Knowledge of the global fresh water budget is presently inadequate. Key terms in the fresh water flux budget—precipitation, atmospheric transport and continental discharge—are poorly measured and global evaporation is usually modeled or calculated as a residual to other terms on the budget. Knowledge of global water storage is similarly poor. We are just now obtaining precise measurements of the major components of global fresh water storage in global ice, and first steps are just now being taken to measure ground water. The easily accessible fresh water resources from rivers, lakes, snow runoff and ground water mining are only measured in the most affluent portions of the world, and frequently with some difficulty. Presently, however, proposals are now suggesting methods of making global measurements of many of these quantities from space. This is important, as improved knowledge of the global fresh water resources under the effects of climate change is of increasing impact as the human population grows and places the availability of fresh water under increasing stress.

This paper reviews the state of knowledge of the global fresh water budget, evaluating the accuracy of the various global water budget measurement and modeling techniques. This review includes evaluation of the accuracies of the major satellite measurement capabilities, comparison with validation studies and alternative measurement approaches, and comparisons with analytical model analyses. Based on this analyses, and on the goal of improved knowledge of the global fresh water budget under the effects of climate change, we suggest priorities for future improvements in global fresh water budget monitoring. These priorities are based on the relative importance of potential new approaches to improved global fresh water budget evaluations, and on the need to measure and evaluate the potential speed-up of the global water cycle under the effects of climate change. The discussion includes discussion of the technological feasibility of improving various observational and modeling components of global fresh water budget monitoring.