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.

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.