Rain water harvesting year 2 results

Perth gets around 750mm of rainfall a year, although recent years have seen a sharp decline. This might sound like a lot to some people, but the pattern of rainfall is not favourable either for growing food or storing the water. Around 80% of our rainfall falls in the 3 months of winter (June-August), with some in May and September to November and virtually none between December and April (unless we're lucky enough to get a summer storm). This means Perth is heavily reliant on scheme (mains) water through our hot, dry summers with vast quantities going on lawns, swimming pools and gardens (44% of residential water is used for these purposes). Unfortunately the population is rising, people are using more water per head and the drop in rainfall has caused a crash in inflow to dams which store Perth's scheme water. Last year Perth had its dryest winter on record and only 13 gigalitres (GL=1 billion litres) flowed into dams, compared to the average of 100 GL. Dams now account for just 25-45% of Perth's water supply, down from historical levels of over 90%. This means the water utility, Watercorp (and government via taxpayers) has had to fund other ways to source water. These sources are not nearly as sustainable as dams. Desalination plants have been built to turn sea water into drinking water, which is very costly and emits vast amounts of greenhouse gases due to high power consumption. Desal accounts for 20% of Perth's water supply now, with plans for more plants. Perth is lucky enough to have large amounts of potable groundwater in aquifers and this now accounts for up to 50% of Perth's water supply (source: watercorp). It is still under debate whether these levels of extraction are sustainable in the long term and what effects it will have on the ecosystems which rely on groundwater.

One way to help the broader community is for people to install rain water tanks and grey water systems in order to be more self sufficient and therefore put less demand on our struggling infrastructure. So, two years ago we decided to install a rain water tank. It's a 14,000L poly tank, which cost us a fair wack of dough but we feel it's worth it to try to make our patch a bit more sustainable. It certainly wasn't for the money we save on our bills (ha, I worked out the payback time to be about 200 years!). Our tank is about 10m downhill from the house. This means that we have the space for it and also the slope allows us to gravity feed water in from the roof. We have 4 downpipes feeding in to 1 central pipe which is buried about 300mm underground. This then goes up the side of the tank and feeds the water in. This is what's known as a 'wet system' since there is always a certain amount of water sitting in the pipes which doesn't get to the tank until the next rain forces it through. This is opposed to the normal 'dry system', where water goes from pipes above the tank down into the tank. We went for the wet system because it usually gives you the ability to feed water from a larger roof area into your tank and to give us room to install a big tank. If you don't have a sloping block then you can bury your tank underground to get the necessary drop (but this will cost you more).

Here are some tank pics:

Our mate Ryan preparing the sand pad (we paid him!)

Compacting the pad

The tank installed with Quin doing bog laps...



A tadpole filter on the downpipe, with a removable insert containing a coarse screen and fine screen to catch debris.

Pinching the neighbour's water...

  

The tank with the following: big pipe on the left is the water inlet, then water out (with gauge), then power cords for the pump and float valve, then the overflow pipe on far right.

And here are the juicy geek numbers for you:

First of all rainfall. We have a rainfall gauge and have our own figures since May 2010 (data before from nearest BOM site- Murdoch). The following chart has 2 years of data and 2 Y axes, cumulative rainfall on the right and weekly rainfall on the left. In our first year we had a great (relatively speaking!) 757mm (blue line), but this year we've only had 511mm (pink line). Thankfully we've had some decent falls since May this year to help bump up our figures after a shocking 6 months before. When we were away over east our friends got about 60mm in the 3 weeks of our stay (a dry period for them), which was what we had had in the previous 5 months!You can see from the spikey weekly rainfall the pattern of rainfall, with most rain falling in winter and flatlining in summer.

NB: No we don't have two Decembers here in Perth! I just have Excel learning difficulties...

So how much water do we harvest and use? Well I had to laugh when I read a pamphlet from the Watercorp (the experts...) recently,the winter 'Watermark'. They are promoting rain water tanks, which is great. Unfortunately they haven't a clue how much water a tank can supply to a household. They say a 2,00L tank not plumbed in to anything can save up to 26,000L a year; and that it would save up to 59,000L a year if it was plumbed into a toilet and washing machine. The 'up to' bit probably covers their arses, but I know for a fact that these numbers are much too optimistic for Perth. Our 14,000L tank plumbed into everything (whole house, garden, etc) harvests around 65,000L a year (I know this because guess what, I have a gauge on it!). I would guess that a 2,000L tank not plumbed in would save a maximum of 10,000L a year. This is based on the assumption that you could use 400L of rainwater a week for half the year (the other half of the year the tank would be empty), which is a fair bit: say 20L a day drinking/cooking water and 4 x 9L watering cans a day for the garden (remember, there's no pump in the tank so you can't water automatically or use in the shower, etc.). Another factor to consider is that if you go 5 weeks with no rainfall (common in Perth) your tank's empty. How about the 2,000L plumbed in scenario? A toilet and washing machine use 7% of household water, which equates to 30,000L a year for a 4 person household. In 6 (wet) months that adds up to 15,000L (assuming you get enough rain every 2 weeks to fill your tank, which is doubtful), so if you add the 10,000L from scenario 1 to this you get 25,000L. This makes both the figures quoted by the Watercorp at least double what is realistic.

Anyway, enough of my Watercorp bashing rant, what about some real numbers? As a household we used an average of 830L a day in year 1 and 840 in year 2. This compares to the Perth average of 870L for a 3 person household (according to Watercorp, 2008-09 data). This means we have a lot of improvement to make to our consumption. However, I think we did quite well in year 2 for two reasons. Firstly, it was a very dry year and you'd expect that to result in much higher water use for the garden. Secondly, we are always adding more retic, which means we used less water per area of reticulated garden in year 2. One interesting fact is that our use inside is around 2-300L a day or around 30% of our total usage. I know this because this is our usage when the garden retic is off. This compares to Perth's average of 52% usage inside. This means we're comparatively good with our usage inside, but we use a lot more than most people in the garden. This is because we grow lots of veggies and have 20 fruit and nut trees to water (plus a small lawn). Personally I think this is a good use of water and it's why I ignore the state water restrictions. The restrictions state that you can only water automatically twice a week for 15 minutes on each station in summer and ban automatic watering in winter. This would mean sure death to most of our veggies through summer unless we hand watered for long periods every day (sometimes twice a day is needed to keep plants alive). We do our best to conserve water by using drip irrigation and by building up the water holding capacity of our soil with compost and soil wetter. The alternative to growing our own veggies would be to buy them, with the resulting food miles and extra water use. Market gardeners use about ten times as much water as home growers (no restrictions for them), so I just don't think it adds up for us not to grow our own.

The chart below shows our water use for year 1, split into tank (green) and scheme (red), with rainfall (blue line) to indicate the times we are harvesting water. Over the whole year we used 21% tank water. We had some teething problems with our pump in this year, so at times we had a full tank but used mainly scheme water due to the pump tripping out. Even if the pump is working we always use a bit of scheme water because the waterswitch needs a tiny bit of mains pressure before it can deliver tank water (the waterswitch switches between scheme and tank depending on the amount of water in the tank). From the end of November the tank was empty for 17 weeks until mid March when we got a huge storm which half-filled the tank (it would have filled it if I'd been well prepared and cleaned gutters and filters). The long dry spell is normal for Perth and has big implications for the size of tank to buy. For us to go 'off-grid' and be self sufficient we'd probably need a tank of 80-100,000L, which is just not feasible for a suburban block. What we have done to combat this is to get the biggest tank possible and use it as efficiently as possible. The key to this is to plumb absolutely everything in to the tank. This means that there is the least waste when it's wet (via overflow to soakwells) and the maximum turnover of tank water. It results in us running out of water more quickly in spring, but this isn't that important. The rainwater switch automatically switches to scheme water when the tank runs down.

In year 2, we actually harvested as much rainfall as year 1 despite 33% less rainfall (21% tank water as a proportion of total use again). Why is this? I think there are a few explanations. One factor is that higher rainfall will usually lead to more loss via overflow, which means that the extra rainfall is not used. Another reason is that we started year 1 with an empty tank (it was brand new), but this year we had a full tank. This means that the of the 64,640L we used in year 2 only 50,000L actually fell in year 2. Next year I might start readings in January when the tank is likely to be empty to avoid this problem. The last factor explaining the amount of rainfall used this year is that we have had a lot fewer problems with the pump, which means we are constantly using rainfall and losing less to overflow.

By the way, if you're wondering about the water quality it's great. We got a filter installed with a separate tap in our kitchen for drinking water. The guy who installed it said "Oh yes, you'll need one of these because you'll be on scheme water some of the time!" In other words tank water is better than mains water if you follow a few simple rules: try to avoid debris entering gutters by pruning overhanging branches; maintain and clean your gutters at last once a year, especially before the main rainy season (you can also install a guard to stop coarse debris falling in); install filters on your downpipes (eg leafeaters or tadpoles, and clean these regularly; install a 'first flush diverter' to divert the first rains away from the tank (it's dirty because there is more crap from your roof then); install a waterboy to pump water from the cleanest part of your tank; install a bottom draining overflow and a 'calming' rainwater inlet. I haven't actually done the last 3 points, but we're planning to do them (especially the overflow tip, great idea!). The last few tips are from a great kiwi website, Waterboy.  

Here are my top tips for installing a rainwater tank:

Install as big a tank as you can afford and have the space for.

Plumb it into absolutely everything to maximise usage and efficiency.

Hook up as large a roof catchment area as possible to the tank (we even harvest from a neighbour's carport).

Maintain/replace gutters as needed (with a good drop to use all the rain and avoid stagnant water) and clean them periodically.

Download a spreadsheet to calculate the right sized tank for your rainfall zone, water usage and roof catchment available (like this).
Poly tanks are my preference. They're cheaper and have the lowest carbon footprint (no concrete pad needed).
Put a gauge on for the geek factor.

So that's about it. I hope you appreciate this post, it took a while to write (gestation period blew out due to the Tour de France)...

Where to from here? We are now planning to install a grey water system in order to cut our summer use and keep our productive trees and decadent lawn healthy. Watch this space.

Go on, install a rainwater tank. You won't regret it and the planet will make a small sigh of relief.