Reliability Schmeliability

One of the most common arguments I hear from a subset of urban water managers who don’t like rainwater tanks is that tanks are too unreliable to be considered a viable urban water source.  So, perhaps now, a week after our tank lay empty for 8 days, would be a good time to dissect this argument and review what our system is doing for Melbourne’s water supply.  We’ve been using the tank since 30 July 2014: 452 days, and the tank has been empty for 9 of those days.  It has therefore provided us with water with 98% reliability.

Exactly! I can hear my urban water manager nay-sayers saying – that is not a reliable enough water source to base a water supply on.  Well, maybe that is true, if I wanted to make the tank my sole source of water, but I don’t – I want to (a) reduce my impact to my local river and (b) while I’m at it reduce my demand on the large, ultra-reliable water source that Melbourne has at its disposal: 1500 gigalitres of water in our dams, and (regrettably) a desalination tank that could (if it is ever turned on) provide an additional 150 gigalitres of water a year.

Now, let’s put the desal plant aside, and think about how the dams work.  Each year, Melbournians draw ~400 gigalitres a year to make the city do what cities should do (let a lot of people live in good health, comfort and prosperity), and each year our rivers and streams top up our dams with ~600 gigalitres on average. On average then, the dams should provide us with plenty of water for a long time in to the future.  But those are long-term averages.

Melbourne’s usage doesn’t vary all that much, but it went down quite a lot during the last drought, and is likely to climb slowly again as the population grows and as governments have less motivation to encourage people to reduce their demand.

But the inflow volume is very variable, and every now and then we have a string of years in which the inflows are less than the amount Melburnians are demanding from the dams.  In the 2000s–the Millennium Drought–, for the first time, we had 10 years in a row when the inflows were less than the demand, and our dams approached a dangerously low level. (It was at this time that the politicians panicked and the desal plant was born – but that’s another story.)  With climate change, droughts like that are tipped to become more common.  So let’s say we have another 10 years in which inflows into the dams drops to 300 gigalitres a year, like they did in the Millennium drought (actually in the drought, the average only dropped to 375 gigalitres a year, but let’s be extreme to put a safety margin on the back of my envelope calculations).

With us still using 400 gigalitres a year, and only 300 gigalitres a year flowing into our dams (that currently hold 1300 gigalitres), they would be down to 500 gigalitres in 8 years, and our politicians would start panicking again.  But what if everyone had a not-reliable enough harvesting system like ours for our non-potable purposes?  Our 98% reliable rainwater tank and raingardens use ~43,000 litres per year, and our household uses ~91,000 litres of water from Melbourne Water’s dams each year.  So the tank is providing 32% of our house’s water supply.  If everyone had a system like that (and many bigger houses could have harvesting systems that supply a larger percentage of their larger demand), then Melbourne’s demand on our dams would reduce to ~270 gigalitres per year.  That would be enough of a reduction in demand to keep our ultra-reliable dams near full, even in the driest years (300 in, 270 out) and easily able to cope with the temporary rises in demand when the city’s tanks are empty for 2% of the time.

So, I say reliability schmeliability.  It is all about yield; and the potential yields from rainwater harvesting in Melbourne are huge.

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