Rainwater Harvesting

It’s been a wet summer, by all accounts. Rainfall has been well above average and Brisbane has experienced the worst flood since 1974. Keeping your garden well irrigated is probably the last thing on your mind. This would be a mistake.

All life needs adequate amounts of water to survive and flourish, but the availability of water is variable. The Brisbane climate is fairly moderate, but there is a distinct wet and dry season. Australia in general is prone to extended periods of below average rainfall, typically for years at a time.

If you structure your garden like most suburbanites, you will rely on town water to keep your garden going during the dry. This has become problematic as dams have been unable to meet demand. During the wet season, heavy rains run off saturated soil and down hill or into the storm water system. The garden gets some water, but much of it is wasted to the sea, picking up pollution and soil as it goes. When it rains it pours, so the saying goes. What can be done? The answer is storage.

The first action is to see water as a resource, not a problem. There will be storms that cause issues, but as you will see many of these can be mitigated and in fact, taken advantage of. Even dirty water coming off a road can be used safely. The second is to catch as much of it as possible and manage the overflow. There are several ways to store water, with different costs and benefits. Let’s look at some of the basic principles of catching and storing rainwater.

It’s best to start at the highest places in your catchment. At this point, the amount of water you have to manage is the smallest in volume, highest in quality and possibly lowest in velocity, depending on your site. As the water travels downwards, it joins with the water falling on your site, growing in volume. As it travels over your site it also picks up your soil, vegetation, and detritus. These are valuable – you don’t want to lose them. As you reduce the amount of run-off up hill, the volume of water lower in your catchment will reduce and become easier to manage.

High quality rainwater harvesting using water tanks
A water tank catches high quality run-off from a house, filtering out debris using a leaf eater and first flush diverter. Photo copyright All You Can Eat Gardens.

 

You also want to slow the water down, to a stand still if possible. Not only to catch it but to drop its load and prevent it from causing damage. In natural water courses the widest parts are where the water travels slowest and also where sand banks and fertility build. Spreading the water out is one way to slow it down. Using containers is another. Spreading and containment can be achieved using many different materials including soil, vegetation, stone, wood, and man made materials.

 

Storing water in the soil using a french drain
A french drain accepts water from a patio and absorbs it into the soil for plants to use.
Photo copyright All You Can Eat Gardens.

Containers will fill up and overflow during a rain event depending on the available capacity and the duration and intensity of the downpour. The overflow is still a resource and if not properly managed can cause damage. Keeping the overflow as slow as possible by diffusing its energy will limit the damage. Directing it to further containers will maximise the water you can store. Keep on looking out for places to store more water – get creative!

Swales capture large volumes of water and store it in the soil
Swales capture large volumes of water and store it in the soil.
Photo copyright All You Can Eat Gardens.

Finally, you need to protect the water you have stored from evaporation, leaking and pollution. Nothing is static: vegetation will grow, soil, rocks and logs will erode, shift and rot, and pipes will leak and break. You need to make inspection and maintenance a regular activity so that your rainwater harvesting system is operating at peak performance when that big downpour comes.

Basins catch run-off sheeting off adjacent paving
Basins catch run-off sheeting off adjacent paving.
Photo copyright All You Can Eat Gardens.

The specifics of each harvesting and storage technique could be an article in itself, but this should get you thinking about your own situation and what you might be able to do. Get out in the garden while it’s raining. See where the water goes, and look for opportunities to harvest, store and use that water. Soon your garden won’t even notice the inevitable droughts!

On a large scale, these strategies - and their absence - can have dramatic consequences. By catching rainwater and using it during dry periods, you are moderating floods and droughts. When land is cleared for farming, paved with roads or otherwise drained, run-off is concentrated. Volumes and flows increase. The result is increased severity of floods. For Brisbane, expensive dams – rainwater harvesting beyond a sensible scale - have not prevented damage because the run-off is too much and the storage relatively small. Remember: start at the top of the catchment. Do your part and catch your water.

Floods are the result of inadequate rainwater harvesting higher in the catchment
Floods can be mitigated with rainwater harvesting higher in the catchment. Many small dams are cheap to build and it all adds up.
Photo copyright All You Can Eat Gardens.