Abstract
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 values of Kp and the observed 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 the 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.
Highlights
Reference evapotranspiration (ETo) is an essential component for use in water supply planning and irrigation scheduling (Snyder, 1992) since the crop evapotranspiration (ETc) is estimated by ETo multiplied by the crop coefficient (Kc)
The estimated and the observed values of Kp, obtained from the relationship between ETo measured in a weighing lysimeter and evaporação do tanque Classe A (ECA) measured in a Class A pan, were compared by regression analysis
Pereira et al (1995) proposed a model for the Kp which was based on the relationship between ETo and ECA, both estimated by Penman-Monteith equation, adopting a maximum Kp value equal to 0.85
Summary
Reference evapotranspiration (ETo) is an essential component for use in water supply planning and irrigation scheduling (Snyder, 1992) since the crop evapotranspiration (ETc) is estimated by ETo multiplied by the crop coefficient (Kc). One common method to estimate ETo is converting the class A pan evaporation (ECA) into ETo by using a pan coefficient (Kp), which varies depending on the site and the weather conditions as showed by Doorenbos & Pruitt (1977) and Allen et al (1998). According to Snyder (1992) the equation proposed by Cuenca is complex, and in some cases the output is quite different from the original data found in Doorenbos & Pruitt’s table (Doorenbos & Pruitt, 1977). The predicted Kp obtained by the authors showed better fit than Cuenca’s or Snyder’s equations in relation to the data from Doorenbos & Pruitt’s table
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