Reference evapotranspiration (ETr) is defined as the potential water lost to evaporation and plant transpiration from a reference surface:

The reference surface is a hypothetical grass reference crop with an assumed crop height of 0.12 m, a fixed surface resistance of 70 s m

^{-1}and an albedo of 0.23. The reference surface closely resembles an extensive surface of green, well-watered grass of uniform height, actively growing and completely shading the ground. The fixed surface resistance of 70 s m^{-1}implies a moderately dry soil surface resulting from about a weekly irrigation frequency.^{1}

To calculate ETr, we use the Food and Agriculture Organization’s Penman-Monteith ETr algorithm. This method allows ETr to be calculated with using either hourly or daily meteorological observations collected at our Mesonet stations. With this method, daily ETr is calculated as:

\[ET_r = \frac{0.408 \Delta (R_n - G) + \gamma \frac{900}{T + 273} u_2 (e_s - e_a)}{\Delta + \gamma (1 + 0.34 u_2)}\]where:

- $ET_r$ is the reference surface evapotranspiration (mm/day)
- $R_n$ is the net radiation at the crop surface (MJ/m
^{2}/day) - $G$ is soil heat flux (MJ/m
^{2}/day) - $T$ is the average temperature (°C)
- $u_2$ is the wind speed at 2 meters height (m/s)
- $e_s$ is the saturation vapor pressure (kPa)
- $e_a$ is the actual vapor pressure (kPa)
- $\Delta$ is the slope of the vapor pressure curve (kPa/°C)
- $\gamma$ is the psychrometric constant (kPa/°C)

For more details about how ETr is calculated and how meteorological data are used to derive these variables, please visit the following webpage: https://www.fao.org/3/x0490e/x0490e08.htm