About forty years ago, Gene Rasmusson published two seminal articles in which he discussed the water cycle for North America. Those articles summarized his years of research on the water cycle, and identified the fundamental aspects that impeded achieving further progress in the subject. He noted that the quality of water cycle estimates deteriorates rapidly as the size of a basin is reduced, owing to three basic reasons: (a) errors due to diurnal variations, (b) poor representation of the small scale features of the flux fields, and (c) local station peculiarities (now replaced by grid resolution). One last unresolved issue was the lack of agreement between the atmospheric and terrestrial components of the water cycle (i.e., agreement between moisture flux convergence and river streamflow). Today those same research areas are considered fundamental in hydroclimate studies and are being investigated both in observational and modeling studies, particularly those that deal with the predictability of the hydrologic cycle.

With the development of programs like the Global Energy and Water Cycle Experiment (GEWEX), early on Gene Rasmusson understood that a major advance in water cycle studies could be achieved from diagnostic analysis based on regional fields produced by mesoscale models. As models and data assimilation systems improved, so did the reliability of the hydrologic cycle. Today, research along this path has shown success in representing the hydrologic cycle at scales about one order of magnitude finer than those of the 1960s, and a much closer agreement has being reached between the terrestrial and atmospheric branches of the hydrologic cycle. It can be argued that the development of the North American Regional Reanalysis is the result of diagnostic studies of the water cycle like the ones Gene envisioned many years earlier.

The progress in understanding the water cycle has been remarkable, but still significant challenges are being faced. The next stage involves the role of the hydrologic cycle in Climate Change research, Earth System modeling and Water Management activities. The three topics require a rigorous understanding of the hydrologic cycle at global and regional scales, its interactions with the atmospheric physics and chemistry, its variability and finally, its eventual intensification. As the World's water supply is getting scarce and the World's population is increasingly more vulnerable to changes in hydroclimate, the subject is rapidly moving to the center of attention in many disciplines.