In the context of a changing climate and increasing environmental pressures, the demand for data to understand the mechanisms of change and their interactions with the hydrological cycle has grown significantly. Further, a gap in data availability remains between the Global North and the Global South. This thesis contributes to the research into the capabilities of citizen science as a recent approach to data collection in water resources. Data obtained via citizen contributions, mainly through social media mining, has been shown to achieve accuracy comparable to authoritative sources and provide valuable inputs for hydrodynamic modelling. Nonetheless, aligning the spatial and temporal resolution of citizen-generated data with modelling requirements remains a challenge.....

Hydrological processes can be highly influenced by changes in land use land cover (LULC), which can make hydrological modelling also very sensitive to land cover characterization. Therefore, obtaining up-to-date LULC data is a crucial process in hydrological modelling, and as such, different sources of LULC data raises questions on their quality and applicability. This is especially true with new data sources, such as citizen science-based land cover maps. Therefore, this research aims to explore the influence of LULC data sources on hydrological models via their parameterization and by performing sensitivity analyses. Kiffissos catchment, in Greece, a poorly gauged and highly urbanized basin including the city of Athens, is the case study area. In total, 12 continuous hydrological models were developed by mainly varying their structure and parametrization (lumped and gridded) and using three LULC datasets: coordination of information on the environment (CORINE), Urban Atlas and Scent (citizen-based). It was found that excess precipitation is negligibly contributed to by soil saturation and is dominated by the runoff over impervious areas. Therefore, imperviousness was the main parameter influencing both sensitivity to land cover and parameterization. Lastly, although the parametrization as lumped and gridded models affected the representation of hydrological processes in pervious areas, it was not relevant in terms of excess precipitation.