How do satellites measure water?

Since the first Earth-observing satellite was launched sixty years ago, satellite remote sensing has become a crucial part of weather observation and forecasting systems worldwide. In more recent decades, the use of satellites to observe surface and soil water conditions has developed into practical solutions. Ideally, satellite measurements are calibrated to on-ground measurements where they exist to increase their accuracy. Once calibrated, they can provide information much faster, over much larger areas and with much greater detail than the on-ground measurement network alone.

All data discussed in this report were developed using methods that have been published in the scientific literature:

  • Precipitation, air temperature and air humidity are estimated by combining the latest satellite observations with all globally available weather station data and information from weather forecasting models (Beck et al., 2022). The data record extends from 1979 onwards.
  • Soil water is interpreted from measurements by passive and active satellite microwave (radar) instruments and made available by the EU Copernicus Climate Change Service (link, v202012 combined product). The data record extends from 1991 onwards.
  • Surface water occurrence, including lakes, rivers and other forms of (temporary) inundation was mapped using NASA’s MODIS satellite imagery (link)
  • River flow estimates are based on the automated measurement of river width in satellite imagery (Hou et al., 2022). Where historical ground station measurements of river flows are available, those are used to calibrate so-called satellite gauging reaches. Where ground stations are not available, historical modelled data were used for calibration. The data record extends from 2000 onwards.
  • Lake and reservoir storage data are estimated by combining satellite measurements of water level and water body extent by different satellites, along with information on local topography where necessary (Hou et al., 2022). The data record extends from 1984 onwards.
  • Vegetation vigour (NDVI) responds to water availability and is observed by NASA’s MODIS satellites (link).
  • Terrestrial water storage, including groundwater, soil water, surface water, snow and land ice, are derived from gravity measurements by the GRACE satellites (Boergens et al., 2019).