Water Sensitive Urban
Design

In the

Home Garden

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. Raised garden beds, water transpiration by plants and the maintenance of healthy soils is discussed in this report.

Ted Floyd

July 2003

Water Storage in Gardens

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.

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 microrganisms.

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 and some councils have Development Control Plans where the installation of rainwater tanks is mandatory in new houses. The NSW State Government has introduced new regulations to help encourage the use of rainwater tanks.

New technology is now available to store and use rainwater. Rainsaver Technology stores water at gutter level enabling rainwater to be used in the home for flushing toilets and washing clothes. Excess water is applied onto the garden and encouraged to infiltrate into the soil.

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.

Often when using below surface devices the importance of water use by plants is not considered. In many home gardens the residents wish to grow plants and often irrigate the plants with town potable water. This report will discuss the importance of gardens and the use of plants for the disposal of stormwater.

Raised Garden Beds

Water runoff from the roof of a house 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 the top of a raised garden bed.

The top of the raised gardens should be dish shaped to allow water to be held during rain periods, encouraging water to enter the soil. Overflow from the top of the raised gardens is allowed to flow down to a lower terrace where water is again encouraged to infiltrate into the soil.

Terraces should be made from a permeable sandy soil or gravel with little clay. Plants should be nurtured to help encourage rapid water infiltration into the soil. A dense plant canopy should spread over the ground surface to protect soils from beating raindrops. Maintenance of a thick leaf litter on the soil surface will greatly help to encourage water infiltration.

A portion of the water stored in a raised garden may percolate down to the water table. Transpiration of water back to the atmosphere by plants should be encouraged. Growing plants and rapid transpiration will dry out the soil enabling easy water absorption into the soil during the next storm. 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 evaporation to occur through the walls and to enable plants to be established in the walls. 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. Near the drainage area plants with a high transpiration rate should be planted.

A rubble drain may be constructed under the raised bed to direct drainage away from the bed. Drainage should be directed away from buildings and a small pond may be excavated to hold excess water. Water could be pumped from the pond back onto the raised garden. If the water table is shallow a well may be dug and water pumped from the well onto the raised garden or other parts of the garden.

Volume of Water Stored in Soils

Total water storage in soils is approximately 1/3 of the total volume of soil. Three cubic meters of soil is needed to store 1 cubic meter of water.

Water absorbed by soils drains to water table or is returned to the atmosphere by evapotranspiration. In Sydney, average yearly evaporation is 1800 mm and evaporation in June is 78mm and 239mm in Dec.

Transpiration

Native Trees

Deep rooting native trees may be planted close to the bed. Native trees with deep roots have a high transpiration rate, even during short dry spells. These trees are able to remove large volumes of water from the deep subsoil. In a small garden it is common for parts of the garden to be shaded. Tall plants grow upwards into the sunlight and the extra light increases transpiration. Tall plants can transpire faster if winds are present and remove excess humidity surrounding leaves. In small gardens trees may be unsuitable and climbing vines may be more suitable. Vines can be trained to climb up into the sunlight and can be trained to cover walls, sheds and other small buildings in the garden.

The larger the area of a tree canopy, the higher the transpiration rate. Transpiration continues to occur from leaves covering nonpermeable paving and buildings. Transpiration rate is greater in trees with a large canopy surface area and is not restricted by nonpermeable surfaces.

Water Transpiration by Plants

Growing plants absorb large amounts of water from soils, which travels up the roots and stems to the leaves. The water then evaporates through stomates, which are small pores in the leaf surface into the atmosphere. This process is called transpiration and the water travelling up the stems transports minerals from the soil to the leaves.

The volume of water flowing up the stems is far greater than water needed for growth and the excess water evaporates into the atmosphere.

The driving force for transpiration is the energy in sunlight and if conditions are ideal, plants can transpire a volume of water nearly equal to water evaporated from open water surfaces. Deep plant roots enhances transpiration. Many Australian native trees have deep roots reaching the groundwater enabling the tree to continue growing and transpiring water during dry spells.

In natural catchments near Sydney, 60% of rainfall is returned to the atmosphere by evapotranspiration from soils and runoff to rivers is only 15%. In high density urban catchments total stormwater runoff is up to 90% and the sum of evapotranspiration and seepage to groundwater is only 10%.

Evapotranspiration is the sum of direct evaporation from soils and transpiration from plants.

The potential to reduce stormwater runoff by increasing transpiration and encouraging plant growth is an untapped resource.

Precipitation/Evaporation P/E

In Sydney, yearly evaporation is greater than rainfall. Average annual rainfall at Observatory Hill is 1200 mm and evaporation is 1800 mm per year and the yearly P/E ratio is O.67. During the colder months, May June July, evaporation is low and P/E is greater than 1. In the hotter months evaporation exceeds rainfall and the P/E is lower than 1.

The above data demonstrates evaporation is highly significant in Sydney and has the potential to reduce stormwater runoff. The climate in many cities in the world is different and evaporation is lower especially in the Northern European and American cities of London and New York. The importance of encouraging plant growth and increasing transpiration as a stormwater management tool is probably more significant in Sydney and similar Australian cities than in many colder Northern cities.

In Sydney rainfall is higher in the first 6 months of the year and summer storms often result in flash flooding. In these summer months evaporation is high.

Water Absorption by Soils

Permeable Paving

Water infiltration into soils is encouraged if footpaths and driveways are layed with permeable paving. At the ground surface, moisture movement occurs both into and out of the soil. When a soil is moist, evaporation occurs from the surface into the atmosphere. Many permeable pavements will allow water movement into the soil and greatly restrict evaporation of moisture from the soil. When designing a garden to maximise the loss of stormwater, careful consideration should be given in designing pavements and selecting paving materials.

Leaf Litter

Leaves falling onto the soil surface build up a leaf litter. Leaf litter is a very dynamic eco-system with many small animals and microbes. The leaf litter and soil is natures recycling centre where small animals and microbes eat fallen plant material releasing nutrients into a soluble form which is available to growing plants.

A healthy, dynamic leaf litter also improves the physical fertility of a soil. Small soil animals and microbes create good soil structure improving soil aeration and water infiltration. Burrowing animals, especially earthworms greatly improve water infiltration. Leaf litter protects surface soil from beating raindrops and erosion. A deep leaf litter is natural mulch and is food for earthworms, every gardeners friend.

Improving Water Infiltration, In the Home Garden

Water infiltration into garden soils can be improved. Good management of garden soils can greatly increase the amount of water soils absorb. Proper management is also necessary to prevent soil loss by erosion.

 

Ways to improve water infiltration * Increase plant cover to protect soil surfaces * Encourage plants that have deep roots * Improve soil structure by adding organic matter * Add mulches, green manures, and compost * Encourage earthworms and micro-organisms * Add soil improvers such as gypsum when needed * Maintain rough soil surface * Protect the soil surface from the impact of raindrops * Reduce soil compaction * Improve sub-surface drainage * Use organic fertilisers and animal manures * Encourage build up of leaf litter

Pollution Attacked by Microbes

Healthy soils with dynamic microbial ecosystems are able to break down many organic pollutants at a faster rate than in sterile mediums. Organic pollutants may be broken down by chemical or microbial actions. Soils are an ideal medium for microbes to grow in. Bacteria prefer to grow in the thin layer of moisture surrounding clay particles and fungi grow better in large soil pores.

Microbes need a continual supply of organic food to grow and growing plants provide leaf litter to the soil surface. Different microbes eat different plant materials. Bacteria generally prefer smaller and more soluble molecules and fungi feed on tougher woody material containing cellulose and lignin.

A gram of healthy soil contains up to 3,000 million bacteria and 500,000 fungi plus actinomycetes, algae and protozoa.

When microbes are encouraged to grow, many organic pollutants will be broken down at a faster rate. Microbes generally grow faster in fertile soils similar to the ideal conditions for plant growth. The soil needs to have adequate moisture, aeration and good drainage. Most microbes need similar inorganic nutrients to plants. Nearly all microbes differ from plants and cannot manufacture organic material by photosynthesis and need a supply of organic matter to grow on.

Stormwater, Useless or a Useful Resource?

Stormwater is treated as a major disposal problem. Water is essential for all life and water runoff should be highly prized and not drained away as quickly as possible, as if it were a nasty infectious disease.

In home gardens water should be encouraged to infiltrate into soils and plants nurtured to grow rapidly and absorb water from the soil.

Growing plants consume a large amount of water and will reduce stormwater runoff. The irrigation of plants by townwater is reduced if rainwater is absorbed by soils and transpired by growing plants. Rainwater consumed in the garden encourages the beneficial use of water and reduces nuisance stormwater runoff. This is very simple and is simply ignored in most drainage schemes.

Further reading on 'mimicking nature'

Australia – restoring wetlands, Friends of the Earth International, Amsterdam. Link Issue 102, January 2003.

Urban Catchments Enhanced by Green Corridors, Stormwater Industries Association, Bulletin No 108, Dec. 2002.

Transpiration Benefits for Urban Catchment Management, Stormwater Industries Association, Bulletin No 92, Aug. 2001.

Whites Creek Wetlands – Background Studies, Series of Reports, Friends of the Earth Sydney, 2003.

Soil Water Infiltration, Stormwater Industries Association, Annual Conference, Sydney, Sept. 1996.

Water is life – Whites Creek Catchment, A series of three leaflets describing how to restore urban waterways by maintaining healthy soils. Friends of the Earth Sydney, 1996.