The need for better quantitative understanding of energy fluxes and water exchanges is basic to all climate studies, yet our quantitative knowledge of the global hydrological cycle remains surprisingly poor. Evaporation minus precipitation or, similarly, the net flow of water from land to oceans and the net advection of moisture from the marine atmosphere to the terrestrial atmosphere is known at best to a factor of two to three (J. Schaake, personal communication, 1991). The regional and temporal distribution of the components of the hydrological budget are even more uncertain.
Improved understanding of the global hydrological cycle has been limited in part by the absence of reliable global data on such essential quantities as precipitation, evaporation, and atmospheric transport; acquisition of these data is a prerequisite for the study of global energetics and hydrology. Space agencies in the United States, Europe, and Japan have announced plans to launch a series of global Earth observing satellites in the late 1990s to monitor these essential atmospheric and surface parameters and to complement existing operational meteorological and hydrological networks.