Efforts to collect and utilize rain water for irrigation are on with full gusto all over the world. Loss of this precious natural resource, rain water, because of surface run off during rain storms is taking a toll on water management systems catering for a growing world population. Only 2.3% of the available water on earth is able to be used as fresh water. To prevent more fresh water flowing into the sea where it becomes saline, there are many dedicated manufacturers all over the world who are designing rain water storage systems and marketing them to consumers desperate for fresh water. It is a relatively simple process to engineer basic storage of precipitation when it occurs and to reroute stored water to the desired area of need. In order to collect rain water for irrigation the system installed is the same, on a small or large scale, as per the requirement

In rural areas in Australia, a common sight used to be the old windmill drawing water from a bore or sub-surface aquifer, but this water is often heavily mineralized and may not be suitable for all purposes. Most farms have at least one dam, where a hole has been excavated with sloping sides so that surface rainwater flows into the dam.

Kenya Station Windmill, courtesy National Museum of Australia.

For small scale irrigation, tanks provide useful water storage which may be designed to be above ground or underground. Manufacturers of these tanks which may be made from concrete, polyurethane or steel, take care to ensure that the materials used suit the purpose and that there is no scope for contamination that could affect the quality of crops or harm people or animals. Tanks used to collect and store rain water for irrigation should be fit for sourcing water for fruit or vegetable gardening and other food crops. Check with your manufacturer or supplier. Tannin filters can be used if the collected rainwater becomes contaminated by trees containing tannins, such as eucalypts, which could affect plant growth.

A Kingspan Steel slimline tank suitable for irrigating the home garden.

The ability to collect rain water for irrigation has enabled many a small and medium scale irrigation project such as a plant nursery to source water during dry summer months. The collection systems are available in varying sizes that range from a few hundred gallons or litres up to 375,000 litres, in the case of very large steel panel tanks. The rain water saved may enable an owner to store sufficient water for an entire year’s gardening.

Irrigation can be gravity-fed (if the tank is situated higher than the garden or there is sufficient water in the tank). Some tank owners use a smaller header tank installed on the rooftop or elevated rain collection surface and connected to a larger storage tank which may be located on the same level as the garden. This arrangement provides sufficient pressure to push the water into the irrigation pipelines. Drip-feeding is an even simpler process where the tank outlet is connected to a main hose line and the water simply runs out into the driplines and waters the garden at the speed the gardener desires.

In those two cases, a pump is not necessary, unless the garden is located at a higher level than the tank.

If more rapid irrigation is required, or the water needs to be delivered faster, or if the garden is situated uphill from the tank location, a suitable pressure pump will be necessary.

The system designed to collect rain water for irrigation incorporates internally fitted filters such as the leaf/mosquito screen. Gutter leaf screens, tanning filters and/or leaf-shedding rainheads can be fitted to the downpipes or sourcing devices to avoid contamination and to keep the pump running smoothly. Check the inlet screen and overflow outlets regularly. Tank outlets have integrated  fittings for simple connection to a hose and discharge. Use your tank water on a regular basis, even if you believe the garden has received sufficient rainfall. This keep the system flushed and prevents stagnation of the tank water. Estimate how much rain water you are likely to need for irrigation and approximately how much water you can potentially collect and then choose the system which best suits your needs.

The collection of rain water for large scale irrigation is an important consideration that many government authorities all over the world are addressing for triple and multiple cropping in one agricultural year. For small scale irrigation such as the home garden you will find your Local Council or Permaculture group to be a good source of information and advice.

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