Inferring changes in terrestrial water storage using ERA-40 reanalysis data: The Mississippi River basin.

Inferring changes in terrestrial water storage using ERA-40 reanalysis data: The Mississippi River basin.
Date Published
ERA-40 Project Report Series
Document Number
S. Seneviratne
P. Viterbo
D. Lüthi
C. Schär
Event Series/Collection
ERA Report
Abstract 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 with the water-balance method, using the following three variables: water vapour flux convergence, atmospheric water content, and river runoff. The two first variables are available with high resolution and good accuracy in 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-year (1987-1996) case study for the Mississippi river basin. Data used include ERA-40 reanalysis data from ECMWF (water vapour flux and atmospheric water content) and runoff observations from the United States Geological Survey. 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, as 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 vapour convergence. Nevertheless, the results suggest that the critical domain size for water-balance computations using high resolution reanalysis data such as ERA-40 is much smaller than for raw radiosonde data. The Illinois domain has a size of only 2x105 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.