Rainfall in Australia is erratic with floods and droughts. In the early 2000s large areas of Australia suffered from very severe droughts and water restrictions were introduced in Sydney. Restrictions on how and when home gardens could be watered were applied and the price of water was increased. Water authorities have predicted some water restrictions will continue indefinitely.
Rainwater falling on the home block can be utilised in the garden by installing water sensitive urban design technology to reduce runoff into stormwater drains. Home gardeners can mimic nature by encouraging soils to absorb and store water then carefully nurture healthy growing plants to transpire water back into the atmosphere.
Excess rainwater flows from house blocks into stormwater drains causing flash flooding. Stormwater pipes and drains are often constructed from concrete and are biologically dead. Pollution accumulates in concrete drains and is easily carried down the drain into natural rivers and harbours.
A major problem in urban creeks is flash floods. Runoff from roads contain many oil based pollutants. Floodwater seeps into sewage pipes causing sewage overflows into drains and creeks. Collection and storeage of rainwater for irrigation of gardens reduces flooding and pollution.
Home gardens are often watered using the local town water supply. Town water is valuable and is needed for drinking and cooking and other household purposes.
Water is stored in large dams and potable water required for human use is treated to kill bacteria and other harmful micro-organisms.
Watering home gardens with potable town water is a waste of a valuable resource and magnifies environmental problems caused by large water storage dams.
Water collected from rooftops can be used for household purposes and to water the garden. Rainwater tanks are very common in country areas and in many households rainwater is the primary source of water in the home. In Sydney, rainwater tanks were, until recently, illegal. Many councils now encourage the use of rainwater tanks. The NSW State Government has introduced new regulations to help encourage the use of rainwater tanks. Rainwater can be disposed into rubble drains, leaky wells, soakways, trenches and other below surface devices. There are several products used to help construct below surface storage devices and drains including drainage cells and geotextiles. It is expected water in below surface drainage devices will percolate down to the watertable.
There are many ways water can be conserved in the urban home garden. Every garden is different and suitable ways for conserving water will differ for individual gardens.
Stormwater collected in council drains is a problem. During heavy rainfall flooding may occur. Many drains are polluted and especially runoff from roads and car parks is heavily polluted. In Sydney leaking sewage pipes and sewage overflows during heavy rainfall are a major source of pollution. Often stormwater drains are polluted with a wide range of toxic chemicals by illegal dumping.
The collection and storage of rainwater in the home garden will reduce runoff into stormwater drains and help to reduce downstream flooding. Water harvesting creates a useful resource and reduces a nuisance.
Gardens can be designed with low water needs and every drop of rain should be captured and encouraged to soak into the soil for use during dry times.
Leaf litter on the surface of soils is natures perfect mulch.
Mulches are a layer of loose plant material or other residue spread on the soil surface to help conserve moisture. Common materials used for mulching are wood or bark chips, straw, animal manures and compost. Garden waste if chopped into small pieces makes a good mulch. Artificial materials are sometimes used as a mulch. A layer of black plastic, paper or small stones will stop weed growth and conserve moisture.
Mulches made from plant material will be eaten and broken down by soil animals and micro-organisms and incorporated into the soil humus. The speed of breakdown varies greatly depending on the mulch. Wood chips breakdown slowly and remain on the surface for a long time. Straw from horse stables will breakdown more quickly if it contains animal manure. Many mulches, especially mulches containing animal manures will add plant nutrients to a soil when they are broken down and buried in the soil. It is important not to introduce weed seeds or diseases to the garden from mulching.
Mulches shade soils from sunrays reducing evaporation from soil surface. Evaporation is also reduced by the insulation properties of mulches. Suppression of weed growth reduces moisture loss by transpiration.
Water infiltration is improved when soil is protected from raindrop impact reducing puddling and surface crust formation. The addition of organic matter, healthy growth of soil organisms and development of improved soil structure will increase infiltration. Compaction of surface soils is decreased by mulches.
Under a mulch, soil temperatures are warmer in winter and cooler in summer. Warmer temperatures stimulate plant growth and seedling germination. The colour of a mulch can vary the effect of mulches. Dark mulches absorb heat while light coloured mulches will reflect sunrays. Frost damage can be reduced by mulches.
Earthworms are encouraged by mulches. Organic mulches provide food for earthworms and improved moisture and temperature conditions are favourable for earthworms.
Evaporation from salty soils draws salts up from the subsoil depositing them on the surface. Mulches reduce surface evaporation and reduce surface salt accumulation.
Plant roots often will grow profusely under a mulch.
Selection of suitable plant species helps to reduce the amount of irrigation water needed. Many native plant species are more drought tolerant than most exotic plants. Plants with deep roots are often drought tolerant. Many grasses used in lawns have shallow roots and need constant watering to produce a desirable lush lawn in the hot summer months.
Landscape planning and design can reduce the severity of droughts by incorporating water harvesting in home gardens. Many urban gardens have drainage systems collecting runoff from rain and then disposing of the rainwater into the councils stormwater drains. The rainwater should be either encouraged to soak into the garden soil or stored for later use. Water drained away is a big waste and stormwater is a nuisance adding to flood and pollution problems.
Water harvesting is not new and was used by ancient civilizations in the Middle East over 4,000 years ago.
Water tanks are becoming very popular and Government authorities are introducing schemes to encourage the use of tanks. One thousand liters of water (KL) is collected from 100 square meters of roof when 10 mm of rain falls. Sydney has an average annual rainfall of 1200 mm and it is possible to collect a little over 1 KL of water from every square meter of roof area in one year. In practice the volume collected will be less because in heavy storms water tanks fill up and overflow.
Water from garden tanks can be used in the home to reduce reliance on town water supplies. Toilets do not require highly purified potable water and can be connected to water tanks.
In early Sydney, before town water was piped to every home, many houses possessed a well. Water from the house roof was collected and stored in a well. The well was the primary source of water for household needs. If a well is spring fed, ground water will flow into the well after water is pumped out and the depth of water will remain at the depth of the water table. The reintroduction of water storage in wells could be an attractive method of conservation in some home gardens.
A well can be dug in a garden and then covered over so as to be invisible and not take up garden space. Large water tanks can be unsightly and fill up garden space. Runoff from paved areas and excess stormwater flow from gardens can be directed to flow into a well. Subsurface drains can be directed into a well. A pump will be needed to pump water from the well onto the garden.
Infiltration of rainwater into soils is decreased when there are large areas of impermeable surfaces. Concrete paths, paving and driveways seal the surface preventing rain entering soils. Permeable pavers will help to increase water infiltration. Concrete paths and paved surfaces should be reduced to the minimum area needed. Paved surfaces should have a gentle slope so runoff flows onto gardens or lawns.
Water runoff from rooftops may be stored in raised garden beds. Terraces built up with highly permeable sand and gravel can store water while it is raining and the water is then available to growing plants during dry periods. Rainwater from the roof of the house, or the overflow from a water tank is led onto a raised garden bed.
Terraces should be made from a permeable sandy soil or gravel with little clay. Plants should be nurtured to help encourage rapid water infiltration deep into the soil. Maintenance of a thick surface leaf litter or mulch will greatly help to encourage water infiltration.
A soil can store a volume of water equal to about one third of its total volume.
The terrace walls can be constructed from common materials used in gardens. Local rock, poles, logs, sleepers and other suitable forms of timber are useful materials for constructing walls in gardens. The terrace walls should be permeable to enable seepage to occur through the walls and to enable plants to grow. The garden terraces should be designed to enhance the landscape architecture of the garden. The position and shape should fit into the design of the whole garden.
Terraces built along fence lines will help solve privacy issues.
In small enclosed gardens, sunlight may be blocked by fences and neighbouring buildings. Terraces may help in improving sunlight access to plants growing on raised gardens.
A rubble or agricultural drain installed under the raised bed will direct water away to a low point in the garden. Drainage should be directed away from buildings. Drains may lead to a garden pond or well.
A home garden should have a contoured layout to create a small water catchment. All rainwater falling onto the home block should be captured with no drainage to councils stormwater system and overland flow onto neighbouring properties should not be allowed. Rainwater should be encouraged to infiltrate into garden soils and excess stormwater stored in watertanks or wells.
Gutters are not always necessary on roofs. Where applicable roof water can be allowed to flow directly onto a suitably designed garden. This reduces the cost of gutters, downpipes, cleaning and maintenance. Water is distributed more evenly onto the garden when there is no gutter and downpipe. Rainwater from downpipes should be directed away from buildings onto pervious gardens where water easily soaks into the soil. Stone filled pits and trenches encourage water to flow away from buildings and to infiltrate into soils.
Rainfall and runoff can be collected from impervious surfaces, including rooftops, paved areas and driveways and directed to garden areas where water is readily soaked up by the soil. The ground surface and paved surface should slope away from the house and buildings. Paved areas should have a positive slope towards a permeable garden soil.
A low point in the garden is selected where all water will drain to. If necessary a levee bank may be needed along neighbouring boundaries to prevent surface water flow onto the neighbours property. The low point needs to be several meters away from the house to prevent the build up of moisture under the house. The low point acts as a water storage basin while it is raining and as a on-site recharge basin. In the low point a garden pond would be ideal for capturing excess water. The low point needs to be large enough to store all excess water from the heaviest predicted storm and water should soak into the soil within a day after rain.
During heavy rain it will be expected parts of the garden or lawn will flood. Garden layout should ensure flooding occurs where it is not a nuisance and away from the house. The soil in low points should contain little clay and be highly permeable. Plants requiring a high amount of water can be grown in the low points. Water should be rapidly absorbed by the soil and not lie on the surface for more than 24hrs after rain.
In a contour garden there will be high spots and low spots with wet areas and dry areas. Plant selection should be appropriate for the wet and dry areas. Native plants, resistant to droughts may be more appropriate on the high spots and exotic plants with high water needs may survive better in the wetter low spots. Contour gardens will have sunny warm spots and shady cool areas. A contour garden creates a variety of microclimates suitable for a range of plants.
Depth of plant roots should be considered in garden layout. Many annual plants only have shallow roots and need frequent watering. Perennial plants often have deep roots and grow better when irrigated with long, deep penetrating waterings, less frequently. Native trees can have very deep roots and may penetrate twenty or more meters down to the water table.
The garden layout and drainage lines should encourage water flow into the low point. Subsurface drains laid in trenches will remove excess water from garden soils.
Raised garden beds can be constructed on the borders of the garden with drains leading to the low point.
A water harvesting scheme for a home garden could initially capture rooftop runoff in a water tank overflowing onto a raised garden bed. The contour layout of the garden directs water to a low point with a fish or frog pond and excess water drained into a well. In dry times water from the well can be pumped into the water tank and used for watering the garden.
|Lucerne||6 or more meters|
|Wheat, Oates, Barley||3.3|
|Maize||1 to 2|
|Potatoes||0.6 to 2|
|Lettuce||0.1 to 0.2|
|Fruit trees||0.6 to 1.3|
|Grape vines||0.5 to 1|
|Gum trees||20 to 40 meters|
Many back yards are flat with green areas of lawn surrounded by flower or vegetable gardens. Often in a landscape design the land is leveled and any natural slopes are eliminated. Most of Sydney is hilly and where the parent rock is Hawkesbury Sandstone very steep hills occur with rocky outcrops. The local native plants thrive in an ecosystem with steep slopes and rocks.
In a droughtproof garden with local plant species advantages will be obtained if the garden is not entirely flat and bush rocks will encourage native plants and animals. Raised garden beds, rock piles and sunken gardens will help create a garden with ecosystems imitating local natural environments.
A creative contoured landscape should be more stimulating for children to play in. For older members of the family a contoured garden could be designed as an exercise yard to help lose weight and stay healthy.
Plants with deep roots can tap a larger reservoir of water and are more drought resistant. Plants with a shallow root system can only absorb water from the surface soil and are more susceptible to droughts.
When irrigating, enough water should only be applied to wet the soil to a depth slightly deeper than the root zone of the plants. Excess water will drain to a depth below the roots and will not be available to plants.
Roots do not penetrate into dry soils. When watering plants sufficient water should be applied so the water wets the soil to a depth a little greater than the root depth. Plants should be encouraged to grow deep roots and to become drought proof.
Poa and Kentucky bluegrass are annual lawn grasses with very shallow roots and need to be watered at frequent intervals and are not drought resistant. Lucerne is a pasture legume with very deep roots and is drought tolerant. Gum trees often have very deep taproots over 20 meters long, reaching the water table.