Recent advances in documenting the atmospheric water cycle over basins of the Americas

The GEWEX Continental-scale International Project (GCIP) has as one of its major scientific objectives to document the temporal and spatial variability of the hydrological cycle over continental and subcontinental scales, with a primary focus on the Mississippi River basin. Probably one of the most challenging aspects has been to obtain reliable estimates of the moisture budgets, i.e., the atmospheric component of the hydrological cycle. However, significant progress has been achieved with the recent availability of operational regional analyses from the National Centers for Environmental Prediction (NCEP) Eta model. Now, moisture budgets for subbasins of the Mississippi River can be estimated with the same internal consistency as those estimated from radiosonde observations at continental scales. These advances are probably due to the model's ability to resolve, in time and space, mesoscale circulations like the Great Plains low-level jet.

Recently, GCIP and VAMOS (Variability of the American Monsoon Systems) started cooperating to transfer to other regions techniques that were successful in the Mississippi River basin. Cuenca del Plata is a basin in southeastern South America that is the water resource for a largely populated area, and is well known by its agricultural production and other factors that sustain the region's economies. Because of its similarities with the Mississippi River basin (e.g., size, location east of a mountain barrier and at the exit region of a prominent low-level jet), it is assumed that it could be a good candidate to transfer the experience already gained in GCIP. However, a careful analysis of moisture sources suggests that also differences can be significant, and they should be taken into account for such transfers to be effective.

In this vein, Eta model forecasts over South America were used to diagnose the components of the moisture budget over Cuenca del Plata. It is shown that while basin-averaged moisture flux convergence is consistent with physical processes, a quantitative estimate of basin-averaged precipitation is still the component that needs to be improved. It is proposed that a blend of observations and high resolution satellite estimates will be needed to complete the description of the atmospheric water cycle.