Discharge is a fundamental concept in hydrology, and important to any understanding of hydrodynamics and flow modeling. It refers to the flow rate of water in a channel, usually measured in cubic meters per second. Another way of picturing this is to imagine standing at one spot on the bank of a river and measuring the water flowing by: What volume of water passes you within one second?

Discharge is determined by the size and speed of a river, and includes any chemicals, biological materials, or solids in the water. Modeling discharge can thus help us understand how runoff from fertilizers and industry affect downstream habitats, predict where flooding is most likely during heavy rainfall, and monitor the water level in reservoirs.

NASA's Surface Water and Ocean Topography (SWOT) mission uses radar interferometry to measure global freshwater levels. SWOT emits radar waves that reflect off the surface of bodies of water. By measuring the precise angle and speed at which those signals return to the satellite, scientists can determine the precise elevation of the water.

The Gravity Recovery and Climate Experiment/GRACE Follow-On (GRACE and GRACE-FO) missions also help track water balances across the Earth's surface, measuring tiny variations in gravity that can indicate surface water movement. NASA supplements these remote sensing observations with in-situ data from stream gauges to build up a global picture of hydrology and stream flow.

NASA data on river discharge support streamflow models, flood predictions, and nutrient cycling research around the world. NASA also provides tutorials on working with SWOT data to find and access river discharge data.