Saturday, July 21, 2012

Rain water harvesting year 3

We've had a pretty good year of rainfall, with 691mm. This is about the average for Perth these days. Last year we had 515mm, the year before 757mm. We harvested and used 77,000L of rain water with our 14,000L rain water tank, which is the most to date.

29% of our water use was rain water this year, which is way up on the 21% of the previous two years.The chart below shows the weekly figures for rainfall (right axis) and water consumption (left axis), with the proportions of tank water (green) and scheme water (red) used. Rainfall in September and November to January allowed us to harvest more in this period than in previous years. Also, the pump broke down less often this year, which always helps.

This next chart is a new one and shows the level of the tank (left axis) and weekly rainfall (right axis). As you would expect the tank is fairly full in winter (june-august) and still has water right through spring. Then as rainfall drops off the tank empties as we turn on the retic to water our vegies and productive trees. Through most of summer the tank was empty until the return of rain in April. Since then we've had water in the tank most of the time. In May we had a warm, dry spell and turned the retic back on. This emptied the tank, but soon after the tank refilled. This shows the value of having the tank plumbed into everything (house and garden) since we're using as much of the rain water when it's available. If we had a few appliances (eg toilet and washing machine) running on tank water and the rest on scheme as is common, then we wouldn't have used much of the water in May. When it rained again a lot of that rain water would have been lost to overflow. I estimate that we would use about half or a third the tank water we currently use if we only plumbed in a few appliances. This highlights the need for people to plumb tanks properly to maximise the benefits to themselves and the broader community.

Best of all, we cut our water usage by 100L a day this year. We did this despite adding yet more retic (out the front on our productive trees). I can't really explain this. It could just be that we had less dry spells when we needed to turn the retic on (about 70% of our water usage goes on the garden). 

1 comment:

  1. Hi Adam

    We have had a similar experience in Melbourne. We have a storage capacity of 11,000 litres and over three years we harvested and used 87000 litres per year. Our rainfall is somewhat higher than yours during the drought we were at about 650 mm per year but the last 18 months we are at about 1000 mm per year. Even so I found that in the wetter period we did not harvest much more water as what was limiting how much we could harvest was the rate that we could use it. So I agree that the key to getting good performance from you tank is to use the rain water as much as possible. We have found that our mains use has dropped by 90% compared to pre tank which is saving us about $650 a year in water charges.