Numerical models are an important tool for simulating temperature, hydrodynamics, and water quality in lakes and reservoirs. Existing models differ in dimensionality by considering spatial variations of simulated parameters (e.g., flow velocity and water temperature) in one (1D), two (2D) or three (3D) spatial dimensions. The different approaches are based on different levels of simplification in the description of hydrodynamic processes and result in different demands on computational power. The aim of this study is to compare three models with different dimensionalities and to analyze differences between model results in relation to model simplifications. We analyze simulations of thermal stratification, flow velocity and substance transport by density currents in a medium-sized drinking-water reservoir in the subtropical zone, using three widely used open-source models: GLM (1D), CE-QUAL-W2 (2D) and Delft3D (3D). The models were operated with identical initial and boundary conditions over a 1-year period. Their performance was assessed by comparing model results with measurements of temperature, flow velocity and turbulence. Our results show that all models were capable of simulating the seasonal changes in water temperature and stratification. Flow velocities, only available for the 2D and 3D approaches, were more challenging to reproduce, but 3D simulations showed closer agreement with observations. With increasing dimensionality, the quality of the simulations also increased in terms of error, correlation and variance. None of the models provided good agreement with observations in terms of mixed layer depth, which also affects the spreading of inflowing water as density currents and the results of water quality models that build on outputs of the hydrodynamic models.

With the aim to improve current knowledge on physical processes in tropical and subtropical reservoirs, we explored the dynamics of prevailing hydrodynamic processes in a small- to medium-sized reservoir based on 1-year continuous measurements of temperature, flow velocity, and turbulence. The mixing regime of the reservoir is characterized as warm polymictic, and extended periods of comparably strong density stratification were observed during all seasons, interrupted by several mixing periods between autumn and winter. The stratification was affected by lateral, density-driven flows from the main river inflow. A forebay through which the river enters the reservoir influenced the density currents and reduced the occurrence of underflows. Flow velocities in the middle of the reservoir were generally small, and higher velocities (currents >3.5 cm s⁻¹) were driven by wind. These currents were more frequent during stratified than during mixed periods. No basin-scale internal waves (seiches) were observed, but, high-frequency internal waves propagating, with periods between 2 and 17 min were present in all seasons. Dissipation rates of turbulent kinetic energy were generally small (10⁻¹⁰–10⁻⁹ W kg⁻¹). Wind-driven currents were associated with enhanced dissipation rates at the bottom-boundary layer but represented only 10% of the total monitored period. We identified several aspects in which the observed hydrodynamics of the reservoir differed from those of more commonly studied temperate aquatic systems. We expect that our findings are representative for a large number of small- to medium-sized lakes and for a growing number of reservoirs located in the tropical and subtropical zone.

Chlorophyll-a (chla) is an important parameter to assess water quality in lakes and reservoirs, since it is a proxy for phytoplankton biomass and primary production. The increasing availability of data with high spatial and temporal resolution allows assessing short-term dynamics and small-scale variations of chla within larger water bodies. In freshwater reservoirs, the nutrient concentrations and the physical conditions that control phytoplankton growth vary along their longitudinal extend. Here, we analyze how the flow paths of the inflowing river water into density-stratified reservoirs affect the vertical and longitudinal distribution of nutrients and chla. We combine spatially resolved and high-frequency measurements of chla from satellite remote sensing and in-situ sensors, with numerical simulations using a three-dimensional hydrodynamic model to assess the influence of density currents on chla dynamics along a subtropical drinking water reservoir in the south of Brazil. Chla did not have pronounced seasonal dynamics (4.9 ± 1.2 µg L⁻¹, at the continuous measurement station); instead, spatial variability along the reservoir was more pronounced (4.4 ± 2.1 µg L⁻¹, all monitored points within the euphotic zone). Most of the nutrients from the inflowing river were consumed in the upstream region, and phytoplankton in the lacustrine zone depended on internal loading. Temporal variability was observed only in the upstream region, and phytoplankton produced in that area was transported downstream by density currents, resulting in large concentrations of chla below the euphotic zone. The results of a hydrodynamic model that simulates the present state are in good agreement with the observations. Two simulated scenarios, where the density current patterns were altered, resulted only in slight variations in density currents, indicating that the influence of the main inflow was of minor relevance in chla concentrations in downstream regions of the reservoir. Our results highlight the importance of two-dimensional hydrodynamic processes in regulating phytoplankton dynamics in reservoirs.

In thermally stratified reservoirs, inflows form density currents according to the interplay between inflow temperature and reservoir stratification. The temperature of inflowing water is affected by catchment properties, including shading by riparian vegetation. We hypothesize that the degree of shading in the catchment can affect the inflow dynamics in downstream reservoirs by changing inflow temperature and consequently the nature of the density current. We test it for a subtropical drinking water reservoir by combining catchment‐scale hydrological and stream temperature modeling with observations of reservoir stratification. We analyze the formation of density currents, defined as under, inter and overflow, for scenarios with contrasting shading conditions in the catchment. Inflow temperatures were simulated with the distributed water‐balance model LARSIM‐WT, which integrates heat‐balance and water temperature. River temperature measurements and simulations are in good agreement with a RMSE of 0.58°C. In simulations using the present state of shading, underflows are the most frequent flow path, 63% of the annual period. During the remaining time, river intrusion form interflows. In a scenario without stream shading, average inflow temperature increased by 2.2°C. Thus, interflows were the most frequent flow path (51%), followed by underflows (34%) and overflows (15%). With this change, we would expect a degradation of reservoir water quality, as overflows promote longer periods of anoxia and nutrient loads would be delivered to the photic zone, a potential trigger for algae blooms. This study revealed a potentially important, yet unexplored aspect of catchment management for controlling reservoir water quality.

Numerical models are an important tool for simulating temperature, hydrodynamics and water quality in lakes and reservoirs. Existing models differ in dimensionality by considering spatial variations of simulated parameters (e.g., flow velocity and water temperature) in one (1D), two (2D) or three (3D) spatial dimensions. The different approaches are based on different levels of simplification in the description of hydrodynamic processes and result in different demands in computational power. The aim of this study is to compare three models with different dimensionality and to analyze differences between model results in relation to model simplifications. We analyze simulations of thermal stratification, flow velocity, and substance transport by density currents in a medium-sized drinking water reservoir in the subtropical zone, using three widely used open-source models: GLM (1D), CE-QUAL-W2 (2D) and Delft3D (3D). The models were operated with identical initial and boundary conditions over a one-year period. Their performance was assessed by comparing model results with measurements of temperature, flow velocity and turbulence. Results show that all models were capable of simulating the seasonal changes in water temperature and stratification. Flow velocities, only available for the 2D and 3D approaches, were more challenging to reproduce, but 3D simulations showed closer agreement with observations. With increasing dimensionality, the quality of the simulations also increased in terms of error, correlation and variance. None of the models provided good agreement with observations in terms of mixed layer depth, which also affects the spreading of inflowing water as density currents, and the results of water quality models that build on outputs of the hydrodynamic models.