Understanding the Water Quality Dynamics in a Large Tropical Reservoir Under Hydrological Drought Conditions

Castanhão is a multipurpose reservoir located in the semiarid region of Brazil. From 2012 to 2019, a severe drought occurred in the area, leading to a significant degradation of the water quality in the reservoir, spanning from oligotrophic to eutrophic/hypereutrophic state. In addition, this deterioration resulted in massive fish mortality events and highly-increased water treatment costs. Those negative impacts motivated an in-depth water quality monitoring program of this reservoir, which started in 2016. Although numerous studies have addressed the water quality issue in tropical reservoirs, limited availability of water quality data during drought periods has created a knowledge gap regarding the understanding of water quality dynamics under these conditions. Therefore, this study aimed to comprehensively understand the spatio-temporal dynamics of water quality in the Castanhão reservoir during a hydrological drought, with the objective of providing scientifically sound references for water management. The findings of this study indicate that total phosphorus and nitrogen concentrations in the reservoir follow approximately a complete-mixing pattern on a monthly basis, with temporal variations in nutrient levels being much more significant than spatial variations. This indicates that it is possible to apply a complete-mixing model at a monthly time-step to evaluate the nutrient dynamics of the reservoir. Additionally, the reservoir exhibits stronger chemical stratification of dissolved oxygen compared to thermal stratification, with dissolved oxygen levels decreasing from upstream to downstream. Furthermore, hypoxic conditions are predominant during the rainy season. In summary, this study offers an advance in the knowledge of water quality dynamics in a large tropical reservoir during a hydrological drought period. The findings highlight the vulnerability of tropical reservoirs to water quality deterioration during droughts and provide valuable insights into the spatio-temporal dynamics of water quality in such water bodies. These findings are crucial for improving water quality management in similar tropical reservoirs