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Local Water Catchments

 

 

 

 

 

 

 

 

Where do our streams and creeks flow, and how much rain will fill our lakes?

This is all guesswork by an amateur and should not be taken seriously.

Click here to see a map of catchments.

Click on a catchment or lake to see details of area and where the creeks flow.

These creeks flow to Lake Tooliorook, Gnarpurt or Corangamite.

Water to the west of Derrinallum flows to Mt Emu Ck.

Water to the east of Cressy flows to the Barwon River.

 

Whats Happening with our Waterways & Wetlands?
 

Recently there has been concern with the local lakes drying up and creeks not flowing, leaving some to wonder what’s happening to our waterways and wetlands. An assessment of rainfall patterns and some approximate figures relating to the hydrology of our area might help to explain why the lake levels are low.

The approximate volume of Lake Tooliorook is 15300 megalitres (ML), Deep Lake 3000 ML and Shallow Lake 2300 ML), giving a total volume of 20600 ML, while the approximate area of catchments contributing to all 3 lakes is 23500 hectares.

Catchment yield can be estimated by multiplying the average annual rainfall by a percentage of runoff.  South of the Dividing Range the (former) Soil Conservation Authority considered about 15% of rainfall as runoff.

At Derrinallum (station Craigmore) the long term average annual rainfall is 560mm (Bureau of Meteorology).  Note “Craigmore” has been used as it has 116 years of data (1898-2014), whereas Derrinallum Post Office only commenced collection of data in 1956.

Catchment Yield is 23500ha x 560mm x 15% = 1974000 (then divided by 100 to convert to) 19740 megalitres. (which is about the volume of the lakes).

Direct rainfall onto the Lakes contributes about 560mm per annum (based upon average rainfall) but there is significant loss due to evaporation of about 1300mm per year. This gives a net loss in the water level of at least about 740mm per year.

Dams in the catchment are roughly approximated to hold 400 ML.  With a total surface area of all 3 lakes of 744ha (Tooliorook 400 ha, Deep Lake 77 ha, Shallow Lake 267 ha) this would equate to a depth of about 53mm of water across the 3 lakes. Hardly enough to account for the prolonged low lake levels!

To generate runoff either the soils must be saturated and / or the rate of rainfall exceeds the infiltration rate of water into the soil.  Generally the Autumn and Winter rainfall fills and saturates the soil and the late winter and spring rainfall generates the runoff to fill lakes. (unless some exceptionally large and heavy or intense rainfall events occur at other times to produce runoff).

In 2014 the soils were saturated by the end of July, and runoff commenced. But given there were no significant follow up rain events and an extremely low (decile 1) Spring rainfall, little to no runoff was generated. Had we received average rainfall in August or even September (which is about 62 mm and 57 mm respectively) the lakes would likely have filled.

Looking at longer term rainfal
l trends shows a more revealing (or concerning) picture.  The report “Climate variability and change in south-eastern Australia” (BOM and CSIRO in 2012), identifies a declining trend in long term rainfall across south eastern Australia.

Generally speaking a decline in Autumn rainfall commenced about mid 1990’s, a winter decline started in the early 2000’s, and a Spring decline started about the mid 2000’s. While the declines in Autumn and Winter rainfall are (just) within natural variability, the Spring decline is outside of natural variability.

Modelling of the impacts of climate change show that one of the more concerning effects in this area is the likely reduction in Spring rainfall. Even a seemingly small decline in rainfall can have significant impact, with modelling showing that a 10% reduction of rainfall can result in a 50% decline in runoff. Such a reduction will affect the health of all waterways and wetlands, and the species dependent upon them.

Although dams take some water from the lakes, this is probably much less than is contributed to the lakes due to the use of raised beds to remove water from paddocks, and the draining of many natural wetlands with the consequent loss of their important habitats. Many of these dams now provide an important function by returning some wetland habitats to our landscape for rare and threatened species such as the brolga and growling grass frog.