As the climate warms and rainfall becomes less predictable, water harvesting in gardens will become more and more valuable. Everybody knows about the value of rainwater tanks, but did you know there are other ways to harvest rainwater and runoff too?
Read on to learn more about the benefits of water harvesting in Canberra. Watch a video showing how we capture rain and greywater using a couple of grated drainage channels cut through the driveway. Because our driveway is very long, that's a lot of water. We reckon this might harvest as much as our roof does!
In order to help us think about the value of water harvesting, here's a quick thought experiment:
Imagine your property before it was turned into a suburban lot. It was probably originally grassy woodland, though it may have more recently been grazing land. Provided that you're not on much of a slope, most of the rain that landed on your plot would have soaked into the soil - especially if there was a good variety plants growing on it (plant roots create tiny access ways for rain to more deeply penetrate the soil). On the other hand, if you live on quite a steep slope, there was probably some runoff during heavy rain, which in turn would have influenced what plants could grow there.
Fast forward to today, and have a look at where the rainfall goes on your property now. If you have a suburban property here in Canberra, it probably contains a house, a driveway, paths, maybe a garage and a shed or two in the garden. You might even have a second property or granny flat. Together these make up the hard surfaces in your garden - and if you look, you will find that they are nearly all connected to drains that lead into the storm water system. For many older properties with large gardens, the amount of hard surface is around 25% to 35% of the total land area, while in higher density suburbs, the hard surfaces might take up 75% or more of the total area. So, even if you have installed rainwater tanks, there is probably plenty of other infrastructure that still diverts runoff straight into the storm drain system.
What this means is that your garden now sheds anywhere between 25% and 75% of the rain that would have once naturally been taken up in that land. Year after year, this means that your land will be drier than it once was (unless you do a lot of supplementary watering). While structures certainly prevent evaporation, if you still have large trees in your garden that pre-date the building of your house (as we do), those trees may now be existing on less water than they once had access to. Now that summer temperatures are on the rise, and with that, higher evaporation, this situation places many street and garden trees under increasing stress, just when their cooling shade will be needed most.
Many new suburbs now take water-sensitive design into account, so that runoff from hard surfaces and roadways is directed into planted bio-swales and rain gardens. But what about existing suburbs that were designed to shed water as quickly as possible into the storm drain system? Is there much we can do, beyond installing rain tanks? And if there is, can we ensure that we do it safely and don't accidentally flood our gardens or house foundations?
Thankfully the answer to the above questions is a resounding yes. The video below shows a simple design we have used to harvest water off our driveway and divert it into adjacent garden beds. Our approach is an example of passive water harvesting in that there is no mechanical element (like pumps or water treatment) - all the water flows directly into the garden where it soaks away into the ground. In this situation, we are using the soil as our water storage - it's amazing how much water it will take and it's also cheaper than a tank!
Why is water harvesting becoming increasingly important?
As the climate warms, rainfall patterns in the ACT are changing. Scientists predict that winter and spring are likely to become drier, though autumn will probably become wetter. And nobody is quite sure whether summers will be wetter, drier or about the same.
That said, the ACT has highly variable rainfall already, with some years being very dry (like 2019!) and others very wet. This year-on-year variability will probably continue – this means in practical terms that Canberra will continue to alternate between drier and wetter years. The key difference is that the dry years will be drier than before, and wetter years may be very wet – due to a warmer atmosphere holding more moisture (individual rainstorms may get very intense). At the same time, there will be greater evaporation in all seasons from higher average temperatures.
Over coming decades, we are likely to see more intense droughts followed by intensely wet years. Can passive water harvesting help in both of these extremes?
Water harvesting as a resilience strategy for both dry and wet weather
It's fairly obvious that in a drying climate it makes sense to harvest as much water as possible when it does rain, but did you know that the act of harvesting water in your garden can also alleviate problems associated with too much rain as well?
Basically, the more water we can keep on our land, the less water goes into the storm drain system when it does rain. While one or two gardens won't make much difference, if a large number of us implemented water harvesting, this will take a lot of pressure off the storm drain system during extreme heavy rain events.
And you guessed it, even if Canberra does get some very arid seasons, those climate models tell us to expect more extreme rainfall events, especially associated with summer and autumn thunderstorms. This is because a warmer atmosphere holds more water, increasing the likelihood of sudden, severe rainfall. Like they get in the tropics. So while every year may be different, we can expect the rain to come down pretty hard when it does rain - again, making it challenging to manage when it does come.
Remember the year the ANU flooded in that major thunderstorm? That’s the sort of weather event that could happen more. If we can all direct at least some of our storm runoff into our garden, there's less chance of downstream flooding.
A quick caveat: a critical part of any water harvesting approach is to ensure that it can safely seep into your garden in a controlled manner. A key component of any design is to make sure that any water flowing directly into your garden and soil are slowed down first - fast flowing water erodes landscapes, while slow moving water tends to soak in. So all approaches to directing water into your garden in permaculture strongly emphasise ways to slow water down and let it soak in gently. And once any water harvesting system is full, it is best practice to make sure any overflows do go back into the storm drain system - making use of this system as a useful emergency overflow once your garden is full, rather than it being the first port of call for any water shedding off hard surfaces in your garden.
So to summarise, water harvesting in our gardens actually gives us two benefits:
- Storing water in your garden (both in tanks and directly in the ground) gives your plants a free drink and makes the garden more resilient to periods of dry weather.
- Reducing the amount of water going into the storm drain system means less chance of downstream flooding in low lying areas (e.g. at the ANU).
I hope that was a useful little foray into the wonderful world of water harvesting! There is plenty to know and there are of course myriad ways to effectively store water in your garden. But I'll save the nitty gritty of specific techniques for another post. As we implement more and more of these in our own garden, we also hope to provide some workshops on water harvesting in the future to explain the theory and see it in practice as well!
Wishing you all the best this winter and hope you are enjoying the much-needed rain as much as we are!