Phd in ENVIRONMENTAL AND HYDRAULIC ENGINEERING

Thesis title: On the use of satellite rainfall measurements in ungauged areas for hydrological applications

Rainfall has a crucial role in natural sustainment of ecosystems and human societies, as it influences weather and climate, and it is the main driver in the evolution of natural landscapes. Accurate and reliable rainfall information, hence, are of pivotal importance. Yet our knowledge on rainfall relies entirely on to records we are able to collect. Rainfall is one of the most difficult meteorological parameters to measure, due to its high variability in space and in time. Rainfall has traditionally been measured by rain gauges placed on Earth’s surface but in the last decades observations provided by radar and satellite estimates have become available. Even though our knowledge and confidence on rain gauge-based measurements is higher than those on radars and satellites, rain gauge monitoring networks have been declining in the last decades, due to their high maintenance costs. Moreover, their distribution on Earth’s surface is uneven, with higher density on more populated areas. Satellite retrievals, on the contrary, provide free observations on a quasi-global coverage, reaching oceans and remote areas, even though the estimates they provide are indirect, as they measure the upwelling radiation from Earth’s surface or cloud top temperature and then adopt some algorithms to derive rain rates. Radar observations are indirect as well but, conversely from satellite retrievals, they are not open source and limited to some areas of the world. Thanks to accessibility and improved quality of satellite retrievals, their use in hydrological applications has increased exponentially in the last years, allowing for the knowledge of the scientific community on hydrological processes to widen in areas that were completely ungauged so far. On the one hand, thus, satellite-based observations have high potential to be valuable in hydrology, as they can help reducing uncertainties in the predictive system for ungauged areas. On the other hand, despite many studies have been carried out recently, the goodness and accuracy of satellite-based observations still need to be evaluated for some hydrological practical applications. The purpose of this dissertation is to investigate to what extent satellite-based observations can replace ground-based measurements for some hydrological applications in ungauged areas. To this aim, three common applications are explored: the detection and hydrological modelling of severe rainy events, design and optimization of rain gauge monitoring networks, definition of design discharge via IDF curves. The results indicate that the use of satellite estimates can be valuable in hydrology, under some conditions. First, the satellite product to employ has to be chosen based on its main characteristics, in relation with the main features of the study area, as their performance are influenced by climate, topography and proximity to water bodies. If the satellite product is properly selected, it can reproduce the spatial pattern of rainfall variance and identify the most convenient locations where to measure rainfall, in terms of amount of information provided. Second, satellite observations at fine spatial and temporal scales can describe, in summary, the spatial and temporal structure of rainy events, but they present biases in the temporal and spatial evolution of the events themselves, which can compromise the performance of hydrological modelling. Third, biases of satellite products at fine temporal and spatial scales, together with the reduced data length of these data sets, can lead to incorrect and uncertain estimation of precipitation intensity, especially for high return periods.