Cobrança pelo uso da água bruta da união: proposta de modelo baseado na escassez hídrica em reservatórios com múltiplos usos

Abstract

The rational and efficient use of available water resources is fundamental for regional development, especially in arid and semi-arid regions, where frequent droughts exacerbate the imbalance between water supply and demand. This study aimed to develop and evaluate an approach for charging for raw water use in multipurpose reservoirs, considering water scarcity. The methodology involved three main steps: (i) a review of scientific literature and legislation to identify progress and gaps on the topic; (ii) the application of charging models used in Brazilian basins, such as those of the São Francisco, Paraíba do Sul, and Doce rivers, to the Curema-Mãe D’Água system, operated based on the Standard Operating Policy (SOP) and observed data from 240 months (2002-2021); and (iii) the development of an approach (CABCEH model) that associates marginal scarcity costs with the system's water status, considering scarcity costs estimated through the Willingness to Pay method and an implicit stochastic optimization model. This proposal was applied to the Curema-Mãe D’Água water system to promote greater efficiency and equity in water resource management, considering two scenarios: without and with the São Francisco River transposition. The operation of the Curema-Mãe D’Água system based on historical data revealed failures in meeting demands in 85 out of the 240 evaluated months, with emphasis on the severe drought of 2012-2017, which affected water sustainability and revenue collection in 35.4% of the months. This critical period was also recorded by AESA (2021) and other academic and journalistic sources. Water demands for various uses were fully met during the first 126 months of the analysis, with failures concentrated between the 127th and 226th months. The average annual revenue was approximately R$ 1.37 million, R$ 12.71 million, and R$ 3.58 million using the models of the São Francisco, Paraíba do Sul, and Doce rivers, respectively. The system collapse in 2015 and 2016 severely impacted industrial and irrigation uses, while human supply maintained its priority, even without revenue collection during these critical periods. The difference in public prices and charging methodologies resulted in higher revenues in the Doce and Paraíba do Sul river basins compared to the São Francisco model, highlighting that low prices can compromise investments and incentives for rational water use. Although the charging models share the same structure, they exhibited gaps, such as limitations in protecting water quality and the system during water scarcity situations, emphasizing the need to align public prices and revenue collection mechanisms with local hydrological conditions and users' realities. The application of the CABCEH model successfully correlated the hydrological state of the Curema-Mãe D’Água system with scarcity costs, allowing for the identification of the most critical months and the costs associated with different water uses and rationing levels. The comparative analysis between scenarios with and without the São Francisco River transposition (1.5 m³/s) demonstrated that the transposition significantly reduced scarcity costs in the most vulnerable sectors, such as livestock, irrigation, and aquaculture, ensuring greater water stability. Without the transposition, costs were higher, and vulnerabilities increased, highlighting the importance of the transposition for efficient water resource management throughout the year. By reviewing and applying traditional charging models and proposing the CABCEH model, this study contributes to water management in semi-arid regions, helping mitigate the impacts of scarcity and improve water sustainability.