I am surprised by the opening remarks of Moustafa T. Chahine in his article, “GEWEX: The Global Energy and Water Cycle Experiment” (Eos, January 14, 1992, p. 9), which stated that “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.” The author here refers to a personal communication made to him in 1991 by J. Shaake.
It seems that the author of this article and his reference are not aware of important advances made in the subject of global water balance during the last decades, particularly during the International Hydrological Decade Program sponsored by UNESCO and carried out successfully with the cooperation of 110 member countries. One of the major outcomes of this program was the establishment of World Water Balance based on a vast amount of meteorological and hydrological data [Korzun, 1978;Budyko, 1980]. A wealth of detailed information about the water balance of continents, oceans, major river basins, and islands, including atmospheric moisture fluxes to and from the continents, is found in the UNESCO publication edited by Korzun , often with residual values of the water balance that are less than 10 and very often less than 5% of the main water balance components. This is in serious contradiction with the statement that these components are known at best to a factor of two to three as suggested in Chahine's article.