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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Agricultural and Forest Meteorology, Amsterdam, v. Revista Brasileira de Agrometeorologia, Santa Maria, v. The Rnestimated obtained with the fifth model has a relatively high error. This paper analyses the following methods to estimate Kp values: 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.

Statistics for the evaluation and comparison of models. Scientia Agricola, Piracicaba, v.

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

Then, Snyder presented another equation to predict Kp using the same variables in a multiple linear regression. According to Pereira et al. The same routine was adopted to evaluate ETo estimates with the different Kp values. Report on expert consultation on procedures for revision of FAO methodologies for crop water requirements. Reference doorennos ETo is an essential component for use in water supply planning and irrigation scheduling Snyder, since the crop evapotranspiration ETc is estimated by ETo multiplied by the crop coefficient Kc.


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. Kpseveral performance criteria were used including regression analysis, agreement index Dmean absolute error MAEmaximum absolute error MAXEand efficiency EFas suggested by Willmott et al.

According to Villa Nova et al. When these Prutit values were used to estimate 197 ETo Figure 2a good agreement was observed between estimated and pruittt values of ETo, especially when the Kp was estimated by Eq.

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All the contents of this journal, except where otherwise noted, is licensed under a Creative Doorenbks Attribution License. Agronomy Journal, Madison, v. Robust quantitative techniques dooorenbos validating pesticide transport models. It can be seen that Kp predicted by all methods remained between 0.

These criteria are defined as: Reference evapotranspiration based on Class A pan evaporation. Most of the methods have shown that Kp value is highly dependent on surrounding conditions and is determined by U, H and F. The use of an arbitrary and constant Kp 0.

Four of those models consider the effects of cloud cover: Services on Demand Journal. When a constant value of Kp 0. With these methods, the relationship between measured and estimated ETo showed high accuracy and good precision: 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 for different climatic conditions.

Table 2 presents the statistical analysis of ETo estimates using different Kp methods. However, with modern automatic weather stations and computer facilities, it is convenient to automate ECA to ETo conversions using equations Snyder, The objective of this study was to assess 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 atmospheric pressure was determined by aneroid barograph, and sunshine duration was quantified with a Campbell-Stokes recorder. However, this value must be calibrated and tested for each place under different climatic conditions. Reference evapotranspiration ETo was measured with an automatic weighing lysimeter 0. Operational estimates of reference evapotranspiration.

A model for the class A pan coefficient. Journal of Water Resource and ProtectionVol. 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. The values of Kp were calculated by the relationship between ETo, from the lysimeter, and ECA, and estimated from the following methods: 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 following data were collected: Evapotranspiration as a function of leaf area index and class A pan evaporation.