Rainwater Calculator

Calculating the Tank Size

To calculate the size of the tank you need and the number of Hydrowall modules, you will need to have a look at how often it rains (the period between significant rain). How much water you use, and what will happen if you run out of water? If you have mains water then it is not a big problem because you can switch to mains water if you run out. But, if you rely solely on rainwater then you will need to factor in periods of drought or little rain.

The time between significant rain is a bit of a guess, and you will need to use your best guess based on your past experience. You can also use the rainfall table to give you a better idea of the amount of rain you get and when. Don’t forget these are AVERAGES, so a worst case scenario could be DOUBLE, possibly more.

To Calculate how much water you need

To calculate how much water you consume, use the table above. You can either be general and simply use the ‘average household’ figure divide by 12 (12 months in the year) or if you want to be more specific adjust the figures to what you think is appropriate. Keep in mind that you will use more water on your garden in Summer, and if you have a pool or ponds then that can change your usage too. The number of people in your home will also improve your usage.

To calculate the tank size you need Period of no significant rain (days) x Daily Consumption (L)= Tank Size

Example: You live in Adelaide and during Summer you can remember the time it hasn’t rained for four weeks (30 days). You have a family of 4 but will use the rainwater for the lawn and garden only.

Daily Consumption: Lawn and Garden= (90,000 + 55,000) / 365 = 402L per day
Storage Reserve Required: 30 days x 402L = 12,060 L
Hydrowalls (590L/ 2.4m module) needed: 12,060 / 590L = 20 modules

To calculate how much water you can collect each year or month, use the following calculation: 

Annual or Monthly Rainfall (mm) x Roof Size (m²) x run-off percentage (%)* - any first flush (L)= Maximum Harvest Volume * 

‘run- off Percentage’ is the percentage of the total roof area that is used for collection.

Step by Step Calculation:

1. Locate the area on the table above closest to where you live. If you are calculating a figure for the monthly rainfall, then select the month you want. Otherwise use the annual figure.

2. Calculate your total roof size in m² (length x width). 

3. Calculate the run-off percentage (the percentage of the roof area that actually collects water)

4. Estimate any other losses you have from diverters, leaks or other reasons

5. Put the figures into the calculation above and determine the maximum amount of water you will collect for the period.

Example: Your house in Broome has a 100m² roof. But you can't collect the water from the gutters on one side of the house. You estimate this to be 25% of the roof area, which means your catchment area is 75m². You also have a Superhead first flush device that will discard 30L each time it rains. 

1. The annual rainfall for Broome is 599.2mm.

2. Roof area is 100m².

3. Run- off percentage is 75%.

4. Diverter discards 30L each time it rains, and you estimate is rains 50 times per year, So 50 x 30 = 1500L.

5. CALCULATION: 599.2 x 100 x 0.75 - 1500= 43,425 litres can be collected each year.

PLEASE NOTE: These are basic calculations intended for estimating purposes only and do not take into account rainfall intensity, frequency or duration. For important applications, you should consider engaging a qualified civil engineer to assist with your calculations.

Rainwater System Design

Now you know how many tanks you need you can plan your rain harvesting system. Things to consider are:

• Space requirements

• Distance from garden and/ or plumbing

• Location of trees etc that may affect water quality

• Visual impact

• Distance from gutters/ downpipes

• Connection to existing plumbing

• The use of filtration devices- gutter screens, first flush rainwater filters, rain- heads, outlet filtration.

• Pumps if required (you will probably need a pump if connecting to household plumbing)

Rainwater Health Issues

Installing and maintaining a Hydrotank is very simple. However, you need to be aware of the potential hazards of storing water, especially if you are using it as drinking water. Some councils don't recommend drinking rainwater so check to see if there are any restrictions first before you buy a tank. Most problems occur because of dirt and pollutants from the roof building up and being washed into the water. Prevention is better than cure when it comes to rainwater tanks. By keeping your roofs and gutters clean and using suitable filters like the Superhead Diverter can prevent most problems. Below is a list of the common problems and ways to prevent or treat them:

• Bird and Animal Droppings

Prune overhanging branches and keep tank covers closed and seal gaps to prevent access. Use a first- flush filter to discard the first few litres of rain collected.

• Mosquitoes and insects

Protect inlets and overflows with mosquito proof mesh. Prevent water from pooling and stagnating such as near the overflow or in gutters.

• Lead Contamination

Check for and remove lead paints and primers on roof and gutters (pre 1970’s).

• Rust and corrosion

(rotten egg or sewerage odours) Rusty gutters and downpipes can taint the water. This is from low pH caused by contact between leaves and metals for long periods so clean gutters regularly.

• Bacterial Growth

(Musty, fishy or ‘rotten egg’ smells, or small white flakes) Some bacteria (biofilms) are OK, but some others are bad. Anaerobic growth in the sediment at the bottom of the tank is usually not good so try to minimise sediment entering the tank and keep the water oxygenated by using it regularly. Also, make sure no light can enter the tank. Chemicals can also be added to treat bacterial growth. Most caravan or boat shops will sell a range of chemicals for treating water tanks.

• Coloured water

Caused by leaves rotting on roofs and gutters, or from sediment coming from the roof tiles or paint. Keep the gutters clean and use a first-flush filter.

• Slime

A light coating of slime (bio-film) is standard and useful for maintaining the quality of water. Heavy slime may be a concern and should be treated with chemicals.

Maintenance

The Hydrowall Sytem low maintenance provided steps are taken to minimise sediment and pollution entering the tank. The slimline design makes it difficult to inspect the inside of the tank so all the more reason to prevent problems occurring.The main problems are usually caused when things like leaves collect on your roof and gutter. They will start to rot and breakdown making them easier to get washed into the tank. The rotting process can also encourage bacteria and insects to breed. So by merely keeping the gutters clean you can make sure your tank is clean and safe. You can even cut a range of devices like the Superhead to help keep your gutters clean.

Rainwater harvesting works on a cycle. Most of us will get rain in the winter months, and preceding that is Summer and Autumn when there is a little rain, and the leaves are falling off the trees. So a bit of common sense says that Autumn is a good time to check that the roof and gutters are clean. Also, let the first rain wash all the dust and dirt off the roof if it hasn’t rained for a while. You should regularly try and use the water stored in tanks to prevent it stagnating and growing unfriendly algae and bacteria. This won’t be a problem if it is plumbed into your toilet and laundry, but if you are just using it for the garden then try to use a little every week.