as it has been aptly termed, consists of about 70% water at its surface. This all-important resource--water--moves eternally and dynamically through changes of state from gas to liquid to solid and back again, permeating to great depths and rising to stratospheric heights. Everywhere it goes, water sustains and nurtures life. Those of us who study the incredible and complex writhings of the hydrologic cycle are termed
. None of us has an expert's knowledge of the entire cycle; each of us tends to concentrate on one or two small aspects of this vast, interdisciplinary subject. The practical difficulties and the high cost of attempting a detailed study of even a small, say 2,000 km2 , river basin are staggering and cannot be appreciated by the layman. Yet efficient management of our water resources becomes more important every day for those of us aboard
Within the Federal Government, the responsibility for effective monitoring of the Nation's water resources is vested in a host of agencies; NOAA is one of those agencies. As part of NOAA, the National Environmental Satellite Service is continuously evaluating and studying new hydrologic applications of satellite data. The ERTS-1 satellite provides hitherto unavailable, synoptic spectral-reflectance data on mesoscale hydrologic features such as snow, water, ice, soil, vegetation and rock. But these features involve disciplines with which even the interdisciplinary hydrologist may not be totally familiar. Nevertheless, this exploring of new ground must be done if we are to use the satellite data wisely. The transition from aircraft remote sensing to satellite data collection is not going to be simple, nor will it be warmly embraced by the operational hydrologist until and unless the advantages are clearly revealed to him in a most convincing manner. ERTS-1 has gone a long way toward demonstrating these advantages.