High temperature and reference evapotranspiration

When the air temperature increases, the reference evapotranspiration tends to increase. We've shown what this relationship looks like across a range of temperatures in California.

Temperature in that case is not changing on its own, however. There are also differences in wind, and in solar radiation, and in day length, and in humidity—all of which are variables that influence the reference evapotranspiration (ETo) calculated by the Penman-Monteith equation.

One way to isolate the effect of different factors on ETo is…