This thesis explored the global C budget of freshwaters and its spatiotemporal variations in the 20th century using the mechanistic IMAGE-Dynamic Global Nutrient Model extended with the newly developed Dynamic In-stream Chemistry Carbon module (DISC-CARBON). This model couples river basin hydrology, environmental conditions and C delivery and generates C flows from headwaters to mouths. DISC-CARBON is a spatially explicit global model with 0.5 by 0.5-degree resolution that simulates the concentrations, transformations and transfer fluxes of dissolved inorganic C (DIC), dissolved organic C (DOC) and terrestrial and autochthonous particulate organic C (POC) from headwaters to river mouth in a single integrated model. Chapters 2 and 3 of this thesis show that the simulations of spatiotemporal river export and CO2 emissions are in good agreement with observations and literature, and Chapter 4 shows that the model also properly simulates the C cycle for an individual river, the Rhine. With the DISC-CARBON model developed, the long-term C budget of freshwaters can now be estimated with confidence. The last part discusses the shortcomings of the model, and possible improvements needed to better quantify the C budget of global freshwaters.At the end of the 20th century, total global C inputs from land to freshwaters add up to 3.5 Pg C year-1, burial is 0.4 Pg C year-1 and CO2 emission is 2.2 Pg C year-1. Increasing global C inputs, burial and CO2 emissions reported in the literature are confirmed by DISC-CARBON. Global river C export to oceans has been stable around 0.9 Pg C year-1. This indicates that global river basins have been balancing the increased inputs through enhanced in-stream retention and emission to the atmosphere. The increased C retention in inland waters, from 0.3 Pg C year-1 in the first decade of the 20th century to 0.4 Pg C year-1 in the last decade of the 20th century may be closely related to the increasing number of dams constructed. CO2 emissions from inland water systems showed an increase of 0.2 Pg C year-1 from an average of 2.1 Pg C year-1 in the 1900’s to an average of 2.3 Pg C year-1 in the 1990’s, mainly as a result of an increase in terrestrial carbon (C) delivery.Most CO2 emissions originate from floodplains 1.4 Pg C year-1, where CO2 is produced through mineralization of terrestrial organic C. Rivers contribute 0.8 Pg C year-1 to total CO2 emissions, mainly due to the influx of CO2 supersaturated groundwater. Lakes and reservoirs emit 0.2 Pg C year-1. Although delivery of organic C through soil erosion increased from 106 Tg C year-1 to 168 Tg C year-1 and the volume of reservoirs increased during the 20th century from nearly 0 to more than 3500 km3, DISC-CARBON simulations do not indicate a major influence of these changes on global CO2 emissions from freshwaters, but some key processes such as eutrophication have not been explicitly included.