EVAPOTRANSPIRATION OF SUNFLOWER (Helianthus L) AND ITS RELATIONSHIP WITH THE REFERENCE EVAPOTRANSPIRATION AND GLOBAL SOLAR RADIATION
Abstract
Low water availability is a limiting factor for growth and development of summer crops in Rio Grande do Sul State. Brazil. Studies aiming to determine crop coefficients for water requirements are important for water supply recommendation. The objective o f this study was to determine the maximum evapotranspiration of sunflower (ETm) in relation to the reference evapotranspiration (ETo) and with solar radiation (Rs), to obtain the ratios ETm/ ETo (Kc , coefficient) and ETm/Rs (Kc 2 coefficient), at different crop stages. Field experiments were carried out at Centro de Pesquisa de Fruticultura, in Taquari, 29°48'S latitude, 51°49'W longitude and 76 m of altitude, during 1987/ 88, 1989/90, 1990/91 and 1991/92 crop seasons. The average ratio ETm/ETo for the whole crop cycle (from sowing to physiological maturity) was 0.77, while the average ratio ETm/Rs was 0.48 for the same period. The highest average values of crop coefficients occurred during flowering period with average values of 1.12 and 0.71, respectively, for Kc, and Kc 2 .
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