Water rights in sugarcane irrigation: influence of irrigation criteria and probability levels adopted for ETo and rainfall

ABSTRACT Sugarcane irrigation is one of the main technologies to increase productivity and profitability in the sugar-energy sector. However, to improve the management of water resources in Brazil, growers need to obtain a water rights permitfrom regulatory agencies. The calculation of water rights is determined from data based on estimates of rainfall and reference evapotranspiration (ETo) provided by the Agência Nacional de Águas e Saneamento Básico (ANA). The primary hypothesis of this study was that the method proposed by ANA to calculate water rights did not provide operational security since it lacked an adequate probabilistic character. As a corollary, we included a secondary hypothesis stating that determining flows through the simplified FAO method proposed in the CROPWAT 8.0 manual displayed vulnerability due to the criteria to choose the representative years. This research aimed to compare such methodologies with a more detailed case study (standard method), taking into account the temporal variability of these estimates, the irrigation criteria, and the probability of occurrence of ETo and rainfall to determine water rights for sugarcane irrigation. During the period between Apr and Sept (dry period), the calculation method proposed by ANA met the needs of the project. However, when rainfall was concentrated (Oct to Mar), the calculation method tended to underestimate the monthly values of available flows. The simplified method proposed by FAO and the alternative method proposed here approached ideal conditions.