Observations by instruments on board satellites provide data on radiative characteristics of the earth's surface at various wavelengths. From these characteristics, one may determine parameters of interest for large-scale modeling of the earth's energy balance and surface hydrology, for instance land use, vegetation indices, extent of snowpack, surface temperature, radiative fluxes, and surface soil moisture. Some of the sensors in common use are:

• Landsat Thematic Mapper (TM) for high spatial (30 m)/low temporal (16 day) resolution scanning in different bands of the visible and infrared range (note: Landsat 7, with Enhanced Thematic Mapper, began collecting data in April, 1999; the Landsat program has been in operation since 1972)
• Geostationary (GOES) data for high temporal resolution (30 min) scanning in the visible range for a fixed area
• NOAA Advanced Very High Resolution Radiometer (AVHRR) has lower spatial resolution (1 km) but higher temporal resolution (12 hr) than TM
• Nimbus Scanning Multichannel Microwave Radiometer (SMMR) for low spatial resolution scanning at 12 hour intervals in various bands of the microwave range

NASA's Earth Observing System (EOS) has begun a new international remote sensing-based study of the planet, primarily to study climate change. In December 1999, NASA launched its EOS flagship satellite "Terra" to begin collection of the new data. The satellite has five state-of-the-art sensors for studying interactions among the Earth's atmosphere, land, oceans, life, and radiation.

In another recent landmark, the first high-resolution (spy quality) commercial satellite named Ikonos was launched on September 24, 1999 by Space Imaging. Ikonos orbits the Earth 14 times per day, delivering commercially available 1 m black-and-white, and 4 m color resolution images.

An exciting area of current research is the use of the SMMR microwave data to determine surface soil moisture (up to about 5 cm depth). Soil moisture is important because it regulates evaporation, and is the dominant state variable for other hydrologic processes such as infiltration and runoff. The passive microwave approach uses the effect of moisture on the soil dielectric constant, which determines the emissivity of microwave radiation from the earth (however, this is complicated by surface roughness and vegetation cover). Active microwave (radar) has also be used to determine soil moisture, at higher spatial resolution.

There are many difficulties and uncertainties involved in interpreting the multispectral satellite data, and this is an ongoing area of research and emphasis at NASA. For example, a critical link between land surface hydrology and the surface energy balance is the evaporation flux, a function of solar energy, humidity, and surface soil moisture. However, attempts to determine evaporative flux using satellite data have met with limited success. Further research into such issues is needed since accurate regional and global data are used for parameterization, initialization, and validation of large-scale hydrology and climate models. Click here for a recent review of remote sensing applications in hydrology. Another good review paper for more information is "Multispectral Satellite Data in the Context of Land Surface Heat Balance", by B. Choudhury, Reviews of Geophysics, May 1991.

• NASA Earth Observatory
• NASA EOS site
• Hydrological Sciences Branch at NASA
• National Operational Hydrologic Remote Sensing Center (NOHRSC) at NWS
• The Susquehanna River Basin Experiment (SRBEX) at Penn State
• Global Energy and Water Cycle Experiment (GEWEX)