"Energy only" PET from Milly and Dunne 2016

[Aariq]

Milly and Dune (2016) investigate the Penman-Monteith method for calculating PET and find that it overestimates drought compared to non-water-stressed evapotranspiration (which should theoretically be equal). They propose a simplification of Penman-Monteith that better predicts PET: PET = 0.8(R_n - G) where R_n is net radiation at the surface, and G is heat flux into the subsurface, both converted to units of mm/d. This might be a nice addition to the SPEI package.

Milly PCD, Dunne KA (2016) Potential evapotranspiration and continental drying. Nature Clim Change 6:946–949. https://doi.org/10.1038/nclimate3046