Fire and Water Supplies


By Dr Frank McKinnell

How could fire and water mix?

Of the five classical elements described by early Greek philosophers, fire and water were seen as opposites, After all, we use water to extinguish fire. However in the context of management of water catchments in Western Australia, there is a very close link between the two.

The surface water catchments located in forested country in the Darling Ranges, are a vital part of the water resources for the metropolitan area, the Goldfields and the Wheatbelt. How fire is managed in these forests has a critical influence on runoff, and thus the amount of water reaching the dams.

There are two main sorts of fires

The two main sorts of fires that occur in our forests are large, high intensity “killer” bushfires, and mild, low intensity green burns.

Large, high intensity wildfires have a devastating effect on forested water catchments. Large numbers of trees are killed, serious soil erosion is caused and water quality in the reservoirs deteriorates. While in the year after a hot wildfire, runoff increases (by as much as double the normal amount), the regrowing forest, composed of a higher proportion of water-hungry young trees, uses more water than normal for many years, thus causing an overall reduction in water storage in dams.

Green burning (also called “prescribed burning, fuel reduction burning or controlled burning”) on the other hand, prevents the development of large, high intensity wildfires, Green burning is the practice of running mild fires through the undergrowth under mild weather conditions every few years. This allows intense summer bushfires to be readily controlled in areas where fuel loads are low because of past mild burns. Furthermore, water yields increase by up to 30 percent for two years after a green burn. Thus, by burning under mild conditions at 5-6 year intervals in the northern jarrah forest, we can have additional water in our dams and at the same time give our forests and our outer suburbs better protection from hot summer bushfires.

The current practice of the Department of Environment and Conservation is to carry out a prescribed burn in water catchments about every 10-12 years. In our view this is grossly inadequate; water flow into dams is decreased as a result and the risk of damaging high intensity fires is greatly increased

What are the financial implications?

Apart from making our forests healthier, putting more water in our dams, and reducing the risk of wildfire damage to life and property, there are financial benefits from a green burning regime. Let’s do a little back-of-the envelope calculation.

There are about 450,000 hectares (ha) in the surface water catchments, of which about 150,000 ha are in the high rainfall western zone. This high rainfall area yields an average of about 100 mm of water, or 1000 kilolitres (kL)/ha flowing into dams every year. With a 30% increase in runoff, this means an additional 300kL/ha per year.

Prescribed (green) burning costs about $50/ha to implement. A 12-year burning cycle would burn about 12,500 ha a year, while a 5 year cycle would require that 30,000 ha be burned, a difference of 17,500 ha each year. The additional annual cost of burning on a 5-year cycle would be $875,000.

A 5-year burning cycle would result in at least 17,500 x 300 kL=5,250,000 kL additional water into the dams (ie, 5.25 Gigalitres). At a conservative price of 80 cents per kL, the additional volume of fresh water in our dams would be worth at least $4million!  In fact, the increased water yield after burning would last for about two years and the real benefit is about twice that figure.

This benefit: cost ratio will increase with the forthcoming increase in water charges that have been foreshadowed by the government. It’s also cheap water compared with that coming from the first desalination plant, quoted as $1.00 per kL and about $2.00 per kL from the second desalination plant.

A win-win opportunity

Here is a potential win-win situation for the forest, for our precious water supplies, for people living near forests, and for the general householder. We can protect our forests from killer bushfires, while generating more cheap fresh water.

Why the hell aren’t we doing it? (With apologies to Lara Dingle)

For more information on fire and catchment management see the following websites:

Department of Environment and Conservation WA: management/fire and the environment/fire and water production

Water Corporation of WA:       Wungong Whispers Vol 4 and Vol 5.