What are reservoir dynamics?
Reservoir dynamics describes how water in lakes moves around at different times of the year, forming warm or cold layers, and the movement of catchment inflows which can affect water quality.
During summer, lakes soak up heat from the sun and store it in the water. In deep lakes, such as Lake Burragorang behind Warragamba Dam, the surface temperature in summer is up to 15 degrees warmer than the temperature at the bottom.
This natural process of warm surface water and colder water underneath is known as thermal stratification. It develops during the warmer months, and can disappear during the cooler months when the layers of water mix.
In winter, the lake surface cools through contact with the colder air. When the surface water gets cold enough, the lake may 'turn over', mixing the layers of cold and colder water. This can change the water quality throughout the whole lake quickly. For an insight into this process see the video Beneath Lake Burragorang.
Why understanding it is important?
Understanding reservoir dynamics is important because changes in how the lake behaves can affect water quality in different ways at different times of the year, as well as after heavy rainfall or high winds.
When a lake is used to supply drinking water, it is important to know how the behaviour of the lakes can affect quality of water supplied for treatment.
How we do it at WaterNSW
WaterNSW studies and monitors reservoir dynamics, and uses data collected from instruments in streams and lakes to understand how the lake is behaving, and to make decisions about which dams, and from what depth the water is drawn for supply.
This data is also used to develop a complicated three dimensional computer model of the lake, which helps scientists to understand how the water moves within the lake, and how the layers interact with each other.
The data collected is also sent to a three dimensional computer model which can track, interpret and forecast water quality changes, such as after major inflows or the annual lake 'turn over'. This information allows WaterNSW to make decisions to ensure only the best quality water is supplied to customers. See Managing our reservoirs for more information.
Stratification in action
After rainfall, water flowing into a lake can be muddied by eroded soil or contaminated by organic waste from farms, dissolved chemicals from industry, stormwater run-off from towns, or sewage overflows from rural properties or sewage treatment plants in the catchment.
When a lake is stratified, with warm and cold water at different depths, run-off after rainfall may enter the lake at different depths. During these times, WaterNSW will adjust the depth at which it draws water, to ensure the best quality water is supplied.
If the incoming water is colder than the lake, it travels in a layer near the bottom, so we can usually draw cleaner water from near the surface. When the inflows are warmer than the lake, they travel closer to the surface, so we take cleaner water from lower in the lake. By tracking the water as it flows into the lake, we can make changes to how the dam is set up to draw the best quality water available.
When there is a 'turn over' of the lake during winter, mixing may give variable quality water, for a period of time afterwards, so we may supply water from a different dam, and ensure customers such as Sydney Water know that additional treatment may be required.