Simplified Estimation of Reference Evapotranspiration from Pan Evaporation Data in California

Evaporation pan $\left(E_p\right)$ data are often used to estimate reference evapotranspiration $\left({\mathrm{ET}}_0\right)$ for use in water resource planning and irrigation scheduling. This paper reviews equations to estimate ${\mathrm{ET}}_0$ from $E_p$ and provides a simpler method to make this conversion for arid climatic conditions like in California. The new method accounts for fetch differences by first adjusting the $E_p$ rates to values expected for $100\mathrm{m}$ of grass fetch. Then it relies on an empirical relationship between ${\mathrm{ET}}_0$ and the adjusted $E_p$ to determine $K_p$ values; thus, eliminating the need for relative humidity and wind speed data that are often unavailable. The method is conceptually simpler, easier to code into computer applications, and within California, it gave better results than methods based on relative humidity and wind speed. However, the method might require calibration in more humid or windier climates.