SWOT: map water height across a swath

Surface Water and Ocean Topography, or SWOT, launched in December 2022. NASA and CNES developed the mission with contributions from the Canadian and United Kingdom space agencies. It brings together ocean science and the study of inland waters. Changes in the height and extent of lakes, reservoirs and rivers provide a different perspective from ocean-only applications.

Traditional nadir altimeters observe along a track directly beneath the satellite. SWOT’s Ka-band Radar Interferometer, KaRIn, observes broad swaths beside that track. Two separated antennas receive reflected radar signals, allowing their relationship to help determine the water surface’s elevation. The observing geometry adds spatial detail; it does not mean the spacecraft observes every water body continuously.

Water level is an elevation. Stored volume also depends on the shape and area of the basin containing the water. River discharge — open definition is a volume passing a location per unit time and requires additional information and modeling. These related quantities have different units, so a water-height map should not be relabeled as a direct measurement of flow without explaining the inference.

In a fictional reservoir with a constant surface area of 2 square kilometers, a uniform 0.5-meter rise corresponds to 1 million cubic meters of added volume. This simplified calculation multiplies area by height change. Real reservoirs may widen as water rises, requiring a more complete elevation-area relationship. The example is a calculation exercise rather than an observation made by SWOT.

A height map is the beginning of an inference

SWOT’s science orbit has a 21-day repeat cycle, with observation opportunities depending on location and overlapping swaths. A repeat cycle is not a promise that each location has exactly the same revisit interval or no observations between repeats. Read the product’s timestamps and coverage instead of assigning a universal sampling schedule to every river.

Imagine a short flood peak occurring between usable satellite observations. A before-and-after pair may constrain broader changes without capturing the maximum level. Ground gauges and other observations can fill temporal information gaps. Combining sources works best when their reference levels, locations and uncertainties are aligned. SWOT adds an important spatial perspective while preserving the need for a complete account of when and how water was observed.