EVAPOTRANSPIRATION OF SUNFLOWER AND ITS RELATIONSHIP WITH CLASS A PAN EVAPORATION

Abstract

Rio Grande do Sul is a State characterized by climatic variations specially regarding to amount and distribution of rainfall, with frequent periods of low water availability for spring and summer crops. Studies to determine the water relationship with the soil-plant-atmosphere system become very important for the hydric supply recommendation, as well as to help agricultura) planning and agroclimatic zoning. The purpose of this research was to establish the maximum evapotranspiration of sunflower (ETm) and to connect it with Class A Pan evaporation (Eo) to obtain the ratio ETm/Eo (Kc coefficient), at different sub-periods. The field trial was conducted at Centro de Pesquisa de Fruticultura, in Taquari/RS, Brazil, 29° 48'S, 51° 49' W and 76 m of altitude, during four growth seasons: 1987/1988, 1989/90, 1990/91 and 1991/992. The ETm was measured in drainage evapotránspirometers Thomthwaite-Mather typc. The total ETm at the cycle (sowing- physiological maturation) changed from 371.5 mm to 488.5 mm along the four years, with an average of 408.5 mm. The sub- period with highest daily mean consumption was from beginning to end of flowering, with an average of 6.5 mm/day in the four years . The mean values for the ratio ETm/Eo at the cycle ranged from 0.63 to 0.75 with an average in four years of 0.67. The highest values of the ratio ETm/Eo were also observed during the fiowering sub-period, varying from 0.90 to 1.05, with an average of 0.98.