The mystery of Thirlmere Lakes
During the past decade, water levels in the Thirlmere Lakes have varied from full in 2016 to completely dry between October 2018 and February 2020. These variations have raised concerns with the local community and left them wondering; "Where has all the water gone in Thirlmere Lakes?"
Thirlmere Lakes National Park, located south-west of Sydney in an ancient river meander, contains five lakes – Lake Gandangarra, Lake Werri Berri, Lake Couridjah, Lake Baraba, and Lake Nerrigorang (Figure 1).
Two WRL research teams (EcoEng and Connected Waters) have investigated the water balance budget and surface-groundwater interaction in Thirlmere Lakes. These investigations were supported by coordinated research projects with ANSTO, University of Wollongong, and the NSW Department of Planning, Industry and the Environment (DPIE). In collaboration with these groups, WRL engineers undertook extensive fieldwork between 2017 and 2020 to monitor the site, including remote sensing bathymetry surveys, deploying micro-meteorological stations for measuring evapotranspiration, and installing a piezometer network for groundwater investigations.
In February 2020, the WRL research teams presented their findings at the Annual Science Day meeting in Picton, NSW. Hosted by the NSW DPIE, interesting outcomes presented to the community included:
- The lakes are more sensitive to climate than other anthropogenic activities. When evaporation is greater than rainfall, the lakes lose water and eventually dry out. This balance drives the lake water levels.
- The recent drought is not unprecedented. Evidence from the water balance calculations, which are supported by satellite data, indicate that the lakes have dried out previously. Previous dry periods include the World War II drought and other earlier periods (Figure 2).
- All lakes are set within a complex fabric of geological layers deposited over hundred thousands of years (Figure 3). Some layers have a high hydraulic conductivity e.g. sand, which potentially allows groundwater to move, while silt and clay layers with a low hydraulic conductivity inhibit the movement of groundwater.
- Peats found in the lake beds have a variable but generally low hydraulic conductivity. At low water levels the surface water is perched on this peat.
- During the recent drought the differences between surface water and groundwater levels indicated that water flows from the lakes towards the groundwater.
- The low hydraulic conductivity of the peat, silt and clay layers however prevent substantial downward movement of water to the regional aquifer (Figure 4), which suggest it may take years to decades for water to move through the sediments.
- However, every lake has a different degree of groundwater-surface water connection depending on its local geology.
Investigation into the hydrogeologic processes has revealed that the rainfall and evapotranspiration has mostly driven the recent lake level declines.
|Figure 3: Conceptual hydrogeological model of Lake Baraba and Lake Couridjah indicating the hydraulic situation of May 2019. Hydraulically conductive layers (i.e. sand which allows easy travel of water if present) are displayed in yellow, while unconductive silt/clay layers (which form a barrier for water flow) are shown in grey and peat layers in the lakes are shown in brown. During May 2019 the more elevated Lake Baraba contains water while the lower Lake Couridjah is dry. This indicates that there is no hydraulic connection between the two lakes. Two bores, while drilled to different depths, show a similar groundwater level, indicating a possible regional aquifer spanning below the two lakes, but being hydraulically disconnected from both by the clay, silt and peat.||Figure 4: Time series of surface water levels of Lake Baraba (red) and Lake Couridjah (blue) and respective groundwater levels measured in adjacent bores in absolute elevation above sea level. The two lakes are separated by a small hill (sill), where Lake Baraba has a higher elevation than Lake Couridjah. The time series in green are from two waterNSW bores close to Lake Couridjah. The time series covers the time from 18 December 2017 to 27 February 2020. Because of a severe drought, all levels show a decreasing trend. Lake Couridjah falls dry in October 2018, while Lake Baraba remains partially filled. All but one measured groundwater level is below the respective lake water level, indicating downward hydraulic gradients. This indicates that the lakes are connected to shallow groundwater in their direct vicinity but disconnected from deeper aquifers.|