Remote sensing data

Technology has provided an expanding array of alternative approaches for acquiring hydroclimatic data using instruments at a distance from the location being measured. The practice of distant measurement known as remote sensing is accomplished using instruments on satellites, aircraft, or the ground. In most cases, the remote sensing technique involves the interaction of electromagnetic radiation with the Earth's surface or the atmosphere. Aerial photography utilizing visible wavelengths of the electromagnetic spectrum was the earliest use of remote sensing (Engman, 1993). A wider array of the electromagnetic spectrum is utilized currently by multispectral scanners, thermal sensors, microwave sensors, lasers, radar (radio detection and ranging), and lidar (light detection and ranging) to measure or infer the fluxes of energy, precipitation, and evapotranspiration and to infer soil moisture and runoff over an area. In addition, sodar (sound detection and ranging) uses short sound pulses to determine wind characteristics.

Radar, lidar, and sodar are active remote sensors in that they emit a pulse of electromagnetic or acoustic radiation to probe the atmosphere and surface characteristics. They detect with a receiver a portion of the radiation that is reflected or backscattered by the target (Angevine et al., 2003). Passive sensors detect radiation emitted from the Earth's surface or from the atmosphere. A radiometer is a passive instrument that measures the emitted radiation in a specific region of the electromagnetic spectrum.