Future projections of water level and thermal regime changes of a multipurpose subtropical reservoir (São Paulo, Brazil, 2021)

Projeções futuras de mudanças no nível de água e no regime térmico de um reservatório subtropical multiuso (São Paulo, Brasil, 2021)

Carolina Cerqueira Barbosa, Maria do Carmo Calijuri, André Cordeiro Alves dos Santos, Robert Ladwig, Lais Ferrer, Amorim de Oliveira, Ana Carolina Sarmento Buarque

ABSTRACT

The increase in global air temperatures as well as variability in rainfall shifts due to climate change has been affecting the dynamics of water level fluctuations and thermal regimes in lakes and reservoirs. It is expected that at the end of this decade, such impacts will be evenmore noticeable andmay harm the inlandwaters use. However, little is known about the possible consequences of climate change in multipurpose subtropical reservoirs.

Using data generated by a regionalized climate model (RCM) as input to a simple hydrological model and a onedimensional vertical hydrodynamic model, we forecast potential changes in the Itupararanga reservoir, São Paulo, Brazil, in an exemplary time period (2028–2030) in the next decade. Two Representative Concentration Pathway (RCP) scenarios were considered: an optimistic one corresponding to a CO2 increase of about 650 ppm (RCP 4.5) and a pessimistic scenario where CO2 exceeds 1000 ppm in 2100 (RCP 8.5).

We found a significant reduction in the reservoirwater level for both scenarios of 35% compared to current conditions. The surface water temperature is expected to increase (+0.6 °C); on the other hand, there would be a cooling of the hypolimnion (RCP 4.5=-0.3 °C; RCP 8.5=−1.2 °C). Another consequence is an increase of the duration of stratification periods thatwould start earlier in the dry period (between July and August), aswell as the intensification of the stability of thewater column (+43% compared to current conditions) and a deepening of the thermocline.

The hydrodynamic modeling results suggest that the water level drop may threaten the reservoirmultiple uses, in particular drinking water supply and power generation. Furthermore, the heating of surface water layers and increase of the number of stratified days and thermal stability can have negative impacts on water quality.

Keywords: Kohonen neural network, 13C NMR, FT-ICR-MS, Stable isotopes, Humic substances, Source discrimination

RESUMO

O aumento da temperatura global do ar, bem como a variabilidade nas mudanças de precipitação devido às mudanças climáticas, têm afetado a dinâmica das flutuações do nível da água e dos regimes térmicos em lagos e reservatórios. Espera-se que, no final desta década, tais impactos sejam ainda mais perceptíveis e possam prejudicar o uso das águas interiores modelo hidrológico simples e um modelo hidrodinâmico vertical unidimensional, prevemos mudanças potenciais no reservatório de Itupararanga, São Paulo, Brasil, em um período de tempo exemplar (2028-2030) na próxima década.

Dois cenários de Caminho de Concentração Representativo (RCP) foram considerados: um otimista correspondente a um aumento de CO2uso. No entanto, pouco se sabe sobre as possíveis consequências das mudanças climáticas em reservatórios subtropicais multifuncionais. Utilizando dados gerados por um modelo climático regionalizado (RCM) como entrada para um modelo climático simples _de cerca de 650 ppm (RCP 4,5) e um cenário pessimista onde o CO2 _excede 1000 ppm em 2100 (RCP 8,5).

Encontramos uma redução significativa no nível de água do reservatório para ambos os cenários de 35% em comparação com as condições atuais. Espera-se que a temperatura da água da superfície aumente (+0,6 °C); por outro lado, haveria um resfriamento do hipolímnio (RCP 4,5 = -0,3 °C; RCP 8,5 = -1,2 °C). Outra consequência é um aumento da duração dos períodos de estratificação que começariam mais cedo no período seco (entre julho e agosto), bem como a intensificação da estabilidade da coluna de água (+43% em comparação com as condições atuais) e um aprofundamento da termoclina.

Os resultados da modelagem hidrodinâmica sugerem que a queda do nível da água pode ameaçar os múltiplos usos do reservatório, em particular o abastecimento de água potável e geração de energia. Além disso, o aquecimento das camadas superficiais da água e o aumento do número de dias estratificados e da estabilidade térmica podem ter impactos negativos na qualidade da água.

Palavras-chave: Rede neural de Kohonen, RMN de 13C, FT-ICR-MS, Isótopos estáveis, Substâncias húmicas, Discriminação de fontes

Introduction

Sustainable development goals have assumed a new paradigm with Earth’s life-support system, society, and economy including targets for 2030 (Griggs et al., 2013). Concerns about clean water, as well as the health and production of ecosystems are intrinsically linked to climate change.

Some of the consequences of climate change are a rise in air temperatures and alterations in rainfall patterns (IPCC, 2014). The water levels of lakes and reservoirs have shifted due to the increase in the frequency of occurrence of weather extremes. More frequent drought and flood periods have been reported worldwide (Brasil et al., 2016; Soares et al., 2019; Jeppesen et al., 2015). Scientific research has focused on a relationship between volume fluctuations and water quality degradation, in particular the nutrients dynamics, trophic state, and phytoplankton community (Jeppesen et al., 2015).

There is also evidence that climate change influences the thermal dynamics of inland waters (Sahoo et al., 2016). The air temperature increase has been reported as a driver of heating surface water temperature (Zhang et al., 2020) and another consequence are alterations in the heat budgets of lakes (Woolway and Merchant, 2019). All of those alterations can lead to cyanobacteria dominance (Kosten et al., 2012) and blooms (Wells et al., 2015; Huisman et al., 2018).

In this way, global and regionalized climate models have been applied under different concentration pathway scenarios to predict climate change impacts (Eccles et al., 2019; Fenocchi et al., 2018; Prats et al., 2018). Using this forecast data has enabled a better understanding of the global warming effects on aquatic ecosystems. Especially coupling climate models to aquatic ecosystem models has been used to predict the consequences of climate change on aquatic environments (Moe et al., 2016). Furthermore, these coupled models are used to test adaptive water management measures for the potential mitigation of negative impacts on the ecosystem (Ladwig et al., 2018).

Climate change effects have already been highlighted in global lake ecosystems (Jeppesen et al., 2017; Woolway and Merchant, 2019). However, further studies are needed to incorporate the likely impacts of climate change in vulnerability assessments and lake management efforts (O’Reilly et al., 2015). Although a water temperature increase is prospective to be felt most strongly at low latitudes (Kraemer et al., 2017), there are incipient local studies of likely impacts on lakes and reservoirs located in subtropical regions.

The present study attempts to highlight whether potential climate projections could affect the water levels and the thermal regime in a multipurpose subtropical reservoir at the end of 2020s. Data generated by a regionalized climate model was used for hydrological and hydrodynamic simulations.

The overall trends of CO₂ emissions rise have led to a change in the pattern of climate forcing data and were evaluated to understand how the reservoir water level and the thermal regime would respond in the near future. This study facilitates our current knowledge of the possible implications of climate change on subtropical lakes hydrodynamics to target possible management efforts.

Section snippets

Study site and input data availability

Itupararanga Reservoir is located in an urbanized region in the Alto Sorocaba basin in São Paulo State, Brazil. The reservoir’s main inflow is the Sorocaba River which itself is formed by three streams: Sorocabuçu, Sorocamirim, and Una. The Sorocaba river is very important for the whole state because it is one of the main contributors to the Tietê River, a major river of São Paulo city.

The Itupararanga reservoir was built for multiple management purposes, e.g. public water supply (~2.15 m³ s⁻¹

Model performance

The Sensitivity analysis for water level and water temperature presented variations between the parameters and the evaluated multiplicative factors (Fig. 3). The water level had high sensitivity to the inflows and outflows factors (SI = 0.54 and 0.69, respectively) and medium sensitivity to the rain factor (SI = 0.15).

Regarding water temperature, the sensitivity analysis highlighted the model’s medium sensitivity to the wind factor (SI = 0.18) and high sensitivity (SI = 0.21) to the latent heat 

Evaluation of model performance and hydrodynamics features of Itupararanga reservoir

The water level simulation results suggest a good fit with the observations (RMSE < 0.63 m, r > 0.8). Previous studies using 1D models have shown similar agreements (0.2 m < RMSE < 0.74 m; Fadel et al., 2017; Melo et al., 2019; Bueche et al., 2019). Furthermore, the model showed a good capability to predict short-term water fluctuations, especially between 2014 and 2015, when the reservoir volume decreased to ~29% of its normal capacity (Fig. 4).

The achieved model fit for temperature profile

Conclusion

The current study investigates the impacts of future scenarios on the hydrodynamics of a Brazilian multipurpose reservoir. Meteorological projections based on a one low CO₂ emissions (RCP 4.5) scenario and a high-emission pathway with no climate policy (RCP 8.5) scenario were incorporated in a simple hydrologic model and a process-based hydrodynamic model.

In two scenarios of GHG emissions and minimum water withdrawal by the reservoir management, the water level was projected to decrease and

Code and data availability

The GLM source code and several test cases are available, accessible at https://aquatic.science.uwa.edu.au/research/models/GLM/latest_release and the SMAP model is available online at http://pha.poli.usp.br/default.aspx?id=76&link_uc=disciplina. The climate projections data regionalized by the ETA model are available online at https://projeta.cptec.inpe.br/.

CRediT authorship contribution statement

Carolina Cerqueira Barbosa: Writing – original draft, Writing – review & editing, Conceptualization, Methodology, Software, Visualization. Maria do Carmo Calijuri: Supervision, Resources, Writing – review & editing, Writing – original draft. André Cordeiro Alves dos Santos: Writing – review & editing, Data curation. Robert Ladwig: Writing – review & editing, Visualization. Lais Ferrer Amorim de Oliveira: Writing – review & editing, Methodology. Ana Carolina Sarmento Buarque: Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are grateful to the INMET, CETESB, UFSCAR-Sorocaba for provision of the available data. The authors express their sincere thanks to Dr. Davi G.F. Cunha, Dr. Simone P. Casali, Dr. Munique A. B. Moraes and Msc. Gabriela A. Marafão for providing the water temperature profiles for the Itupararanga reservoir. The authors also acknowledge the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for the financial support.

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