In those cases where a rainwater tank is to represent a supplementary source of water, the size of the tank will depend on a balance of cost weighed against the range of uses required (drinking, cooking, bathroom, laundry, toilet etc).
If the rainwater tank is to represent the only source of potable or domestic water cost will be less important than the size of tank required to provide security of supply.
In this circumstance the size of the tank will depend on:
- the volume of water needed,
- the amount and pattern of rainfall,
- the area of the roof catchment,
- the security of supply required.
The amount of rain combined with the area of the roof catchment will determine the maximum volumes of water that can be collected. If this is not sufficient then either a greater catchment area will be required (eg garage or shed) or alternatively water demand will need to be reduced. A number of water conservation measures could be applied including dual flush toilets, dry toilets (if permitted), water efficient devices, reduced flow shower heads, washing machines with suds savers etc. If necessary, a separate lower quality supply could be considered for uses such as toilet flushing.
If after the implementation of these measures the volume of rainwater that can be collected is not sufficient to meet demand, additional water will need to be obtained from another source.
Experience is always a useful guide and advice should be sought from neighbours particularly in areas where reliance on rainwater tanks is common.
It should be noted that in some areas local authorities specify minimum requirements for water storage and in many rural areas there may also be storage requirements associated with firefighting. This may vary from one area to another.
Water demand will depend on:
- the number of people using the water,
- average consumption per person,
- the range of uses (drinking, cooking, bathroom, laundry, toilet etc),
- the use of water conservation devices.
- In areas supplied with reticulated water the average indoor use per household is estimated to be in the range of 400-740 litres per day or alternatively 100-200 litres per person per day.
These volumes are being steadily decreased with the application of water conservation measures. Advice on water usage could be sought from the local water or water resources authority.
In general the most accurate source of this information is the Bureau of Meteorology. In addition to average rainfall (annual and monthly) it is important to determine the season of the rainfall and the occurrence and length of dry spells.
Calculate the area covered by the parts of the roof from which the water is to be collected.
Note that it is the flat or plan area (including eaves) that should be determined. The slope or pitch of the roof and the actual number of square metres of tiles or metal is not important.
The average roof area for a small house is about 100 m2, for a medium house about 150 m2 to 200m2 and for a large house over 250 m2.
The continued supply of water requirements under almost all conditions, including extended dry spells, will require a larger tank than that required to supply demand under normal or average conditions.
Some State Government Departments have tables of calculated tank sizes based on rainfall data. In Queensland this information has been available from the Department of Natural Resources, in South Australia from the South Australian Water Corporation or the Department of Environment, Heritage and Aboriginal Affairs and in Western Australia from the Department of Agriculture. Other departments with responsibilities for water resources or water supply may also provide this information.
Suggested Tank Size
By far the most common question faced when purchasing a water tank or looking at a new water tank design is – what size of water tank is needed?
For the average home and average water needs, a water tank around the size of 1000Lt to 3000Lt is usually more than sufficient. The most common water tank design found today is the plastic or polymer water tank, which can be found in the traditional round tank, with many water tank designers moving into rectangle or square designs.
Larger requirements such as extensive gardens and the requirement to water your lawns will require a much larger water tank, possibly up to or exceeding 10,000 litres.
Maximum Amount of Collectable Rain Water
If you wish to ensure that you don’t waste any of the rainfall you can apply known formulas to work out the maximum amount of collectable rainwater and then from that you can calculate the required tank size to ensure you collect every drop.
Once the area of roof catchment has been determined and the average rainfall has been established the maximum amount of rain that can be collected can be calculated using the formula:
Run-off (litres) = A x (Rainfall – B) x Roof Area
A = is the efficiency of collection and values of 0.8-0.85 (ie 80-85% efficiency) have been used.
B = is the loss associated with adsorption and wetting of surfaces and a value of 2 mm per month (24 mm per year) has been used
Please Note that Rainfall should be expressed in mm and Roof Area in square metres (m2)
Calculate the Size of Tank Required
The next step is to calculate the size of the tank. The tank needs to be large enough to ensure that:
- the required volume of water can be collected by the tank.
- the volume of water in the tank will be sufficient to meet demand during the drier months or through periods of low or no rainfall.
The simplest way of checking a tank size estimated to provide water throughout an average year is to use monthly rainfall data and to assume that at the start of the wetter months the tank is empty. The following formula should then be used for each month:
Vt = V t-1 + (Run-off – Demand)
Vt = theoretical volume of water remaining in the tank at the end of the month
V t-1 = volume of water left in the tank from the previous month.
Run-off should be calculated as discussed above (A = 0.8, B = 2 mm).
Starting with the tank empty then Vt-1 = 0. If after any month Vt exceeds the volume of the tank then water will be lost to overflow. If Vt is ever a negative figure then demand exceeds the available water. Providing the calculated annual run-off exceeds the annual water demand, Vt will only be negative if periodical overflows reduce the amount of water collected so that it is less than the demand.
Tank size is not necessarily based on collecting total roof run-off. If the water demand is less than this some overflow may occur while demand is still met. If water demand is to be met throughout the year the tank should be large enough so that Vt is never negative.
Calculations should be repeated using various tank sizes until Vt is = 0 at the end of every month. The greater the values of Vt over the whole year, the greater the security of meeting water demand when rainfalls are below average or when dry periods are longer than normal.
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