Rainwater Harvesting in the Tropics

Rainwater Harvesting in the Tropical north

In north Australia where rainfall is mostly very seasonal, with relatively well defined wet and then drier periods of the year it is possible to collect significant amounts of rainfall – rainfall harvesting. The hardest issue to contend with is the need to store relatively larger percentages due to the seasonally dry conditions.
This is particularly so in the north west of Australia – Darwin and areas west around the Kimberley coast where 4 – 8 months may have zero rain, and rainfall declines as you move away from the coastal areas.
However..............look at the data. Columns 2 and 3 show the amount potentially falling on the roof, in Kilolitres [KL] under a range of roof areas , for two different annual rainfall amounts

Roof area [sq metres] Rainfall – 1000mm/yr Rainfall - 1500mm /yr
200 200KL 300KL
250 250KL 375KL
400 400KL 600KL

If you assume a capture rate of 80%, the amounts potentially available are shown in the tables below. A capture rate of 80% could be considered in the low range area, as in the tropics much of the rain falls in significant storms, and in this situation a smaller proportion is wasted in first flush diversion or similar systems that divert the first smaller volumes of rain to ensure clean water is captured and goes into storage.

Roof area – sq m 80% capture of 1000mm/yr -KL 80% capture of 1500mm/yr-KL
200 160 240
250 200 300
400 320 480

The roof areas selected have been used as examples of small, medium and medium/large areas under roof with gutters and collection systems, and include the additive potential from a house roof, garage and sheds. They apply equally to urban and semi-rural locations.

For straight “in house” domestic use, with sensible management, a family of four could use less than 100KL. And there are a lot of options and management ideas to be considered.

For use outside the household areas, a modest lawn and garden could be developed with usage in the 100 – 200KL/year, especially if sensible grey water and effluent use was practised, using systems that allow subsurface disposal of these products.

One option is the use of KISSS subsurface irrigation systems [see www.iwtech.com.au] along with a range of approved alternative wastewater treatment systems – that is, a system somewhat superior to the older style septic tank, which treats effluent at least to secondary standard or better.

There are a wide range of these, and in Australia each State or local government region has approval systems for various brands and types, so you need to check what is legally available and approved.

The really tricky part is to work out how much you need stored around the end of the rainy season.

Some sort of monthly water balance is required, to calculate input and usage, but as a very broad guide, somewhere between 100 and 150KL would be needed at around the end of the rainy season.

More storage allows more extensive outside greenery development, naturally. Also remember that this stored water is a valuable resource for any fire fighting, and bushfires can be a threat in the drier months.

Tank Management

A few short comments.......
· Many users of tank water do not worry about water treatment, but there are some simple treatments if needed. Use in line or in tank UV treatment – this seems to be now seen as the most efficient way to kill any nasties in the water
· Clean water in – means clean water in the tank, so keep gutters clean and use pre treatments such as first flush diversion, leaf guards and similar systems.
· Keep mosquito larvae out of the tank.......screening has usually been seen as a simple solution, and of course this also keeps some particles out of the tank. The screen must be cleaned regularly to allow easy flow of the water into the tank, and two stage mesh systems do work well. In parts of Asia some areas use copepods [small animals that eat mosquito larvae] in tanks to prevent development of larvae.

Mosquito and mosquito larvae management is vital in those areas known to have the dengue mosquito present.

This is not a complete guide to using rainwater and tanks, but to show it is very definitely possible in this region, based on rainfall, and usage patterns and known areas of roof cover.

Even if there is no need to provide all of a households water needs, there are opportunities to collect and store rainwater for outside use – filling swimming pools, washing cars, garden watering to reduce using expensive treated potable water.

If planning a new house..........a suitable place for a tank may be under the driveway or under the lawn areas, rather than an above ground object in the yard! Many designs allow for traffic across the top of the tank.

It is not always difficult and can be an option where groundwater supplies are poor, or there are other issues around groundwater usage.