Harvest rainwater on your land
The fact that water harvesting must be site-specific could either make the exercise quite simple or very complex. Sloping sites, especially, can lose rainwater to run-off all too easily. Rainwater can be harvested best on such sites in the following ways: • Terraced areas A series of flat or slightly depressed terraced areas – supported by gabions, interlocking retaining wall blocks, rockeries or slopes planted with shrubs and groundcovers with roots that retain the soil – is a good solution. Put down erosion-control blanketing or soilsaving biodegradable mesh before planting to hold the soil in place while the plants become established. • Berms and swales Often used next to each other, berms (raised areas) and swales (shallow depressions) are highly effective in slowing and capturing runoff. They hold rainwater and allow it to filter into the soil slowly.
In large gardens with good soil drainage, long swales can be planted with trees or fruit trees. Alternatively, if constructed at an angle to run-off flow and planted with lawn or made to look like dry streams, they can be used to direct heavy run-off elsewhere and at the same time slow the flow of water. The same principles are used in furrows and raised soil circles around the drip lines of trees and shrubs.
Another example is making a lawned area slightly saucer-shaped. Such depressions can be shallow, but the larger the area they encompass, the more water will be held. Design them to hold enough water to penetrate to the roots of trees and shrubs.
A 20mm rainfall will penetrate the soil to a depth of about 30cm. This is sufficient for the roots of groundcover plants, so a shallow depression of 2cm is sufficient. Shrubs and tree roots grow down to about 60cm and 100cm, respectively. Depressions under or in the vicinity of these plants would need to be a minimum of 5cm. Berms tend to erode down, so consider making them higher – 15cm – so that you don’t have to rebuild them as often. • French drain If heavy run-off occurs at the lowest section of a property, build a long French drain without piping 2-5m from the boundary (depending