Abordagens quali-quantitativas na gestão dos recursos hídricos no semiárido cearense

Abstract

Issues related to water quality and social participation have become central in discussions on water security, due to the impacts of anthropogenic activities on river basins. In this field of research, hydrological processes and water governance in semiarid regions have been studied about adaptation to the alternation between extremes of drought and water input, combined with the complexities of climate change. In view of the uncertainties generated in these scenarios and the limiting factors for water governance, researchers have intensified the application of qualitative and quantitative models and approaches. In this context, the present study evaluated qualitative and quantitative issues for water resources management in a region of the Brazilian semi-arid zone with more than 30 years of participatory management and with severe water quality problems. Firstly, the aim was to evaluate the relationships between nutrients, land use and land occupation, and precipitation, at the river basin scale, comparing 11 hydrographic regions in the state of Ceará. In the second stage, water allocation processes were evaluated in a sample of the reservoir network of the river basin of the largest multipurpose reservoir in Latin America, Castanhão. Historical data (2004–2022) on water storage and release from 26 reservoirs were analysed using measures of agreement at daily, monthly and seasonal scales, together with records of meetings of the river basin committees. Additionally, unsupervised clustering of hydrological data, demand data and correlations between storage and water release was carried out. Still in this stage, the DPSIR (Driver-Pressure-State-Impact-Response) model was used to systematise qualitative and quantitative aspects of water allocation. In the third stage, this study, in a pioneering manner, applied the conceptual approach of the grey water footprint to determine the storage condition of reservoirs. The proposition was applied to 148 reservoirs distributed in 12 hydrographic regions in the state of Ceará based on data on phosphorus concentration and stored volume between 2008 and 2024. The data on concentration limits and the pollutants adopted are arbitrary variables during the formulation, which ensures replicability for other regions and with other pollutants. Positive, negative and marginal effects of precipitation on nutrient concentration in rivers were identified. Precipitation showed a secondary effect when urbanisation predominates but was associated with a linear increase of nutrients in wetter regions with fewer reservoirs, and with a linear reduction in areas with lower precipitation and greater hydrological disconnection. Multiple regression indicated a greater influence of sewerage coverage and precipitation on total phosphorus, and of urban area on inorganic nitrogen. In water allocation processes, there were weak daily correlations between storage and release, influenced by rapid releases and interruptions, which may generate limitations for the application of predictive models based on stored water volume. The DPSIR analysis revealed the predominance of quantitative respoonses over qualitative ones, favouring pollution and conflicts related to qualitative issues. The reservoirs showed severe deterioration of water quality, and the adaptation of the grey water footprint proved to be an important instrument for ensuring that pollutant concentrations are addressed with greater attention by the parties involved in water management decisions. Overall, this study brings together important findings for water management, proposing new approaches for hydrology to advance in considering qualitative and social aspects and, consequently, in implementing adaptation and mitigation policies that integrate multiple factors related to promoting water security and environmental conservation.