A modeling approach for a cascade of reservoirs in the Juquiá-Guaçu River (Atlantic Forest, Brazil)

In a cascade of reservoirs, the events that occur in the upstream reservoir can be transferred to the downstream ones. Thus, the water quality of the second and subsequent reservoirs usually changes. Based on a zero-dimensional model, this work describes the mass balances of 23 limnological variables in a system of 6 cascade reservoirs located in a well-preserved hydrographic basin (within the Brazilian Tropical Atlantic Forest). Ecosystem services are also mentioned to stress the importance of this system to improve the water quality of the Juquiá-Guaçu River. Samples were taken from the reservoirś inputs, in the lacustrine region, and in the Juquiá-Guaçu River downstream of each reservoir. According to the zero-dimensional model (continuous stirred tank reactor), it can be concluded that: i) the retention intensities of the elements varied within each reservoir itself and among the reservoirs. These differences occur because chemical and biotic processes concerning retentions are different, and the specific physical conditions (e.g. water velocity, flow, stratification) related to the retentions of each element are different; ii) not all reservoirs presented high assimilation coefficients for all variables, the six reservoirs were very efficient in terms of retaining the elements; iii) the system reduced the amounts of 87% of the variables; iv) for variables related to erosion and runoff the cascade of reservoirs was not able to decrease the values; v) the The high percentage of retention of the limnological variables enables us to evaluate the importance of these reservoirs to reduce eutrophication (nitrogen and phosphorus compounds), turbidity, TS, color, coliforms (total and fecal) from the Juquiá-Guaçu River.