Rainwater Harvesting

What is rainwater harvesting and why is it Important?

Water is our most precious natural resource and something that most of us take for granted. We are now increasingly becoming aware of the importance of water to our survival and its limited supply, especially in such a dry continent as Australia.

The harvesting of rainwater simply involves the collection of water from surfaces on which rain falls, and subsequently storing this water for later use. Normally water is collected from the roofs of buildings and stored in rainwater tanks. This is very common in rural Australia. Water can also be collected in dams from rain falling on the ground and producing runoff.

Either way, the water collected can be considered to be precious.

Rainwater harvesting techniques

The collection of rainwater from the roofs of buildings can easily take place within our cities and towns, not just in rural Australia. All that is necessary to capture this water is to direct the flow of rainwater from roof gutters to a rainwater storage tank. By doing this, water can be collected and used for various uses.

If you are reliant on collected rainwater and are not connected to a towns water supply, then the water collected will be especially important to you. If you are from the city, then it is possible to replace all or at least a substantial portion of your fresh water requirements by the capture and storage of rainwater from your roof. Being largely self sufficient in water supply is possible for a vast majority of Australian households and buildings.

What are the Benefits in Rainwater Harvesting?

By capturing water directly, we can significantly reduce our reliance on water storage dams. This places less stress on these dams and can potentially reduce the need to expand these dams or build new ones.

Collecting and using your own water can also significantly reduce your water bills.

By capturing water, the flow of stormwater is also reduced and this minimises the likelihood of overloading the stormwater systems in our neighbourhoods.

What About Dirty Roofs?

There are a number of devices (first flush devices) which allow for the first flow of water to the rainwater storage tank to be diverted from the tank. By doing this, any dirt on the roofs of buildings that has built up prior to the rain can be excluded from the tank.

Sizing of Rainwater Storage Tanks

The most appropriately sized rainwater storage can be chosen by quantitatively assessing the performance of various sized storage capacities. By assessing the performance of various sized storage capacities, it is possible to make an informed decision as to what would be the most suitably sized storage capacity for the given application. The input for the assessment is historical daily rainfall data, and the performance of a particular storage capacity can be judged by how much water is required to be supplied from other sources to makeup for any shortfall in demand.

Water Balance for Estimation of Rainwater Storage Capacity

The size of the area of capture or roof area must also be known when estimating the amount of rainfall that is able to be collected. The larger the roof area, the more rainfall that is able to be collected.

Assessing Performance of Different Sized Rainwater Storage Tanks

The performance assessment of various sized storages involves the calculation of the amount of water in storage for a given day. This calculation is based on the water balance shown above. This is a simple calculation, however, using a computer allows this calculation to be completed for many consecutive days of rainfall data. This is equivalent to trial sizing a storage tank size over the period of assessment (over many years).

The computer model completes daily water balance calculations, so that any roof runoff generated from rainfall in that day is calculated. The computer model also calculates the daily level status of the water storage used to hold this rainwater.

During any one day the storage could overflow depending on the amount of roof runoff generated. Likewise, the storage could also be emptied if the volume of water used exceeds the amount of water available from the storage. In this case, water must be supplied from other sources in order to fulfil the water demand. The computer model calculates and sums the amount of water supplied from other sources over the period of assessment. This information can then be used for a comparative assessment of the different amounts of makeup needed with use of different size water storages.

Capturing Ground Runoff

The concepts of rainwater harvesting are not only applied to roof catchments. Ground runoff can be modelled and used as input to overall water balance calculations. Additionally, the size and nature of water usage can be modelled. The computer model can also account for the way the water is handled. All of these factors can be incorporated into an overall water balance model so that the best strategy for capturing and managing this most precious of natural resources can be determined.