Terrestrial Water Budget Data Archive:
Monthly Time Series (1900 - 2008)

(Version 1.03)

produced and documented by
Kenji Matsuura and Cort J. Willmott
(with support from IGES and NASA)

For additional information concerning this archive,
please contact us at:

Center for Climatic Research
Department of Geography
University of Delaware
Newark, DE 19716
(302) 831-2294



Archive (Version 1.03) released August 21, 2009


Time series of gridded monthly average air temperature (T) and total precipitation (P) were taken from the Terrestrial Air Temperature: 1900-2008 Gridded Monthly Time Series (Version 2.01) and Terrestrial Precipitation: 1900-2008 Gridded Monthly Time Series (Version 2.01) archives, which also are available through this web site. The length of each grid-point time series is 109 years (from 1900 through 2008) and the grid points cover the global land surface at a 0.5x0.5 degree resolution. Our spatial interpolations were based on Willmott et al.’s (1985a) spherical implementation of Shepard’s spatial-interpolation algorithm, and they also incorporated DEM (digital elevation model assisted) and climatologically-aided interpolation methods (see the related references below). Our estimates of the climatic water budget from T and P (see below) were made at (and are available for) the land-surface grid points, which number 85,794.


Monthly water-budget fields were estimated from the gridded monthly-average T and total P fields according to Willmott et al.'s (1985b) modified version of the Thornthwaite water-budget procedure. The computational algorithm was derived from Willmott (1977). Climatic water-budget calculations were separately made for each of the 85,794 grid nodes, thereby conserving mass (in z only) at each grid node. These 0.5-degree resolution water-budget estimates are based on semi-empirical relationships between observed monthly total P and an estimated monthly potential evapotranspiration (Eo), derived from a monthly average T. Available soil water-holding capacity (w*) was held constant at 150 mm for these calculations. Time-series fields with different w*s may be added in the future. A snow-cover water budget also was evaluated and coupled with the soil-moisture budget according to Willmott et al. (1985b). Water-budget variables estimated and archived here include: monthly Eo in mm, monthly actual evapotranspiration (E) in mm, average-monthly deficit (Eo- E) in mm, mid-monthly soil-moisture depth (w) in mm, mid-monthly water equivalent of the snow pack (ws) in mm, monthly snow melt (M) in mm, and monthly surplus (S) in mm.


There are seven gzipped tar files, one for each water-budget variable. Each tar file includes 109 yearly files.





actual evapotranspiration


adjusted potential evapotranspiration


snow melt




mid-monthly soil moisture


mid-monthly snow cover

All yearly files have the same structure (once they are uncompressed).




Fortran Format


1 - 8

Longitude (decimal degrees)



9 - 16

Latitude (decimal degrees)



17 - 112

Monthly values (mm)




Legates, D. R. and C. J. Willmott, 1990a. Mean Seasonal and Spatial Variability in Gauge-Corrected, Global Precipitation. International Journal of Climatology, 10, 111-127.

Legates, D. R. and C. J. Willmott, 1990b. Mean Seasonal and Spatial Variability in Global Surface Air Temperature. Theoretical and Applied Climatology, 41, 11-21.

Willmott, C. J., 1977. WATBUG: A FORTRAN IV Algorithm for Calculating the Climatic Water Budget. Pubs. in Climatology, 30, 1-55. (Also published as Report 1 in The Use of the Climatic Water Budget in Water Resources Management and Control. Newark, DE: University of Delaware, Water Resources Center, 1977).

Willmott, C.J. and K. Matsuura, 1995. Smart Interpolation of Annually Averaged Air Temperature in the United States. Journal of Applied Meteorology, 34(12), 2577-2586.

Willmott, C.J., C.M. Rowe, and W.D. Philpot, 1985a. Small-Scale Climate Maps: A Sensitivity Analysis of Some Common Assumptions Associated with Grid-Point Interpolation and Contouring. The American Cartographer, 12, 5-16.

Willmott, C. J., C. M. Rowe, and Y. Mintz, 1985b. Climatology of the Terrestrial Seasonal Water Cycle. Journal of Climatology, 5, 589-606.