Evapotranspiração e análise econômica de irrigação em cana-de-açúcar.

Abstract

The use of irrigation in sugarcane requires close attention because the farmer must use the amount of input that provides the maximum economic return. Therefore, to optimize the use of water in irrigation of sugarcane, this study aimed to analyze the levels of this resource that would provide economically viable yields along of the ratoons. The experiment was conducted at the Agrarian Science Center (CECA/UFAL) in Rio Largo, Alagoas, using the variety RB92579 drip irrigated with seven irrigation depths (levels 0, 25, 50, 75, 100, 125 and 150% of daily average reference evapotranspiration-ET0 of dry season), in three cycles of production of sugarcane between January 2009 and February 2012. The crop evapotranspiration (ETc) and the real evapotranspiration (ETr) of sugarcane were estimated by the dual Kc methodology, which separates the effects of transpiration and direct evaporation of soil water. Was calculated the Satisfaction Index of Water Requirements (ISNA) to rainfed treatment and the relative evapotranspiration of crop (ER) for all treatments. The crop and agroindustrial yield was used for adjusting response curves to irrigation depths. The average ETr in the maximum growth phase of the three cycles ranged from 1.73 to 5.04 mm day-1 in the treatments with 0 and 150% of ET0, respectively. The irrigated sugarcane with maximum Kc of 1.5 (150% of ET0) in the first, second and third cycles of cultive produced an average of 48.0 t ha-1 more than the sugarcane cultivated without irrigation. For an average value of 0.55 R$ kg-1 of commercially recoverable sucrose content (CRS), the depth of maximum economic efficiency was 666 (85% of ET0), 290 (65% of ET0) and 397 mm (70% of ET0) with profit of 3,272.07; 2,456.90 and 1,474.51 U$ ha-1 in plant cane, 1st and 2nd ratoon, respectively. The sugarcane, in rainfed conditions in Rio Largo, Alagoas, is exposed to average climatic risk and the ETr, in the irrigated crops, decreases proportionally with the level of irrigation adopted. The amount of CRS per ton of cane varies with irrigation depth, but the final agroindustrial yield per acreage unit is more dependent on stalk yield; however, the economic irrigation depth in the range 0 to 150% of ET0 is directly proportional to the increase in the price of CRS.