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Satellite altimetry has successfully been applied to derive water level time series over lakes and rivers for many years. In the last decade major advancement within the field have been achieved. The SAR altimetry era, initiated with the launch of CryoSat-2, has made it possible to measure much smaller targets more accurately. The standard 20 Hz radar altimetry products generally allow to study targets down to a few hundred meters in optimal conditions. However, recent advancement in the low-level data processing, the Fully-Focused SAR (FFSAR) technique, where the footprint in the along-track direction can be as small as a few meters, has made it possible to measure the water level of even smaller targets. FFSAR processed data from CryoSat-2 in SAR mode is made freely available by ESA through a newly integrated service on the Grid Processing on Demand (G-POD) platform, within the SARvatore Family of custom processors, where the user can order data from an area and specify various parameters to be applied in the processing. The FFSAR processor has been developed within the ESA ESTEC Sentinel-6 Project and adapted to CryoSat-2 for verification and validation purposes. Here we demonstrate the value of FFSAR and evaluate the FFSAR product obtained from the ESA G-POD service over different inland water targets. The evaluation and validation is done via in situ water level data, external processed FFSAR data, unfocused SAR data and laser altimetry data from ICESat-2. More specific, we perform an evaluation over the narrow ( < 100 m) American Rivers: Red River, Little River, and Canadian River located in the Mississippi basin. Where we compare the water level based on different retracker e.g. ALES+ and threshold retrackers. We make a joint water level time series by considering data from CryoSat-2 and ICESat-2 over a reach, and validate the results against in situ data. Preliminary results show that we can capture the main part of the water level signal. Over the Elbe River, we compare the FFSAR based river water levels with results based on unfocused SAR processing and in situ gauges. We extend our analysis with a case study conducted over the Lake IJssel which is located in The Netherlands and covers an area of approximately 1100 km2. We examine the performance of the G-POD processor by evaluating the FFSAR lake height estimates using externally processed FFSAR data that have been validated against in situ data.

In this dissertation I examine the existence of multiple stable treecover states of the Earth’s forest ecosystems, with a primary focus on the boreal region. Combining remotely-sensed observations, data analysis, and conceptual models, I identify areas with alternative vegetation states under the same environmental conditions, and I explore their possible dynamics under current and future conditions. In recent years, it has been found that the distributions of remotely sensed tree cover in boreal and tropical ecosystems have three distinct modes, corresponding to treeless, open woodland, and forest states. In light of this pattern, it has been suggested that these modes reflect the presence of alternative tree-cover states. As a response to climate change, these ecosystems could undergo critical regime shifts. For the tropics, it has been shown that a positive feedback between fire and vegetation can act as switch between the three states. For the boreal forest, it has been shown that the observed multimodality is not caused by temperature and precipitation patterns. In the first part of this thesis, by means of generalised additive models, I show that the relationship between tree cover and eight remotely-sensed environmental variables varies within the boreal region. Using a classification, I identify areas which exhibit alternative tree-cover states under similar environmental conditions. These regions show a reduced resilience and can shift between states. In the second part of the thesis, I develop and employ a conceptual model to show that tree-cover multistability in the boreal region can emerge through competition between species with different evolutionary traits. By forcing the model with varying permafrost conditions, I show that the asymmetry in tree-species distribution between North America and Eurasia could be due to permafrost presence. In the third part of the thesis, employing projected environmental conditions from the Representative Concentration Pathway (RCP) 2.6 and 8.5 scenarios, I identify potentially multistable areas during the last decade of the 21st century. By including a simple effect of CO2 on plant growth in the conceptual model, I simulate the dynamics of multistable zones under projected environmental conditions. I show that the two scenarios exhibit opposite trends regarding the extent of multistable areas, and that the resilience of Eurasian species might increase, while North American forests might lose stability. In each part of this thesis, I consider limits and advantages of the tools at hand, implying that only through their combined use we can advance our knowledge of tree-cover multistability and improve the representation of the boreal forest in climate models.

Previous analysis of the vegetation cover from remote sensing revealed the existence of three alternative modes in the frequency distribution of boreal tree cover: a sparsely vegetated treeless state, an open woodland state, and a forest state. Identifying which are the regions subject to multimodality, and assessing which are the main factors underlying their existence, is important to project future change of natural vegetation cover and its effect on climate.We study the link between the tree-cover fraction distribution and eight globally observed environmental factors: mean annual rainfall, mean minimum temperature, growing degree days above 0 °C, permafrost distribution, mean spring soil moisture, wildfire occurrence frequency, soil texture, and mean thawing depth. Through the use of generalised additive models, conditional histograms, and phase-space analysis, we find that environmental conditions exert a strong control over the tree-cover distribution, uniquely determining its state among the three dominant modes in ∼ 95 % of the cases. Additionally, we find that the link between individual environmental variables and tree cover is different within the four boreal regions considered here, namely eastern North Eurasia, western North Eurasia, eastern North America, and western North America. Furthermore, using a classification based on rainfall, minimum temperatures, permafrost distribution, soil moisture, wildfire frequency, and soil texture, we show the location of areas with potentially alternative tree-cover states under the same environmental conditions in the boreal region. These areas, although encompassing a minor fraction of the boreal area ( ∼ 5 %), correspond to possible transition zones with a reduced resilience to disturbances. Hence, they are of interest for a more detailed analysis of land–atmosphere interactions.