Pan Evaporation to Reference Evapotranspiration Conversion Methods

Reference evapotranspiration $\left({\mathrm{ET}}_0\right)$ is often estimated from evaporation pan data as they are widely available and of longer duration than more recently available micrometeorologically based ${\mathrm{ET}}_0$ estimates. Evaporation pan estimation of ${\mathrm{ET}}_0$ $(=K_p{E}_{\mathrm{pan}})$ relies on determination of the pan coefficient $\left(K_p\right)\text{,}$ which depends on upwind fetch distance, wind run, and relative humidity at the pan site. The $K_p$ estimation equations have been developed using regression techniques applied either to the table presented in FAO-24 or to the original data upon which this table was based (from lysimeter studies in Davis, Calif.). Here, the relative performances of the FAO-24 table and six different $K_p$ equations are evaluated with respect to reproducing the original data table using the FAO-24 table as a standard. Evaporation pan- and CIMIS-based estimates of ${\mathrm{ET}}_0$ are also compared for stations having ranges of mean humidities (48–66%) and mean wind runs (156–193 km/day) located in the Sacramento and San Joaquin valleys, and for a coastal station (Point Heuneme) near Ventura, Calif., having a greater mean humidity (71%). In comparing the means, standard deviations, root-mean-square errors, and linear regression coefficients, five of the six equations reproduced the original data table with approximately the same accuracy as the FAO-24 table. Use of either $K_p$ table slightly underestimated measured ${\mathrm{ET}}_0$ at the coastal site, while the Cuenca, Allen-Pruitt, and Snyder $K_p$ equations most closely approximated the average measured ${\mathrm{ET}}_0$ at all seven sites.