Terrestrial water storage is an essential part of the hydrological cycle, encompassing crucial elements of the climate system such as soil moisture, groundwater, snow, and ice cover. It is however not a readily measured variable and few observations are available of its individual components.

The water-balance method allows to estimate seasonal variations in the terrestrial water storage using three main variables: the water vapour flux convergence, the precipitable water content, and river runoff. The two first variables are available with high resolution and good accuracy in present reanalysis data, and river runoff is commonly measured in most parts of the world.

Here we test the feasability of this approach in a 10-year (1987-1996) case study for the Mississippi river basin. The employed data include ERA-40 reanalysis data from the European Centre for Medium-Range Weather Forecasts (atmospheric water vapour flux and precipitable water content) and runoff observations from the U.S. geological survey.

Results are presented for the Mississippi river basin as a whole, as well as for a smaller domain covering the State of Illinois, where measurements of the main components of the terrestrial water storage (soil moisture, groundwater level, and snow cover) are available. The water-balance estimates of the variations in terrestrial water storage show very good agreement with the observations. The mean seasonal cycle is well represented for the studied period and the interannual variability is in general well captured. Obtaining absolute values of the terrestrial water storage through an integration of the computed values appears to be possible, provided a measurement of the spring storage level is available. In regions where both soil moisture and snow cover measurements are available, estimates of variations in the groundwater level could possibly be provided.

On the whole, these results are very promising and suggest that the employed methodology can give accurate estimates of changes in terrestrial water storage for large river basins. Although a comparison against other data sets (such as the NCEP or ERA-15 reanalyses) has not yet been conducted, it is likely that the high spatial resolution of the ERA-40 reanalysis is important for the high quality of the results.

Current work on this project is focusing on similar computations for other major continental river basins in Europe and Asia (see paper JP2.8). It is also planned to extend the studied period to all the duration of the ERA-40 reanalysis project (1960-2000). Moreover, the computed estimates could also be useful for a cross-validation with the results of the on-going GRACE mission, which aims at the gravimetric estimate from space of changes in terrestrial water storage for the whole globe.