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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Operational estimates of reference evapotranspiration.

Converting from pan evaporation to evapotranspiration. In addition to the sensor readings Rnmeasuredfive different models were used in order to obtain the Rnestimated. The grass was clipped whenever necessary to keep its height between 0.

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Statistics for the evaluation and comparison of models. According to Villa Nova et al. The Rnestimated obtained with the fifth model has a relatively high error.

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. 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. 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.

Report on expert consultation on procedures for revision of FAO methodologies for crop water requirements. Subsequently, Pereira et al. The best ETo estimates were obtained when Kp values obtained by the Pereira’s and Cuenca’s methods were used. A model for the class A pan coefficient. This paper analyses the following methods to estimate Kp values: According to Pereira et al.


We used meteorological data collected between August and Juneon a daily basis and on a seasonal basis wet vs. Scientia Agricola, Piracicaba, 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.

Based on the above discussion, the objective of this paper was to evaluate different methods used to predict Kp values and their influence on the daily estimates of ETo. 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 atmospheric pressure was determined by aneroid 9177, and sunshine duration was quantified with a Campbell-Stokes recorder.

This fixed value is a practical and simple ppruitt to convert ECA into ETo, but this value must be calibrated for each place under different climatic conditions. With these methods, the relationship between measured and estimated ETo showed high accuracy and good precision: Although there are several methods to estimate Kp, few are the papers that evaluated their precision and accuracy under Brazilian climatic conditions.

Journal of Water Resource and ProtectionVol. The same routine was adopted to evaluate ETo estimates with the different Kp values. The fifth was a linear regression model based on Rs. There are several methods to determine Kp values, using wind speed, relative humidity and fetch length doornbos conditions. The best Kp methods to estimate ETo were Pereira et al. There are several methods to estimate Kp, all doorenboss them use mean daily data of wind speed Urelative humidity Doirenbosand fetch length F.


Reference evapotranspiration 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.

Revista Brasileira de Agrometeorologia, Santa Maria, v. Reference evapotranspiration ETo was roorenbos with an automatic weighing lysimeter 0.

Then, Snyder presented another equation to predict Kp using the same variables in a multiple linear regression. Robust quantitative techniques for validating pesticide transport models.

Table 2 presents the statistical analysis of ETo estimates using different Kp methods.

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

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.

A Note on a Recent Paper by J. Agronomy Journal, Madison, v.

Agricultural and Forest Meteorology, Amsterdam, v. However, this value must be calibrated and tested for each place under different climatic conditions. Sentelhas I ; Marcos V. Kpseveral performance criteria were used including regression analysis, agreement index Dmean absolute error MAEmaximum absolute error MAXEoruitt efficiency EFas suggested by Willmott et al. When a constant value of Kp 0.