Rain water harvesting

This can be anything from using buckets to catch water from drainpipes to sophisticated harvesting and purification systems. Any gardener will tell you that rainwater is so much better for the garden than tap water. Rain water harvesting is a good solution where Boreholes and well points are too expensive, not possible or not appropriate and can be a complement to them. Every litre of water that you don’t buy from the municipality saves you 1.7 times the cost as your sewerage bill is 70% of the water bill – whether it goes down the toilet or not.

A fall of 10mm of rain on 150m2 of roof area will yield 1 275 litres of water; taking into account a 15% loss. The average rainfall in the Cape is 550mm pa and Johannesburg around 700mm; yielding a potential of 70 000 and 89 000 litres pa.

You may ask what relevance a rain water harvesting system has in the dry seasons; i.e. summer in the Western Cape and winter elsewhere. In the wet season you can store up rainfall for use in the dry season and even in the dry season rain will occasionally fall. Unless you have a very large water storage capacity it is unlikely that a rain water harvesting system will be able to sustain your whole garden throughout the dry season.

We suggest that you decide which plants, herbs, veggies, etc are important and use the water that is available to only water those. Dead grass will come back once watered. A grey water system can be utilised to compliment rainwater; alternating the water sources is a good idea especially for veggies that are 2 weeks or less from harvesting.

We have run roof gutters directly into a pool with a surface area of 32m2; approximately 120m2 of roof drains into the pool, see attached picture. This means that for every 1mm that falls on the pool another 3.5mm runs into the pool; this pool requires a top up only in January, February and maybe March; then only if there is no rain. The guttering is unobtrusive and is permanently installed; you can install a ball valve to divert the water elsewhere. You can also do a temporary system; see attached picture of cool drink bottles and irrigation piping.

Storage tanks range from 200litre to 20 000litres; you can get 200 litre drums from chemical, food and general industry and clean them and use as storage tanks in a DIY installation. You can always start small and build up as and when you can afford it.

Filtration is an option with rainwater and will depend on the roof type, storage methods and standard of water required; these can be solar or wind driven.

Roof type plays a factor in rain water harvesting: Roofs made from Metal sheeting, Clay tiles, Concrete/cement tiles, shingles and glazed tiles are good for water run-off and are safe. Thatch is not very effective with water run-off. Any roof where lead, is used or that has been coated with paint containing lead, chromate, fungicides or toxins, and bitumen (tar) is not recommended for rain water harvesting unless an appropriate expert has been consulted. Asbestos is not recommended and should be removed by a qualified expert.

Rain water diverted to the garden helps to top up underground aquifers, as in our hard surfaced urban environments, rain water goes down drains and empties into the sea.

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