Rainwater Harvesting

Rainwater Harvesting

Hydrological Cycle

Precipitation (rain, snow etc),  is the recognised starting point of the ‘hydrological cycle’, and is our ‘primary’ source of water. Rivers, lakes and groundwater are ‘secondary’ sources.  In today’s world however we rely almost entirely on these ‘secondary’ sources for our water supplies.

Rainwater Harvesting

The hydrological cycle  

Rainwater Harvesting

Rainwater harvesting is the process of collecting clean rainwater for domestic and other uses. The ‘harvest’ can provide drinking and irrigation water, and the harvesting process itself can reduce storm water discharge, mitigate risk of floods and help avoid overload of sewage installations.  Rainwater harvesting is also useful in schemes to moderate seawater ingress on the coast.

Rainwater Harvesting

Rainwater Harvesting


But more typically Rainwater harvesting involves collecting run off from roofs or nearby catchments, securing the available seasonal flood water from local streams or conserving water by managing the local watershed. In a domestic environment the process revolves around direct collection for direct domestic use. Water harvesting is part of the Sustainable Building package which allows a home to be built in a remote area without mains drainage or a piped water supply.



Rainwater Harvesting

Indian Rainwater Harvesting system

 Indian Water Husbandry

Community rainwater harvesting in rural locations in the Indian Sub Continent is essential to survival. Water is harvested directly from rooftops, and in collection barrels, and is stored in tanks. In more open land rain is collected and stored in artificial wells, Monsoon run offs and water from flooded rivers can be captured and stored in the same way. India alone is capable of harvesting enough water to satisfy all its needs many times over.

Environmental Effects

Theoretically rainwater harvesting can reduce the amount of water draining into the ground and streams in an area. The effect is either beneficial or detrimental depending on the circumstances and climate. On the one hand rainwater harvesting might prevent sedimentation, erosion and pollution and on the other might cause rivers ponds and streams to dry up.  However if the water is used in the same watershed as it is collected the practice is invariably beneficial or at worst benefit neutral as the only practical  effect it can have is to stabilize the flow of water into the ground.

For practical purposes in the UK however environmental cost/benefit effects of rainwater harvesting are zero and it’s difficult to see how purely domestic collection and use of harvested water could have anything other than a beneficial or neutral effect anywhere in the world.

Rainwater Harvesting

Rainwater Harvesting
A large underground plastic collector tank

The Maths

‘Water Harvesting Potential’ = ‘Rainfall (in millimetres)’ x ‘Collection Efficiency’

The amount of rainfall in a locality is known as its ‘rainwater endowment’. Its ‘water harvesting potential’ is the amount that’s collectable. ‘Collection efficiency’ allows for the fact that only a proportion of the water that falls is collectible. Spillage, evaporation, run off coefficient, and first-flush water loss have to be taken into account.


A building in Surrey with a flat, impermeable (leak free) collection area of 75 sq m, with average annual rainfall 625 mm (12 inches).

§   Area 75 sq m.

§   Depth of the rainfall = 0.625 m (625mm)

§   Volume of rainfall over the area in litres = 75 x 0.625 x1000

§   Assume 50% harvest

§   Rainwater harvested in a year= 75 x 0.625 x 1000 x 50% = 23,437litres

This volume is nearly twice the drinking water required in a year by 4 people and the harvesting assumptions are conservative. A family of four could in reality realistically hope to achieve their drinking water requirements in an area of 6 x 6 metres, and Surrey is hardly the wettest of areas. But the figures show what is possible.

At the moment in such a well served urban environment the hypothetical family might consider asking an electrician to come and install an electropump to enable the rainwater harvesting system to work in tandem with the mains water supply. Such an arrangement is in any event the likely way forward for rainwater harvesting if it is to have mass application in future.  As with most innovations its success or otherwise will be driven by financial considerations. But the fact that water metering is becoming more common and the cost of water is increasing so fast is a factor in its favour.

Systems and Safety

Rainwater harvesting systems can go much further than mere collection of water in a water tank, although a simple water butt is not to be sneezed at in a property which has a water meter and a regular use for unfiltered water.

But tanks or drums for example may be used in a house to provide thermal mass. Rainwater harvesting Tanks can be embedded in a building A ‘Trombe wall’ is a wall effectively made from a water tank to fit within a building’s structure. So apart from harvesting rainwater it stabilizes the temperature and captures sunlight.

House incorporating ‘Trombe Walls’

Fundamental to any rainwater harvesting system however is avoidance of contamination so any system has to have appropriate filtration. Rainwater starts off pure but is liable to absorb pollutants as it falls and accumulate pathogens during collection and storage.

It is supposedly very rare for a serious injury to occur but that may reflect the fact that long term consumption of harvested rainwater is virtually unknown in areas where the risk is highest. Acidic rain can bring chemicals and metals off the roof with it. The rainwater harvested may be significantly poisonous over time and the materials contained in it are not removed or neutralized even by boiling. Some of these materials are invisible and carry no smells.

Modular, scalable systems can be installed underground incorporating geo-synthetic drainage cells, which provide for storage.

A Starting Point

At the moment readers might like to consider the basics. A simple tank or water butt in the garden receiving flow from the guttering with a simple basic filter to remove debris can be a source of water for gardening or for washing the car.

Rainwater Harvesting Butts

The next step up may be a shallow dig of flat tank hidden under decking with a pump to extract the water, The next step up again is rainwater harvesting for basic no drinking use inside the home such as flushing toilets and use in the washing machine.

Rainwater Harvesting

A Rainwater Harvesting tank being lowered into position

KCS will be returning again to the subject of the various methods of harvesting water and how the systems fit in with any Sustainable Building Project. We would like to hear from anyone who has any specific Rainwater Harvesting queries so that we can prioritise an article on the subject.