poster - International Conference of Agricultural Engineering

ADEQUACY OF THE PENMAN-MONTEITH METHOD TO IRRIGATED
SURFACE WITH DIFFERENT EXPOSURES AND DECLIVITY
José Eduardo P. Turco 1* , Adhemar P. Milani 1 , Edemo J. Fernandes 1
1 Faculdade de Ciências Agrárias e Veterinárias, UNESP - Univ Estadual Paulista, Via de
acesso Prof. Paulo Donato Castellane, Km 5, Jaboticabal - SP, 14884-900, Brazil.
* jepturco@fcav.unesp.br
Abstract
Among the methods used to estimate reference evapotranspiration (ETo) there are those that
use as input variable the net radiation for example, the Penman-Monteith formula, widely used in
project planning of hydric dotation for irrigated crops, which are recommended by FAO and used
worldwide. Through this study aimed to determine the net radiation on a horizontal grassy
surface and correlate it with the net radiation on grassy surfaces with different expositions and
declivities to establish equations that can be used in the reference evapotranspiration estimates,
considering the positioning of the site measured, for the four seasons of the year, in Jaboticabal-
SP. The research was developed in a structure called "Experimental Hydrographic Basin", from
the Department of Rural Engineering, FCAV/UNESP, Jaboticabal Campus, SP. In this structure
was used 9 surfaces of 10.5 m 2 , being one horizontal, two with northern exposures, two with
southern exposures, two with eastern exposures and two with western exposures, presenting
each set two exposures of 10% and 20 % slope. In the experimental area surfaces Bahiagrass
(Paspalum notatum Flügge) was planted in order to simulate the conventional weather station
areas. For the determination of the net radiation on the surfaces studied, was installed on each
surface a net radiometer, Kipp & Zonnen model NRLITE. The analysis of the results were made
daily, using regression analysis and considering the linear model (y = ax + b), in which the
dependent variable was the net radiation on the studied surface (Q * s) and the independent
variable the net radiation at the horizontal surface (Q * H). In this analysis was considered the
complete model and the one without intercept. It was analyzed the adjustments of the
regression models, by means of the “t” test, at 1% and 5%. For the fall and the winter the more
appropriate model for both surfaces with two northern exposition and the two eastern exposition
and 10% and 20% slope was y = ax + b. In the spring and in the summer the more appropriate
model for all surfaces was y = ax.
Keywords: slope, reference evapotranspiration, Penman-Monteith.
1. Introdution
The determination of the evapotranspiration is a problem shared by several sciences that
study the soil-plant-atmosphere system. Due to the necessity of knowing the water loss of
vegetated surfaces, several researchers have developed methods for estimating
evapotranspiration. A very used way of obtaining the reference evapotranspiration (ETo) in
different situations and locations is by means of estimation methods. In 1990, the methods
recommended by FAO in 1977 (FAO 24) underwent to a review by experts in
evapotranspiration, which concluded that the Penman-Monteith parameterized method for
grass with to 12 cm in height, aerodynamic resistance of surface of 70 sm -1 and albedo of
0.23 showed better results being recommended by FAO as a standard method for estimating
ETo. In Brazil there are several weather stations being used to manage irrigation by
determining the ETo estimate by Penman-Monteith method, which is an indirect technique
that leads in an estimate of the water needs by the plants, since using a culture coefficient.
The Penman-Monteith formula (Allen et al., 1998) uses as input the net radiation. This
method uses the solar radiation measured, in most weather stations, for horizontal surfaces.