Remote sensing uses measurements of the electromagnetic spectrum to characterize the landscape, or infer properties of it, or in some cases, actually measure hydrologic state variables. Aerial photography in the visible wavelengths is the remote sensing technique that most hydrologists are familiar with; however, modern remote sensing is centered around satellite systems and most of the discussions will emphasize satellite data. Over the years remote sensing techniques have expanded to the point that they now include most of the electromagnetic spectrum. Different sensors can provide unique information about properties of the surface or shallow layers of the Earth. For example, measurements of the reflected solar radiation give information on albedo, thermal sensors measure surface temperature, and microwave sensors measure the dielectric properties and hence, the moisture content, of surface soil or of snow. Remote sensing and its continued development has added new techniques that hydrologists can use in a large number of applications.
This review attempts to pull together examples of current uses of remote sensing information in hydrology and to give the reader information on how to pursue this subject further. For a more in depth coverage of general remote sensing applications to hydrology the reader is referred to a recent United Nations Educational and Scientific and Cultural Organization (UNESCO) publication by Schultz and Barrett [1989], a World Meteorological Organization Report by Kuittinen [1992] and a recent book on the subject by Engman and Gurney [1991].