Potential Evapotranspiration (PET): These data represent decadal mean totals of potential evapotranspiration estimates (mm). The file name specifies the decade the raster represents. For example, a file named pet_mean_mm_decadal_CRU_Historical_annual_1930-1939.tif represents the decade spanning 1930-1939. The data were generated by using the Hamon equation and output from a statistically downscaled version of the Hadley Centre’s CRU TS3.0 observational dataset. Data are at 2km x 2km resolution, and all data are stored in geotiffs. Calculations were performed using R 2.12.1 and 2.12.2 for Mac OS Leopard, and data were formatted into geotiffs using the raster and rgdal packages. Users are reminded that the PET estimates include the inherent uncertainty of model projections. In addition, the methods used to derive PET from temperature require a number of assumptions, which future conditions may or may not meet. In particular, these methods as implemented assume that cloudcover and relative humidity remain constant. This may or may not be true. Hamon Equation: These data were derived by applying the Hamon equation as outlined in Lu et al. (2005) to monthly average temperature downscaled by SNAP.
