About 700mm of water comes in from the sky and 1400mmm goes out in evaporation. That means there is an average shortfall each year of 700mm. To get this water requirement from the catchment land ( 7 times the lakes area), would take 100mm off each hectare of land in the catchment That is still a huge amount of water. ( about 10 times water stored in the West Barwon or 100 times the water stored in Colac storages) However we could reasonably expect , to get 50mm average each year in run off, so some doubling of effect would change things. It is not unreasonable to get an average of 50mm of runoff off the catchment, but it’s impossible to get 100mm. The 2 creeks flowing into an about 1/3 reduced size southern section could be expected to fill that area every other year because the catchment to evaporation ratio would be more than doubled. . There has never been enough water available to raise and retain the water level above the evaporation deficit, so about 18 out of every 20 years , the lake level is lower than we would want it to be - around swamp depth . There has never been enough water coming in each year to keep it above 1600mm, even when the catchment had more runoff with fewer dams and much closer grazed pasture.
Something drastic has to happen to increase the runoff ratio and the only way is to reduce the southern lake size.
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