Class A pan coefficients ( Kp ) to estimate daily reference evapotranspiration ( ETo )

The Class A pan coefficient (Kp) has been used to convert pan evaporation (ECA) to grass-reference evapotranspiration (ETo), an important component in water management of irrigated crops. There are several methods to determine Kp values, using wind speed, relative humidity and fetch length and conditions. This paper analyses the following methods to estimate Kp values: Doorenbos & Pruitt (1977); Cuenca (1989); Snyder (1992); Pereira et al. (1995); Raghuwanshi & Wallender (1998); and FAO/56 (Allen et al., 1998). The estimated and the observed values of Kp, obtained from the relationship between ETo measured in a weighing lysimeter and ECA measured in a Class A pan, were compared by regression analysis. The same routine was adopted to evaluate ETo estimates with different Kp values. The results showed that all methods to estimate Kp did not predict it well, with low correlation (R2 < 0.2), which resulted in estimates of ETo with high dispersion (R2 < 0.8). The best Kp methods to estimate ETo were Pereira et al. (1995) and Cuenca (1989), both presenting high efficiency. The use of an arbitrary and constant Kp (0.71) to estimate ETo, produced the same precision and accuracy as the estimates of Kp based on Pereira and Cuenca methods. This fixed value is a practical and simple option to convert ECA into ETo, but this value must be calibrated for each place under different climatic conditions.