Terrestrial Water Balance Data Archive:
Regridded Monthly Climatologies

(Version 1.02)

produced and documented by

Cort. J. Willmott and Kenji Matsuura
(with support from NASA's Seasonal to Interannual ESIP)

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 1.02) released July 1, 2001


Gridded monthly air temperature (T) and unadjusted (for raingage undercatch) precipitation (P) (at a 0.5 x 0.5 degree resolution) were taken from the Terrestrial Air Temperature and Precipitation: Monthly and Annual Climatologies (V. 3.02) which are based on 50 years of monthly T and P observations. Interpolations to the grid were made using CAI. Our estimates of the climatic water balance were made from these 0.5x0.5 degree T and P fields and are available for 85,974 land grid points.


Average-monthly water-balance fields were estimated from the gridded average-monthly T and P fields according to Willmott et al.'s (1985) modified version of the Thornthwaite water-balance procedure. The computational algorithm was derived from Willmott (1977). Climatic water-balance calculations were separately made for each of the 85,974 grid nodes, thereby conserving mass (in z only) at each grid node. These 0.5-degree resolution water-balance estimates are based on semi-empirical relationships between observed average monthly P and an estimated average monthly potential evapotranspiration (Eo), derived from an average monthly T. Soil water-holding capacity (w*) was held constant at 150 mm. A snow-cover water budget also was evaluated and coupled with the soil-moisture balance according to Willmott et al. (1985). Water-balance variables estimated and archived here include: average-monthly Eo in mm, average-monthly actual evapotranspiration (E) in mm, average-monthly deficit (def) in mm, mid-monthly soil-moisture depth (w) in mm, mid-monthly water equivalent of the snow pack (ws) in mm, average-monthly snow melt (M) in mm, and average-monthly surplus (S) in mm.


Water balance climatologies were produced and archived for a single soil water-holding capacity, w*=150mm. There are seven files in the archive. The beginning of each file name describes the content of the file as follows.

def150.clim: deficit
E150.clim :actual evapotranspiration
Eo150.clim :adjusted potential evapotranspiration
M150.clim :snow melt
S150.clim :surplus
w150.clim :mid-monthly soil moisture
ws150.clim :mid-monthly snow cover
All files have the structure.
Field Columns Variable Fortran Format
1 1 - 8 Longitude (decimal degrees)
2 9 - 16 Latitude (decimal degrees)
3-14 17 - 112 Monthly values (mm)


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