Modelo para simulação de nitrogênio e fósforo em sistemas de recursos hídricos, aplicação em região do alto curso do rio Piranhas-PB.

Abstract

The release of domestic effluents in aquatic environments without the proper treatment has compromised the water quality of reservoirs and rivers. This becomes even more relevant when, around these surface water bodies, there is an intense agricultural practice, with the use of nitrogen and phosphorus base fertilizers, where the occurrence of rainfall events can generate runoff and return the excess nutrients present in the soil to nearby rivers and reservoirs. Therefore, this work proposed the development and application of a model for simulation of nitrogen and phosphorus in water resources systems, allowing the interaction of rivers, reservoirs and agricultural areas in terms of water quality parameters. In the structural arrangement of the developed model, two methodological steps are worth mentioning: nitrogen and phosphorus modeling in agricultural areas, considering the main input and output flows of these macronutrients in the soil, and definition of the systems of ordinary differential equations that can represent the behavioral dynamics of nitrogen and phosphorus in the water bodies, ie, rivers and reservoirs that are part of a given system studied. As a case study, a water resource system was selected, located in the Alto Piranhas-PB hydrological planning unit, comprising agricultural areas, rivers and reservoirs, together with the main nutrient contributions to the respective system, besides being taken into account the contribution of the waters of the transposition of the São Francisco river, coming from the Eixo Norte of the transposition. For the application of the developed model, scenarios were analyzed to enable a better evaluation of the response of the studied system to the release of domestic and agricultural effluents, besides the impact of the transposition waters. The results of the simulations indicated that the analyzed water resources are sensitive to the action of domestic and agricultural effluents, mainly in periods of drought, where the volume of the reservoirs and flows in the stretches of the rivers are reduced. With the addition of the transposition waters, significant attenuations were observed in the nitrogen and phosphorus levels in the water bodies, due to the higher dilution power of the effluents provided by this water supply, although critical levels were observed in some months. Therefore, preventive and corrective measures should be adopted for this type of system in order to avoid compromising the aquatic biota and the multiple uses to which the waters are destined.