ESTIMATE OF REFERENCE EVAPOTRANSPIRATION THROUGH CONTINUOUS PROBABILITY MODELLING

Eduardo M Uliana,José G F Da Silva,Demetrius D Da Silva,Micael De S Fraga,Luana Lisboa

doi:10.1590/1809-4430-eng.agric.v37n2p257-267/2017

Abstract

ABSTRACT This study aimed at testing the fit of continuous probability distributions to a daily reference evapotranspiration dataset (ET0) at a 75% probability level for designing of irrigation systems. Reference evapotranspiration was estimated by the Penman-Monteith method (FAO-56-PM) for eight locations, within the state of Espirito Santo (Brazil), where there are automatic gauge stations. The assessed probability distributions were beta, gamma, generalized extreme value (GEV), generalized logistic (GLO), generalized normal (GN), Gumbel (G), normal (N), Pearson type 3 (P3), Weibull (W), two- and three-parameter lognormal (LN2 and LN3). The fitting of the probability distributions to the ET0 daily dataset was checked by the Kolmogorov-Smirnov's test. Among the studied distributions, GN was the only one to fit the ET0 data for all studied months and locations. We should also infer that continuous probability models have a good fit to the studied ET0 dataset, enabling its estimation at 75% probability through a Generalized Normal distribution (GN). Therefore, it can be used for the sizing of irrigation systems according to a given degree of risk.

Highlights

ALLEN et al (1998) defined reference evapotranspiration (ET0) as being the evapotranspiration of a hypothetical crop with a height of 12 cm, albedo of 0.23, and surface aerodynamics resistance of 70 s m-1
According to SAAD et al (2002), ET0 is a fundamental variable for estimation of crop water demand, which will influence on the designing of an irrigation system
It is noteworthy that generalized extreme value (GEV), Pearson type 3 (P3), generalized logistic (GLO), and W distributions had a good fit to ET0 dataset

Summary

Introduction

ALLEN et al (1998) defined reference evapotranspiration (ET0) as being the evapotranspiration of a hypothetical crop with a height of 12 cm, albedo of 0.23, and surface aerodynamics resistance of 70 s m-1. According to SAAD et al (2002), ET0 is a fundamental variable for estimation of crop water demand, which will influence on the designing of an irrigation system. There have still challenges to set insightfully evapotranspiration role on the designing of irrigation systems. The above-mentioned authors concluded that using solely ET0 monthly averages for irrigation scaling might lead to an underestimation thereof while adopting ET0 maximum daily values can over-estimate them. The ET0 can be determined by direct and indirect methods. Field experimental plots, and soil moisture control. Among the indirect ones are those based on evaporimeters (US Weather Bureau and class A pan), equations (Penman-Monteith, BlaneyCriddle, Hargreaves-Samani, etc) among others

Objectives

Methods

Results

Conclusion