Water Catchments

High intensity wildfires have very adverse effects on water supply catchments. The immediate impact is a great increase in runoff from the next rainy period, due to the lack of interception by tree and understorey foliage. While this may appear to be a benefit for dam storage, in fact it is not, as it is accompanied by extensive soil erosion and the transport of large quantities of silt and ash into the water storage facility. A bushfire like this….kw0011.jpgproduces this …..not-a-leaf-at-12-weeks.jpg

The ash is highly detrimental to water quality and can actually cause a dam to be withdrawn from the supply system. This happened in Canberra after the 2003 fire disaster there. The silt and ash flow can also damage or even destroy valuable stream gauging stations (see the paper presented by Colin Terry at the Eaton Seminar in the Background Material section)

In the large Perth Hills bush fire of 2005, monitoring of a Water Corporation experimental catchment showed that water yield increased by a factor of 2.2 times in the first year, but then returned to normal afterward, However, over 350 cubic metres of soil were deposited in the stream bed, silting up the stream and small pools. Large amounts of silt and ash passed through into the Mundaring Weir. A survey by the Department of Environment and Conservation found that aquatic diversity was significantly impaired in the wild fire area. Invertebrates in the wildfire area were also much less diverse than those from areas that had been covered by a low intensity fuel reduction burn.

It was estimated that between 1.5 and 2.3 million trees were killed outright by this high intensity bushfire. The forest has therefore been significantly degraded by this one wild fire. It will take many, many years to replace these trees.

Analysis by the Water Corporation of fuel reduction burn data and water flow over a 40 year period (1960-1999) showed a stream flow increase of between 20-49% for two years after each burn, As most burning is carried out in spring, when the stream zone is moist, there is minimal damage to vegetation and water quality. On the Gnangara Mound, recharge to the water table increased for 5 years after a fuel reduction burn.

In view of the great importance of the forested catchments in the Darling Range and the Gnangara Mound to metropolitan water supplies, it is imperative that high intensity wildfires be excluded from the region. It is so important that this happen that the management priority for the region should be reviewed and catchment protection be given higher priority than the current DEC top priority for biodiversity protection. The entire area covered by the harnessed catchments or groundwater areas needs to be managed on a 5-7 year burning rotation.