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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Class A pan coefficients Kp to estimate daily reference evapotranspiration ETo. Sentelhas I ; Marcos V. 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.
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: 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 roorenbos was adopted to evaluate ETo estimates with the different Kp values.
The best Kp methods to estimate ETo were Pereira et al. The use of an arbitrary and constant Kp 0. This fixed value is a practical and simple option to convert ECA into ETo, but this value must be 197 for each place under different climatic conditions.
Voorenbos 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. There are several methods to estimate Kp, all of them use mean daily data of wind speed Urelative humidity Hand fetch length F. However, with modern automatic weather stations and computer facilities, it is convenient to automate ECA to ETo conversions using equations Snyder, In order to solve this problem, Cuenca suggested a polynomial equation to predict Kp values from U, H, and F.
Then, Snyder presented another equation to predict Kp using the same variables in a multiple linear regression. Subsequently, Pereira et al. Although there are several methods to estimate Kp, few are the papers that evaluated their precision and accuracy under Brazilian climatic conditions.
Class A pan coefficients (Kp) to estimate daily reference evapotranspiration (ETo)
Most of the methods have shown that Kp value is highly dependent on surrounding conditions and is determined by U, H and F. According to Pereira et al.
According to Villa Nova et al. 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.
Reference evapotranspiration ETo was measured with an automatic weighing lysimeter 0. The grass was clipped whenever necessary to keep its height between 0. 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. Class A pan evaporation ECA was also measured in the weather station with a micrometric screw. The values of Kp were calculated by the relationship between ETo, from the lysimeter, and ECA, and estimated from the following methods: Kpseveral performance criteria were used including regression analysis, agreement index Dmean absolute error MAEmaximum absolute error MAXEand efficiency EFas suggested by Willmott et al.
These criteria are defined as: It can be seen that Kp predicted by all methods remained between 0.
When these Kp values were used to estimate daily ETo Figure 2a pruift agreement was observed between estimated and measured values of ETo, especially when the Kp was estimated by Eq. However, the R 2 values, which indicate the precision of the estimates, varied between 0. Table 2 presents the statistical analysis of ETo estimates using different Kp methods. With these methods, the relationship between measured and estimated ETo showed high accuracy and good precision: When a constant value of Kp 0.
This fixed value is a simple and practical option to convert ECA in ETo, without the need of weather data as wind speed, relative humidity and temperature. However, this value must be calibrated and tested for each place under different climatic conditions. The best ETo estimates were obtained when Kp values obtained by the Pereira’s and Cuenca’s methods were used. The Snyder’s method doorenbos determine Kp was the worst to convert ECA into ETo, resulting in the lowest agreement and efficiency and the highest errors.
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.
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