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_MPI_ECHAM5_A1B_annual_2000-2009.tif represents the decade spanning 2000-2009. The data were generated by using the Hamon equation and output from ECHAM5, a fifth generation general circulation model created by the Max Planck Institute for Meteorology in Hamburg Germany. 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.
