Hydrological Management of a Lake with Floating Islands Near Pirron Yallock, Victoria, Australia

The floating islands of Pirron Yallock were formed in 1952 after heavy rainfall raised the water level in a peaty swamp, the outlet of which had been blocked by road construction. The area has since been granted protected status under conservation legislation and, under present policy, the government has a responsibility to maintain the floating islands. The islands reportedly floated uninterrupted for 30 years. However, a drought in 1982 lowered the water level sufficient to ground the islands and they have not been observed to float since. On the basis of a simple annual average water balance, a previous study attributed the failure of the lake to recover its water level to abstraction of water by a nearby motel, which began in 1980. aper illustrates the application of various hydrologic estimation techniques to the development of a continuous model of monthly lake water level. The model demonstrates that the lake level fluctuates seasonally and yearly, largely in response to variations in rainfall and evaporation. Lake level is also linked to groundwater exchange, providing an estimated 10% of annual inputs. It was found that the lake level was maintained from 1952 to 1981 (although brief episodes of island grounding probably occurred) because rainfall during this period was significantly higher than average. In an average climatic year the lake has a water deficit. The lake has not recovered its pre-drought level mainly because rainfall during the last decade has been close to average. Model predictions indicate that the lake level can be restored by terminating abstraction and diverting runoff from a nearby road surface into the lake.