Bushfires and their implications for management of future water supplies in the Australian Capital Territory

Abstract

Periodic, often ENSO-related widespread droughts in eastern Australia frequently generate severe bush fires. These can devastate water supply catchments and have serious impacts on the quality and quantity of domestic water supplies. The implications for water supplies sourced from native forests and their management are profound. The Cotter River Catchment in the Australian Capital Territory (ACT), eastern Australia has been a major source of water for the national capital, Canberra, since 1918. Previous work following the 1939 bushfires in alpine ash (Eucalyptus Regnans) catchments in the southern Australia state of Victoria found large decreases in yields (of the order of 30%) for up to 50 years following the fires as forests regrew. It is shown here that the response of catchments with mixed native forests to fires differs from the alpine ash forests in Victoria. Slight increases in yield appear initially over the first 30 months because of the decreases in evapotranspiration. It is thought that as epicormic regeneration of eucalypts continues small decreases in yield of less than 10% could occur for some years but similar results occur after severe droughts without bushfires. Dramatic increases in turbidity, iron, manganese, organic carbon, phosphorous and nitrogen in the reservoirs resulted from the 2003 fires and these persisted for over 12 months. This made water unfit for reticulation as the water needed treatment and forced water restrictions on the ACT for the first time since 1968. The fires revealed problems in the monitoring of catchment water resources and in the management of water resources in the ACT. From our analysis of the aftermath of the fires we suggest changes to the management of water resources in the ACT to improve planning for and response to bush fires and their consequences for domestic water supplies. The relationship between the occurrence and effects of major bush fires with climate and hydrological variables is investigated in this paper.