Terrestrial Water Balance Data Archive:
Regridded Monthly Climatologies
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
Archive (Version 1.01) released May 12, 1999
STATION AND GRIDDED DATA SOURCES:
Climatic means of monthly air temperature (T) and unadjusted
(for raingage undercatch) precipitation (P) were taken from the
station and oceanic-grid-point records compiled by Legates and
Willmott (1990a and b). These in-situ averages then were used to
spatially reinterpolate average-monthly T and P fields to a 0.5x0.5
degree spatial grid, which covers the entire globe. The
interpolations were, in part, based on Willmott et al.'s (1985a)
spherical implementation of Shepard's spatial-interpolation
algorithm, although an average of 20 nearest stations influenced
each grid-point estimate, instead of the seven suggested by
Shepard. Two separate sets of gridded T fields were alternately
interpolated, first using "traditional" interpolation alone
(Willmott et al., 1985a, as outlined above), and second by "DEM-assisted"
interpolation as described by Willmott and Matsuura
(1995). It is likely that the DEM-assisted T fields are more
realistic over the land surfaces, with regional and seasonal
exceptions. These gridded T and P data fields are available
through this site under the title "Global Air Temperature
and Precipitation: Regridded Monthly and Annual Climatologies
(Version 2.01)." Our estimates of the climatic water balance from
T and P (see below) were made at (and are available for) only the
land-surface grid points, which number 86,609.
GRIDDED WATER-BUDGET ESTIMATES:
Average-monthly water-balance fields were estimated from the
gridded average-monthly T and P fields according to Willmott et
al.'s (1985b) 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 86,609 grid nodes, thereby conserving mass (in z
only) at each grid node. Two complete sets of water-balance
estimates were produced and archived: one based on the
traditionally interpolated average-monthly T fields; and the other
based on a DEM-assisted gridding of average-monthly T. 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, e.g. at 150 mm. A snow-cover water budget
also was evaluated and coupled with the soil-moisture balance
according to Willmott et al. (1985b). 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.
Separate "...Water Balance Climatologies" were
produced and archived for several different soil water-holding
capacities, namely w*=150, w*=100, w*=75 and w*=50mm.
Furthermore, the entire suite of water-balance computations were
performed and archived twice; once, with "..traditionally
interpolated.." air-temperature fields; and, once, with forced with
"..DEM-aided.." air-temperature fields.
Each of the two alternately interpolated air-temperature
fields was used to estimate potential evapotranspiration (Eo
and the other components of the water balance.
Please see the associated README file for additional information.
Each directory (traditionally interpolated and DEM-aided) has seven files.
The beginning of each file name describes the content of the file
as follows. If the name includes "_d", it is based on DEM-aided,
interpolated air-temperature fields. The "n" refers to the soil water-holding
|E(_d)n... :||actual evapotranspiration |
|Eo(_d)n... :||adjusted potential evapotranspiration |
|M(_d)n... :||snow melt |
|S(_d)n... :||surplus |
|w(_d)n... :||mid-monthly soil moisture |
|ws(_d)n... :||mid-monthly snow cover |
All files have the structure.
||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,