Tool for calculating reference evapotranspiration (ETo) and other agrometeorological variables using the Penman-Monteith method, according to FAO 56
Abbate, P.E. (2026). AGROCLIMA: A web version in JavaScript to calculate reference evapotranspiration by the Penman-Monteith method, according to FAO 56 (Version 1.0) [Software]. https://cultivaresargentinos.com/agroclima/ DOI: https://doi.org/10.5281/zenodo.21874400
Agronomic description
This page calculates reference evapotranspiration -evaporation from the soil plus transpiration from the canopy- on a daily basis (ETo, mm/d), without a crop coefficient, using the Penman-Monteith method as described in FAO 56 (Allen, R.G., Pereira, L.S., Raes, D. and Smith, M., 1998. Crop evapotranspiration — Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56).
The FAO 56 method is currently the international standard for calculating ETo. ETo is the basis for estimating the water requirement of a given crop (ETc = Kc × ETo, where Kc is the crop coefficient, which varies over the crop cycle), for irrigation scheduling, for water balance calculations, and for agroclimatic studies.
The calculation requires solar radiation, maximum and minimum temperatures, a measure of air humidity (vapor pressure, dew point, or relative humidity), and wind speed, together with the latitude, altitude, and wind measurement height of the site. When measured radiation is not available, it can be estimated from sunshine hours; when actual vapor pressure is not available, it can be obtained from dew point or relative humidity.
AGROCLIMA was originally presented in 2004, at the 10th Argentine and 4th Latin American Agrometeorology Meeting (Mar del Plata), as a C++ library for calculating ETo from spreadsheets or programming languages. Its performance was validated against the CROPWAT and ET_CSDLL programs at four climatically contrasting locations (Kurnool, India; Santa Rosa, La Pampa; Balcarce, Bs. As.; and Montpellier, France), with very good agreement (r² = 0.99). See: Abbate, P.E. (2004). AGROCLIMA: un programa para el cálculo de la evapotranspiración por el método FAO Penman-Monteith desde planillas de cálculo o lenguajes de programación (full article, in Spanish).
This page uses the same calculation algorithm as the AGROCLIMA add-in for Excel (XLL); it replaces that add-in without requiring any installation. The calculation is for daily periods.
Description of the variables (columns)
Input
- Date: date of the record in dd/mm/yyyy format; alternatively, the day of the year (Jdoy), between 1 (Jan 1st) and 365/366 (Dec 31st).
- Tmax (°C): maximum air temperature.
- Tmin (°C): minimum air temperature.
- Wind (m/s): wind speed. To convert km/h to m/s, divide by 3.6; to convert km/d to m/s, divide by 86.4.
- Radiation (MJ/m²/d): incoming global (total) solar radiation. If not available, choose the estimation mode with the Solar radiation selector.
- Sunshine (h or %): used to estimate Radiation. It is the number of effective sunshine hours (affected by cloudiness), as an absolute value (hours) or as a percentage of the theoretical sunshine duration. Uses the as and bs constants of the Angstrom equation (general values as = 0.25 and bs = 0.50; values typically used in Argentina as = 0.18 and bs = 0.55).
- AVP (kPa): actual vapor pressure of the air. If available, enter it directly; otherwise, choose with the Actual vapor pressure selector how to estimate it — from dew point temperature, from relative humidity, or approximated with Tmin.
- Dew point (°C): dew point temperature, used to estimate the actual vapor pressure when the measured value is not available.
- RHmax / RHmin (%): maximum and minimum relative humidity of the air, an alternative for estimating the actual vapor pressure.
Site parameters (loaded once)
- Latitude (°): between -90 and 90; negative in the Southern Hemisphere (e.g.: for 37°45'S use -37.75).
- Altitude (m): elevation of the site above sea level.
- Wind measurement height (m): height at which wind speed was measured.
Result
- Jdoy: day of the year, calculated from the date.
- AVP (kPa): actual vapor pressure, calculated according to the selected mode (not shown as a result if you entered it directly as data).
- VPDF56 (kPa): vapor pressure deficit of the air — the difference between the saturation vapor pressure (from Tmax and Tmin) and the actual vapor pressure (AVP); an indicator of the evaporative demand of the atmosphere.
- VPDX (kPa): weighted vapor pressure deficit, calculated with a weighting factor of 0.72; it better represents daytime conditions. Used by Abbate, P.E., Dardanelli, J.L., Cantarero, M.G., Maturano, M., Melchiori, R.J.M. and Suero, E.E. (2004). Climatic and Water Availability Effects on Water-Use Efficiency in Wheat. Crop Science, 44:474-483. It is shown when selecting the vapor pressure mode based on relative humidity.
- Radiation used (MJ/m²/d): global radiation used in the calculation (shown when it was estimated from sunshine hours).
- ETo (mm/d): reference evapotranspiration, in millimeters of water per day.
Calculation strategy
- Blue columns are input data and green columns are results.
- Enter the latitude, altitude, and wind measurement height of the site just once.
- Use the Solar radiation selector to indicate whether radiation data is measured or will be estimated from sunshine hours; the input columns change according to your selection.
- Use the Actual vapor pressure selector to choose the estimation method for actual vapor pressure; the input columns change according to your selection.
- Drag the column headers left or right until the order matches the data you are going to enter or paste.
- Insert the rows you need with the [+ Row] or [Insert several rows] buttons.
- Enter the data manually or paste it from Excel or another source with Ctrl+V.
- When entering data manually, move between cells with the Tab or Enter keys.
- Use a period (.) as the decimal separator and do not use thousands separators.
- The date can be written as dd/mm/yyyy or directly as Jdoy. Southern latitude is negative (e.g.: 37°45'S → -37.75).
- To copy results to Excel or another application, use the [Copy] button or select the cells and press Ctrl+C, then paste into the destination with Ctrl+V. Results can also be exported as CSV or Excel.
- The [Reset] button clears the numeric data and any inserted rows.
- This page requires an Internet connection.
- This page does not store data or results, and does not collect any user information.
Site parameters (loaded once)
Available data (data to estimate)
Would you rather keep using the Excel add-in? Download AGROCLIMA for Excel (.XLL, in spanish). The authors of this page are not responsible for the use given to the information provided, nor for any direct or indirect damages it may cause. First version date: . Last update date: . Copyright 2026.
This page is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License. Its information may be used with attribution to the authors.