public attention in local newspapers: a calcite precipitation event. The principal focus of the remote-sensing community has recently been directed towards very turbid waters in order to address the challenges involved with the retrieval of mixed constituent concentra-tions. In this thesis, I highlight some important challenges relative to clearer waters which also re-quire further attention from the community. Specifically, I provide a solution to account for vertical non-uniformities of water constituent concentrations using simple approximation models in Lake Geneva. Finally, I provide a comprehensive comparison between state-of-the-art atmospheric correction methods, which are presumably relevant for inland water monitoring and applicable to the new con-stellation of remote sensors. The aim is to provide reliable recommendations to help forthcoming studies to apply the most suited procedure to their investigation.

In an environment subject to anthropogenic pressures, the understanding of lakes is crucial as they are essential for ecosystems and represent a source of the scarce accessible freshwater existing on Earth. Water quality management is especially designed to protect this precious resource and relies on monitoring programs and model simulations. Case study of this project, Greifensee is a small eutrophic lake located in the northern plateau of Switzerland (ZH). For decades, it has been subject to nutrient-­‐rich runoff from agricultural activities in its catchments, consequence of the fast population growth in the area. These considerable human-­‐related pressures have led to a strong process of eutrophication and significantly altered its water quality. In recent years, although nutrient inputs have been reduced, the lake has still not fully recovered and remains classified as eutrophic. This project developed a 3D coupled hydrodynamic-­‐biological model for Greifensee using the software Delft-­‐3D from Deltares in the Netherlands. Delft-­‐3D has been widely applied in various situations and validated, which makes it an excellent tool for such project. Moreover, the development of the model also relies on two monitoring campaigns, from the Eawag institute and Zürich Kanton. Finally, while the model provides an almost full spatio-­‐temporal representation of the processes happening in this lake, final results simulated fairly well the general trends of chlorophyll-­‐a, dissolved oxygen, orthophosphate and ammonium in terms of mass content and depth distribution. Understanding the ecological dynamics of a eutrophic lake using such kind of simulation will hopefully contribute, on a longer-­‐term perspective, to the better management of the lake towards the recovery of its former ecological status.