User Manual for the Water Balance Toolbox [Revised August 2010, to include note about aligning all input grids]
in PDF format (click here if you need to download the Acrobat Reader.)
Water Balance Toolbox - Demo Version (98 MB)
Zipped toolbox (Water Balance.tbx) developed for ArcGIS (v.9.2) PLUS sample grids to run the model. Zipped with complete folder structure for executing the model.
Update [May 2010]: Temperature grids for months 04 & 09, and the Precipitation grid for month 10 were originally integer format; these have been converted to floating point.  (Precipitation for month 10 is zero -- this is not an error.)
Water Balance Toolbox (175 KB)
Zipped toolbox (Water Balance.tbx) developed for ArcGIS (v.9.2), without sample grids or folders.
Guidelines for Computing Monthly Radiation Grids Using the Solar Radiation Toolset in ArcGIS
in PDF format (click here if you need to download the Acrobat Reader.)

The National Renewable Energy Laboratory provides solar radiation estimates for the U.S. at 10-km resolution.
URL: http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/

The “readme” file for the SUNY 10-km gridded data contains information about file structure and organization.  Global Horizontal Radiation data can be downloaded for a study area and used to parameterize the Diffuse Proportion (D) and Transmittivity (T) values in ArcGIS’s Solar Radiation Toolset.  Since the SUNY 10-km gridded data are provided at hourly intervals, the global horizontal radiation values can be summed to provide a monthly total, which will be directly comparable to monthly estimates derived from the Solar Radiation Toolset.  Refer to these Instructions for Processing SUNY Gridded Radiation [added August 2010].

In the U.S., soil water-holding capacity is available from digitized soil surveys from the Natural Resources Conservation Service (1:12,000 to 1:63,360 mapping scales; the “SSURGO” database).
URL: http://soildatamart.nrcs.usda.gov
The polygon shapefiles can be joined with available water capacity attributes (located in the "muaggatt" table in the accompanying .mdb database; the join will be based on the "mukey" field.)  To perform water balance analysis, these polygon shapefiles can be converted to AWC grids.  In ArcGIS, this would involve using ArcToolbox – Conversion Tools – To Raster – Polygon to Raster:

The resulting grid will need to be converted to millimeters.  In ArcGIS, this would involve using ArcToolbox – Spatial Analyst Tools – Math – Times.

DEMs are available for many locations at ⅓-arc second (~10 m) resolution (or finer) from the U.S. Geological Survey’s National Elevation Dataset.
URL: http://seamless.usgs.gov/
The DEMs that you download will have elevation in meters, with a Geographic Coordinate System (NAD83) projection (units = degrees).  Thus, the x, y, and z values will be in different units.  You will need to reproject the grid to a "meter-based" projection (such as Plate Carree (world), or UTM).  An easy way to accomplish this is to set the projection of the Data Layer that contains the DEM, then right-click on the DEM and select Data – Export Data.  For Spatial Reference, select "Data Frame (Current)."  The output raster should have square cells, and be in Grid format. 
 

Monthly temperature and precipitation grids are available for the U.S. at 30-arc second (~800 m) resolution from the PRISM Group.
URL: http://www.prism.oregonstate.edu/
After downloading and unzipping the grid, change its extension to ".txt"  You can then use ArcGIS's ArcToolbox – Conversion Tools – To Raster – ASCII to Raster.  The new grid will be in Geographic Coordinate System - Spheroid-Based - GRS 80 (ArcToolbox – Data Management Tools – Projections and Transformations – Define Projection).