Rain is best for irrigation so use every drop of this free precious resource that falls on your property
South Africa’s combined average rainfall is 464mm a year. Cape Town’s average is set at 820mm a year, although the region has received well below this amount over the past few rainy seasons.
The South African Weather Service says readings (at the weather station at Cape Town International Airport) show under 40% of normal rainfall for January through to April has been received, but there is time to catch up before the end of the month.
Read: The drought busters
Landscaper Marijke Honig says Cape Town receives two to three times more rainwater than its annual requirement.
“Why are we not using this and where does it go?” she asks. “It runs off roads, paved areas and roofs and literally goes down the drain into storm water pipes and out to sea.”
Honig says capturing water run-off and allowing it to seep into the ground is of great value. “The soil is in fact our largest and least expensive ‘storage tank’, one that provides moisture to plants for weeks and even months after it has rained,” she says.
Rainwater harvesting can be done with simple “earth shaping” – the construction of swales, berms, depressions and other landforms that capture and absorb the rain – instead of letting it run away.
While golf courses are known for their berms, swales and extensive irrigation systems that direct water run-off to storage dams, earth-shaping principles can be used anywhere – in public landscapes, gardens, schools, street planting, parks and farms.
“Earth shaping reduces erosion and is a convenient passive form of irrigation that requires no tanks, mechanical pumps, valves or irrigation systems,” adds Honig. “It functions whenever rain falls and water flows and is especially useful on sloped terrain, heavy soil and ‘oily’ or hydrophobic soils. Earthworks take some effort to install but require little maintenance.”
Honig quotes from rainwater-capture guru Brad Lancaster’s book, Rainwater Harvesting for Drylands and Beyond. “Lancaster’s advice is to start with long and thoughtful observation, and plan a small and simple intervention in the highest point of the property,” says Honig. “The purpose is to slow down and spread the flow of water so it can seep into the soil.”
Berms, swales and rain gardens
Simply put, berms are raised mounds, and swales are depressions. Both are practical and important for water filtration, but they can also be used aesthetically.
Berms can be used to disguise an unwanted view or even create a garden focal point. By choosing the right landscaping material and plants, berms and swales blend into the surrounds and become part of your garden’s overall design.
“Think of the berm as a miniature dam wall that makes an area where water seeps into the ground or collects in a ‘pond’ during a downpour,” says Honig “It is therefore important that berms are designed with an overflow, often lined with stones, so that water can run over the edge without causing erosion.”
Rain gardens are shallow depressions that collect and hold rainwater (usually run-off from downpipes), allowing it to soak gently into the soil over a 24-hour period.
Rain gardens should not be confused with ponds or bog gardens. Their main purpose is to allow water to percolate slowly into ground. Plants for the rain garden must be able to tolerate extended dry periods and cope with additional soaking during the rainy season.
Replace solid paving with permeable paving to allow water to soak into the ground, rather than run off your property into a stormwater drain.
Consider your driveway. A brick driveway can be slightly raised in the centre so water flows to the outer edges. Or consider a concrete or brick berm on one side of the driveway to direct water to the other and into a flower bed or lawn alongside.
No garden should be without at least one rainwater tank. With the myriad shapes and styles available, you can find one to suit your home and garden. Consider how much water you can harvest and how many tanks you will need. Just 5mm of rainwater collected on a 100m² roof will net about 500 litres of water.
“It is important to ensure the water is clean before entering the tank,” says Honig. “If it contains any organic matter, the water will go ‘off’ and smell. Sometimes a simple leaf trap is enough – it depends on the roof type, pitch and how many trees are around. In some cases, pre-filtration is required.”
Consider how to use your harvested rainwater. Honig says it isn’t practical to use rainwater for an irrigation system as this will empty your tank in a few cycles. It goes much further when used for hand-watering.
“In winter-rainfall areas, it’s more efficient to use untreated rainwater for toilet flushing and laundry, as this means you benefit all winter and your tanks are emptying and refilling whenever it rains.”