Throughout history, societies have relied on rivers, lakes and wetlands for their development and growth. Today, over 50% of the world's population lives within 3 km of a freshwater resource, and only 10% lives further than 10 km away (Kummu et al., 2011). However, with a rapidly increasing population and a growing number of arid regions, the ways in which societies use freshwater have changed significantly. Technological advancements since the industrial revolution have made it possible to redirect water from freshwater sources over long distances through the construction of dams, pipes, and canals. Despite these developments, billions of people still face severe water scarcity, with over 800 million people lacking access to safe drinking water according to the World Health Organization's standards (WHO, 2010). Furthermore, some regions are experiencing drastic changes in water availability. For instance, the Mississippi River reached record low water levels in October 2022, and the Colorado River reservoirs in the southwestern United States are severely impacted by a 22-year drought. In Germany, warmer temperatures and longer droughts have caused water levels in the Rhine River to drop, affecting the ability to navigate a waterway that carries up to 80% of the country's cargo (Smith, 2020).

Because water is essential to all life, changes in the water cycle are a major concern in the context of climate change. It is therefore crucial to understand how the volume of freshwater stored on Earth's surface in rivers, lakes, and wetlands is changing over time. For example, The NASA Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) mission, which was launched in 2018, is using laser measurements to monitor surface water from space and improve water management decisions.

In addition and more excitingly, the Surface Water and Ocean Topography (SWOT) mission is set to launch on December 15th, 2022. It will provide the first-ever full view of Earth's freshwater bodies from space, allowing for the estimation of changing volumes of fresh water across the globe like never before. These data will be used to calculate the rate of water gained or lost in lakes, reservoirs, and wetlands, as well as river discharge variations around the world. This will help improve our understanding of surface water availability and enable better management of freshwater resources. It will also allow us to better prepare for extreme hazards such as floods and droughts, and their economic impacts. SWOT will observe rivers wider than 100 m and lakes with a surface area of 15 acres or more. The mission is being developed jointly by NASA and the Centre National d'Etudes Spatiales (CNES), with contributions from the Canadian Space Agency (CSA) and the United Kingdom Space Agency (UKSA). The data will be hosted and distributed by NASA's Physical Oceanography Distributed Active Archive Center (PO.DAAC).

The dataset used for the map above is based on the  SWOT River Database (SWORD); specifically, the shapefile river reaches ‘na_sword_reaches_hb74_v2.’ The symbology of the reaches is represented by a bivariate color scheme that shows the relationship between two variables: the slope and width of the Mississippi River and its tributaries. SWORD combines multiple global river and satellite datasets to define the nodes and reaches that will constitute SWOT river vector data products. It provides high-resolution river nodes (200 m) and reaches (~10 km) with attached hydrologic variables (water surface elevation, width, slope, etc.) as well as a consistent topological system for global rivers 30 m wide and greater (Altenau et al., 2021).

In addition to the data visualized in the figure, simulated SWOT hydrology data products are also available at the PO.DAAC. These simulated sample data are intended to help users become familiar with the format and content of the expected science data products from the SWOT mission. These products should not be used to interpret the performance of in-flight data or to perform scientific analyses. Actual SWOT observations will be made available on the PO.DAAC SWOT mission page once the satellite has launched and such data become available.