We are considering collecting rainwater from our clubhouse roof and using it to irrigate the surrounding lawns, beds and borders. Is there a formula for working out the appropriate size of tank required?

: Water is precious, so collecting rainwater is a worthy exercise. It is also worth noting that the use of rainwater for irrigation systems is not restricted during times of hosepipe bans or when drought orders are in place.

There are formulae for calculating tank size but most relate to year-round use, such as toilet-flushing. Calculating the size of a storage tank for irrigation purposes will be different. The majority of the water will be collected during the autumn, winter and early spring, and will be relied on for use in summer when it is expected there would be less rainfall.

Factors to be considered include the catchment area - and that could include driveways, car parks and other hard surfaces as well as roofs - along with local factors of rainfall, average temperatures and sunshine levels. The amount of irrigation supplied and the likely high-demand periods of use also need taking into account. Even the type of planting will have an effect on the optimum tank size - shrubs, for instance, will require more water than grass. There may also be additional factors to consider, such as the size of site for the tank and the requirement for earth works in the event of the tank being sited underground.

The mathematics involved is complex. Access Irrigation of Northamptonshire is one company that offers help. It recently developed a computer program to determine tank size. The system looks at month-by-month rainfall for the area and also evapo-transpiration data.

I would strongly recommend you call on the expertise of an irrigation consultant to help determine most appropriate tank size, where to site it, and how best to link it to existing or new irrigation systems. Irrigation consultants specialising in the landscape and horticulture industries will be able to produce a design and offer advice on installation so the system can be tailored to your needs and the needs of the plants.

- Sally Drury has reported for HW and its forerunner GC&HTJ for 25 years, and has spent more than five years testing machinery for HW and What Kit? The advice in this helpline is independent.


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