Perseus, "But that also raises an interesting question of the cost of storm water disposal that will be saved by a tank. This should be included as part of the economics of the tank and certainly justifies a large part of the government tank rebate."

Does use of tanks actually lessen the need for storm water disposal capacity?

I suspect that councils would still need to maintain storm water disposal systems which could handle storm runoff once every bodies tanks were full.

I'd heard in the past that one of the reasons tanks were opposed is that storm water runoff was important for maintaing the storm water drainage. High volume flows are needed to flush the pipes. It's quite likely that the same kind of flushing is required in the waterways which storm water systems empty into.

If the above assumptions are true tanks may actually add to the cost by increasing the maintenance required on storm water systems.

Any thoughts on that aspect of the use of tanks?

R0bert

One practical way you can help to achieve the lowest cost of rainwater supply - and supportive Government policy if you want a rainwater supply before selling your existing house - is to ask your local member of parliament whether you own the water you collect from your roof for your rainwater tank; and whether the Government intends to restrict your right to collect and use this water.

No State Government will confirm that water collected from roofs for rainwater tanks is owned by the building owner (Western Australia and Queensland Governments confirm that the water is not owned by the State).

The lowest cost per kilolitre of rainwater is achieved when rainwater tanks are plumbed for at least hot water, laundry and toilet flushing uses and the plumbing is shared with mains drinking water supply.

It is a matter of personal choice if rainwater is also used for actual human consumption.

If water in rainwater tanks is owned by the Government, the uses will be restricted to toilet flushing and outdoors and separate plumbing will be mandatory.

This is because the quality of rainwater is the owner’s responsibility and where the owner is the building owner they have the legal right to use rainwater for any purpose upon their own responsibility.

The current cost of rainwater tanks and their installation including plumbing is a function of demand and current demand is insufficient to warrant investment in large scale manufacturing facilities and installation services to generate lowest costs.

A solution is mandatory reduction in mains drinking water consumption at point of sale of all property in Australia, with voluntary collection of water from roofs for rainwater tanks being deemed to comply. Ownership is the deciding factor.

If this solution is implemented by State Governments, investment in large scale manufacturing facilities and installation services will be warranted because dwellings in Australia are sold on average every seven years which will underpin demand. Payment of Government subsidies will not be necessary because the cost savings generated by economies of scale are worth more than the subsidies.

Greg Cameron

In response to RObert, let’s establish how much water is involved.

At least 800 billion litres (800GL) of water was discharged from roofs of all buildings in Australia in the 12 months to 30 June 2006. Of this amount, an estimated 670GL was from roofs of separate houses.

The ABS predicts Australia’s population will increase by 5 million and reach 25 million people in 2032. This represents a 25% increase in the population of Australia.

530GL of rainwater collected from roofs for rainwater tanks is sufficient to meet the drinking water needs of 5 million Australians at per capita consumption of 290 litres per day.

As discussed above, ownership of rainwater is the primary determinant of the potential for rainwater to provide 20% of Australia’s drinking water requirements by 2032.

Rainwater tanks convert urban stormwater generated from roofs from being a cost to being an asset.

Urban stormwater systems routinely fail when subject to the first rush of water from a rain event - they leak under pressure. This is because roofs collect rainfall and discharge this water at a single point (the stormwater drain) however this water also has considerable velocity and pressure because it is flowing with the force of gravity (off the roof). The pressure causes stormwater drains to leak and if the storm is big enough, the drainage capacity is exceeded and flash flooding occurs.

An empty 5KL rainwater tank will contain the first 28mm of rainfall from an average roof area of 175m. This leaves the stormwater system free to carry the first rush of water discharged from non-roof sources.

Leaking stormwater drains erode roads from beneath, creating potholes, another cost. In the natural environment, 90% of rainfall stays where it occurs. In the man-made environment 90% of rainfall leaves the point where it occurs. Urban designers are now trying to reduce the need for stormwater drains by water sensitive urban design – which means trying to keep rainfall as much as possible where it occurs so that natural systems can take over.

Greg Cameron

Shonga, the easiest way to save water is to divert the water from your downpipes onto your garden. I use the base of a palm frond because of it's curves to divert the flow from the bottom of the pipe before it goes down the grate. This will ensure the lawn gets a good soaking from even a small fall and can easily be removed if a wet season looks like delivering too much water.

The other point to make about the economics is that the cost of a tank is merely a pre-payment of the money you already pay for mains water so on a cash flow basis it is the only sort of household debt that you know you already have the money to cover.

Re the Tully Millstream, it would seem that the easiest way to protect the Great Barrier Reef from excess fresh water (that kills coral faster than silt) and siltation is to build a decent dam so most of the silt remains up stream. The remaining flood flows can then do their bit for fish and prawn life cycles. But don't say that to an ideological green "Bimbecologist" who simply cannot countenance a dam having any beneficial effects.

Robert, most of the problems of urban storm flooding stem from the greater catchment efficiency of roofs and pavement. So if even 50% of houses had tanks the remaining roof and pavement area will still deliver a significant surplus of storm water compared to natural runoff from forest or pasture. And there is always at least one wet season rainfall event that will make the tanks overflow and give the system an annual flush.

Good points, both. And I agree with Greg, write to the Minister for DNRM to ask him who owns your roof runoff.

Perseus/Greg
Thank you both for your invaluable advice, much appreciated.