Global reference evapotranspiration for food-security monitoring (ver. 2.2, March 2025)

The data are a long-term (1980-present), daily reanalysis of reference evapotranspiration, covering the globe at a spatial resolution of 0.625° Longitude x 0.5° Latitude. Reference evapotranspiration is a measure of evaporative demand, or the "thirst of the atmosphere", basically how much moisture from the surface could evaporate into overpassing air, assuming (i) that enough water is available to evaporate and (ii) the surface is covered with a specific reference crop that completely shades the ground (some other conditions also apply). For this dataset, reference evapotranspiration is derived from the daily implementation of the Penman-Monteith reference evapotranspiration equation (Monteith, 1965) as codified in the FAO-56 report (FAO, 1998), which is the international standard for reference evapotranspiration estimation. The equation is driven by data from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2; Gelaro et al., 2017). The MERRA-2 input data are at a spatial resolution of 0.5 deg lat x 0.625 deg long; the reference evapotranspiration output data are at the same spatial resolution. There are two outputs, one for each reference crop: ETos is the reference evapotranspiration from a short crop (0.12-m grass); ETrs is the reference evapotranspiration from a long-crop (0.5-m alfalfa).

Monteith, J.L., 1965, Evaporation and the Environment. 19th Symposia of the Society for Experimental Biology, 19, 205-234.
Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998, Crop Evapotranspiration-Guidelines for Computing Crop Water Requirements-FAO Irrigation and Drainage Paper 56. FAO, Rome, 300(9): D05109.
Gelaro, R., McCarty, W., Suárez, M. J., Todling, R., Molod, A., Takacs, L., Randles, C. A., Darmenov, A., Bosilovich, M. G., Reichle, R., Wargan, K., Coy, L., Cullather, R., Draper, C., Akella, S., Buchard, V., Conaty, A., da Silva, A. M., Gu, W., Kim, G., Koster, R., Lucchesi, R., Merkova, D., Nielsen, J. E., Partyka, G., Pawson, S., Putman, W., Rienecker, M., Schubert, S. D., Sienkiewicz, M., & Zhao, B., 2017, The Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2). Journal of Climate, 30(14), 5419-5454. https://doi.org/10.1175/JCLI-D-16-0758.1

First posted - August 22, 2022
Revised - October 10, 2023 (version 2.0)
Data addition - April 10, 2024 (version 2.1)
Data addition - March 27, 2025 (version 2.2)