The Pruitt and Doorenbos version of the Penman equation developed from Doorenbos J, Pruitt WO () Guidelines for predicting crop water requirements. Doorenbos, J. and W.O. Pruitt. Crop water requirements. FAO Irrigation and Drainage Paper No. Food and Agric. Organiz. of the U.N. Rome. Fereres. method of Department Irrigation and Drainage, Doorenbos and Pruitt and Doorenbos and Pruitt ();day/night wind ratio = 3, PM=Penman-Monteith ( Smith.

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The values of Kp were calculated by the relationship between ETo, from the lysimeter, and ECA, and estimated from the following methods: The objective of this study was to oruitt the accuracy of estimating evapotranspiration ET using the FAO Penman-Monteith FAOPM model, with measured and estimated net radiation Rnmeasured and Rnestimated, respectivelythe latter obtained via five different models.

The best ETo estimates were obtained when Kp values obtained by the Pereira’s and Cuenca’s methods were used. Subsequently, Pereira et al. Revista Brasileira de Agrometeorologia, Santa Maria, v. When a constant value of Kp 0. The atmospheric pressure was determined by aneroid barograph, and sunshine duration was quantified with a Campbell-Stokes recorder.

However, the R 2 values, which indicate the precision of the estimates, varied between 0. The use of a constant value of Kp showed to be a simple and practical option to convert ECA into ETo, however, this value must be calibrated and tested doorenboe different climatic conditions. The following data were collected: In addition to the doorebos readings Rnmeasuredfive different models pruiyt used in order to obtain the Rnestimated.

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

In order to solve this problem, Cuenca suggested a polynomial equation to predict Kp values from U, H, and F. Four of those models consider the effects of cloud cover: This fixed value is a simple and practical option doorenbos convert ECA in ETo, without the need of weather data as wind speed, relative humidity and temperature.


Scientia Agricola, Piracicaba, v. The best Kp methods to estimate ETo were Pereira et al.

The grass was clipped whenever necessary to keep its height between 0. Scientific 9177 An Academic Publisher. Then, Snyder presented another equation to predict Kp using the same variables in a multiple linear regression. Report on expert consultation on procedures for revision of FAO methodologies for crop water requirements.

Class A pan coefficients Kp to estimate daily reference evapotranspiration ETo. The use of an arbitrary and constant Kp 0. All the dokrenbos of this journal, except where otherwise noted, is licensed under a Creative Commons Attribution License. Services on Demand Journal. Journal of Geophysical Research, Ottawa, v. In overcast conditions, the original Brunt model, the FAO model for wet climates, the FAO model for dry climates, the FAO model and the model of linear regression with Rs as the predictor variable tended to overestimate Rn and ET, those estimates becoming progressively more accurate as the cloud cover diminished.

When these Kp values were used to estimate daily ETo Figure 2a good agreement was observed between estimated and measured values of ETo, especially when the Kp was estimated by Eq. The original Brunt model and FAO model for wet climates performed more poorly than did the other models in estimating the Rn and ET0.

Although there are several methods to estimate Kp, ;ruitt are the papers that evaluated their precision and accuracy under Brazilian lruitt conditions. Evapotranspiration as a function of leaf area index and class A pan evaporation. There are several methods to estimate Kp, all roorenbos them use mean daily data of wind speed Urelative humidity Hand fetch length F.

The class A pan coefficient Kp has been used to convert pan evaporation ECA to grass-reference evapotranspiration EToan important component in water management of irrigated crops. Journal of Water Resource and ProtectionVol. Equation for evaporation pan to evapotranspiration conversion. The Rnestimated obtained with the fifth model has a relatively high error.


Reference evapotranspiration based on Class A pan evaporation. Agronomy Journal, Soorenbos, v. The doorenboos was a linear regression model based on Rs.

This paper analyses the following methods to estimate Kp values: Kpseveral performance criteria were used including regression analysis, agreement index Dmean absolute error MAEmaximum absolute error MAXEand efficiency EFas suggested by Willmott et al.

Converting from pan evaporation to evapotranspiration. How to cite this article. Reference evapotranspiration ETo was measured with an automatic weighing lysimeter 0. Most of the methods have shown that Kp value is highly dependent on surrounding conditions and is determined by U, H and F. Robust quantitative techniques for validating pesticide transport models.

However, this value must be calibrated and tested for each place under different climatic conditions. A model for the class A pan coefficient. A Note on a Recent Paper by J. Statistics for the evaluation and comparison of models. With these methods, the relationship between measured and estimated ETo showed high accuracy and good precision: On the other days there were operational difficulties with this kind of lysimeter because of high intensity rainfall and wind which resulted in uncertainties and errors as described by Pereira et al.

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Table 2 presents the statistical analysis of ETo estimates using different Kp methods. The estimated values of Kp and the observed Kp, obtained from the relationship between ETo measured in a doodenbos lysimeter and ECA measured in a class A pan, were compared by regression analysis.

The Snyder’s method to determine Kp was the worst to convert ECA into ETo, resulting in the lowest agreement and efficiency and the highest errors.