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This paper presents a new diagnostic dataset of monthly variations in terrestrial water storage for 37 midlatitude river basins in Europe, Asia, North America, and Australia. Terrestrial water storage is the sum of all forms of water storage on land surfaces, and its seasonal and interannual variations are in principle determined by soil moisture, groundwater, snow cover, and surface water. The dataset is derived with the combined atmospheric and terrestrial water-balance approach using conventional streamflow measurements and atmospheric moisture convergence data from the ECMWF 40-yr Re-Analysis (ERA-40). A recent study for the Mississippi River basin (Seneviratne et al. 2004) has demonstrated the validity of this diagnostic approach and found that it agreed well with in situ observations in Illinois. The present study extends this previous analysis to other regions of the midlatitudes. A systematic analysis is presented of the slow drift that occurs with the water-balance approach. It is shown that the drift not only depends on the size of the catchment under consideration, but also on the geographical region and the underlying topography. The drift is in general not constant in time, but artificial inhomogeneities may result from changes in the global observing system used in the 44 yr of the reanalysis. To remove this time-dependent drift, a simple high-pass filter is applied. Validation of the results is conducted for several catchments with an appreciable coverage of in situ soil moisture and snow cover depth observations in the former Soviet Union, Mongolia, and China. Although the groundwater component is not accounted for in these observations, encouraging correlations are found between diagnostic and in situ estimates of terrestrial water storage, both for seasonal and interannual variations. Comparisons conducted against simulated ERA-40 terrestrial water storage variations suggest that the reanalysis substantially underestimates the amplitude of the seasonal cycle. The basin-scale water-balance (BSWB) dataset is available for download over the Internet. It constitutes a useful tool for the validation of climate models, large-scale land surface data assimilation systems, and indirect observations of terrestrial water storage variations. [ABSTRACT FROM AUTHOR]

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 land ice. On a regional scale, it is however not a readily measured variable and observations of its individual components are scarce. This study investigates the feasability of estimating monthly terrestrial water-storage variations from water-balance computations, using the following three variables: water vapor flux convergence, atmospheric water vapor content, and river runoff. The two first variables are available with high resolution and good accuracy in the present reanalysis datasets, and river runoff is commonly measured in most parts of the world. The applicability of this approach is tested in a 10-yr (1987-96) case study for the Mississippi River basin. Data used include European Centre for Medium-Range Weather Forecasts 40-yr reanalysis (ERA-40) data (water vapor flux and atmospheric water vapor content) and runoff observations from the United States Geological Survey. Results are presented for the whole Mississippi River basin and its subbasins, and for a smaller domain covering Illinois, where direct measurements of the main components of the terrestrial water storage (soil moisture, groundwater level, and snow cover) are available. The water-balance estimates of monthly terrestrial water-storage variations show excellent agreement with observations taken over Illinois. The mean seasonal cycle, as well as interannual variations, are captured with notable accuracy. Despite this excellent agreement, it is not straightforward to integrate the computed variations over longer time periods, because there are small systematic biases in the monthly changes. These biases likely result from inaccuracies of the atmospheric assimilation system used to estimate the atmospheric water vapor convergence and can be corrected in part with the application of a simple detrending procedure. It is noteworthy that the critical domain size for water-balance computations, using high-resolution reanalysis data such as ERA-40, appears to be much smaller than for raw radiosonde data. The Illinois domain has a size of only ∼2 × 105 km2 and is shown to be suitable for the computation of the water-balance estimates. A comparison for other regions would be needed in order to confirm this result. [ABSTRACT FROM AUTHOR]