Tools to forecast climate change effects on lakes's water column

Lakes and reservoirs are environments with many important uses in social activities, such as hydropower generation, water supply, landscape element, irrigation and flood containment. An ecosystem with so many environmental services needs to have its water quality well preserved, and besides that, inland waters have a key role in climate change studies, because of their faster response to variability in external forces.

Water circulation in a reservoir is the result of a combination of morphometry and energetic driving forces. A lake’s hydrodynamic characteristics vary with morphometric, meteorological and hydrological conditions. The heat balance involves exchanges at the surface, which are actively mixed and energised by the transfers occurring at the air-water interface, and also the exchanges in the water-soil interface near the banks and bottom, which depend on currents and internal waves; while in the main water column, the heat transfer is influenced by light penetration.

In the context of climate changes and water scarcity worldwide, the development of tools to better understand, maintain and improve water quality in lakes and reservoirs becomes an essential ally to environmental research and limnology. This research aims to demonstrate the lake mixing regime by a different approach, testing two different methods to forecast the climate change influence on a lakes’ mixing regime, using data from climate models.

The first tool is a thermal limit curve proposed by the authors which can forecast water column thermal limits for stability or mixing condition in a lake, based on wind speed, radiation and water profile temperature data. The second applied tool is a quasi-3D mathematical model, well known and reputed in the simulation field.

The results obtained for different experimental lakes in temperate and tropical zones showed that both methods have a good performance in representing lakes hydrodynamics accurately. The curve allows a faster response and minor need for data input, on the other hand, the quasi-3D models are capable to produce more detailed results. Possibly in the lakes’ management, it would be more indicated the use of those two methods together, using the curve to analyse faster the period's trend and be able to delimitate the exact period which needs more detailed studies.

The climate change simulations conducted for two experimental lakes considering different scenarios of climate changes showed the driving forces' strong influence on the lake's mixing regime. The number of mixing events is an interesting proxy to analyse this influence. It was greater in the pessimistic scenarios but still less than in the current situation. This means longer periods of stratification, which can cause dissolved oxygen depletion in the deeper layers.

The pessimistic scenarios have mixing events with greater amplitude, which results from a powerful stratification in previous periods. Mixing events with greater amplitudes create higher vertical velocities, resuspending more organic load and dropping dissolved oxygen levels along the water column, impairing the water quality.