Boletim de Pesquisa Online Nº 3, out./2000

Gilberto Rocca da Cunha ²
Homero Bergamaschi ³

Abstract

The choice of a method for assessing crop evapotranspiration should be based on the following points: method accuracy, length of the period involved, and availability of meteorological data to ensure its operational feasibility. Aiming to evaluate the efficiency of a number of assessing methods that could possibly be used in maximum evapotranspiration (ETm), daily measurements of ETM in alfafa (Medicago sativa L.), cv. Crioula, sown to rows 0.30 cm apart, were used. Such measurements were obtained in a weighing lysimeter (5.1 m² – 0.1 resolution) installed in the center of a 90 m by 60 m plot, at Eldorado do Sul, Brazil (30°05’S, 51°39’W, and altitude 46 m), from October 1989 through September 1990. Original Penman (1948), Penman/FAO, Equilibrium evapotranspiration, FAO solar radiation, FAO class A pan evapotranspiration, and class A pan evaporation assessing methods were compared with lysimeter measurements, using average periods of 1, 5, 10, and 30 days. Net radiation, when required, was used for each method as an estimation over alfafa (Rna) and over grass (Rng). The evaluation was based on regression analysis by using the standard error of the ETm (SEE) assessed and the coefficient of determination (r²). The results indicated original Penman (1948)-(Rnm) and FAO class A pan evaporation as the most efficient methods, while Penman/FAO was the worst method. Increasing the length of the averaging period (from 1 to 30 days) resulted in a improvement in the efficiency of ETm assessing methods in alfafa by reduction of SEE and increase in r².

Index terms: consumptive water-use, irrigation, weighing lysimeter, Medicago sativa L.