D5 Satellite Remote Sensing

Topic

Satellite remote sensing enables the observation—on a global scale and with consistently repeated temporal coverage—of the same hydrological and surface variables that field methods characterize at specific points: soil moisture, flooding, groundwater, vegetation, and surface temperature.

Moisture in the top five centimeters of soil is measured from space using passive microwave radiometers that detect the brightness temperature emitted by the soil; it is precisely this brightness temperature—directly related to the soil's dielectric constant—that allows for the inference of the soil's water content.

Synthetic aperture radar, meanwhile, measures the backscatter coefficient of the signal it emits and subsequently receives; this coefficient is sensitive to surface roughness, vegetation, and soil moisture. A sudden change in this backscatter coefficient makes it possible to map a flood from space almost immediately after it occurs.

A pair of twin satellites flying in tandem measures minute variations in Earth's gravity caused by the redistribution of water mass on the surface and underground; this gravitational variation allows for the estimation of changes in total stored water within a region, with a precision of just centimeters of equivalent water and a spatial resolution of approximately three hundred kilometers.

Vegetation and evapotranspiration are monitored using spectral indices that summarize the amount of active green vegetation covering any given point on the surface, as well as evapotranspiration models that apply the same surface energy balance—previously described in the context of ground-based flux measurements—from a spaceborne perspective.

Finally, surface temperature is derived from thermal infrared data captured by the satellites, applying corrections for both surface emissivity and the atmospheric conditions the signal traverses before reaching the sensor. Thus, from the soil moisture with which this document begins to the surface temperature with which it concludes—and taking in everything from the synthetic aperture radar that maps floods to the twin satellites that weigh groundwater from orbit—satellite remote sensing enables variables that could once only be measured point-by-point on the ground to now be observed continuously across the entire planet.

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