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176 Observations of the Global Water Cycle – Satellites

Part 15. Global Hydrology

  1. Franklin R Robertson

Published Online: 15 APR 2006

DOI: 10.1002/0470848944.hsa182

Encyclopedia of Hydrological Sciences

Encyclopedia of Hydrological Sciences

How to Cite

Robertson, F. R. 2006. Observations of the Global Water Cycle – Satellites. Encyclopedia of Hydrological Sciences. 15:176.

Author Information

  1. NASA/Marshall Space Flight Center, Huntsville, AL, US

Publication History

  1. Published Online: 15 APR 2006


Satellite observations of Earth's climate system have evolved dramatically since the first crude video images of cloud cover taken over 40 years ago. Today, spaceborne measurements of precipitation, cloud attributes, radiative fluxes, and surface properties make crucial contributions to monitoring key processes and the evolving state of the planet's hydrologic cycle. We illustrate how this maturing capability has enabled study of the hydrologic cycle in a global context that goes beyond consideration of local or regional water balance. Remote sensing has become a key technology for addressing such interdisciplinary questions as whether the global hydrologic cycle is changing and if so how? What key processes linking the atmosphere, biosphere, and hydrosphere are at play, and what are the prospects for predicting extreme hydrologic events of significant societal impact? The discussion is motivated first by considering these key science problems. Following a brief overview of key remote sensing concepts, we then survey existing capabilities to determine various quantities such as precipitation, evaporation, soil moisture, turbulent fluxes, cloud cover, water vapor, and the links to radiative fluxes. Both successes and persisting challenges are discussed. An assessment of where we stand in developing an integrated picture of the hydrologic cycle and climate is presented, along with anticipated improvements in the future measurements.

In the context of climate, we demonstrate that in spite of substantial improvements in determining hydrologic variables and processes from space, some key challenges remain. Our examination here of signals in precipitation, evaporation, and radiative fluxes suggests that at present we can detect regional patterns of interannual variability with considerable skill; we can resolve globally integrated signals with only modest certainty; and for the most part, we have little confidence yet in decadal changes or trends. Nevertheless, the situation is encouraging because in many cases these retrievals have been done without regard to intercalibration of sensor data streams, and in the case of operational retrievals, algorithm modifications which result in spurious trends. Careful reprocessing is likely to yield substantial benefits. Future planned missions with active remote sensing at 94 GHz (CloudSat) and interferometric measurements at 1.4 GHz promise to expand our knowledge of hydrologic processes of crucial importance to climate.


  • remote sensing;
  • global hydrologic cycle;
  • precipitation;
  • land-surface data assimilation;
  • turbulent fluxes