Creekcare

Transpiration by Trees

Introduction

Water is absorbed by plant roots from soils and flows up stems to the leaves. Transpiration occurs when water vapour flows from leaves into the atmoshere.

Water absorbed from the soil, flowing up stems, carries nutrients essential for plant growth. The sun is the driving force for transpiration.

Transpiration is an important part of the water cycle. A large proportion of rain falling on land is transpired by plants back into the atmosphere.

In natural bushland surrounding Sydney, most rainfall enters the soil and 60% of the total rainfall is returned into the atmosphere by plant transpiration. In the suburbs there are not as many trees and only 10% of rainfall is transpired into the air and most rain lands on streets and houses then flows swiftly down gutters.

In Sydney a large gum tree transpires up to 200 litres of water a day. A well maintained garden in Sydney will transpire nearly twice the volume of water as the total rainfall.

Transpiration and water storage in garden soils

Plants in gardens will transpire up to 2,000 litres of water from one square metre every year (equal to 2,000 mm rain). The total average rainfall in Sydney is 1200 mm/year.

Home gardens planted with native species can be grown with a minimum amount of fertilizers and watering. See Native Plant Species List for the Inner West of Sydney

In many gardens vigorous plant growth is achieved by watering with valuable tap water. In times of drought this is an expensive and silly habit.

Water runoff from a house roof can be absorbed by a garden soil in an area equal in area to the house and transpired by plants back into the atmosphere.

Water runoff from of a house roof can be absorbed by a garden soil and transpired by plants back into the atmosphere.

The big trick is to be able to store water in the garden after heavy rain and to encourage all water to enter soils rapidly. A dry garden soil can store water equal to one third the total soil volume. A storm of 100 mm can be stored in 300 mm of soil. All water in heavy storms may not enter the soil in the short time period of a storm.

When suburbs are built many trees are cut down. Porous soils are covered by roads, paving and buildings. Rain falling on an impermeable surface immediately flows over the land into drains.

Surface runoff and flash flooding is increased when trees are cut down and soils covered by impermeable surfaces.

In Sydney a large gum tree transpires nearly 200 litres of water a day.

Native Australian trees have the advantage of having deep roots and a degree of drought resistance. Often the roots of gum trees reach down to the water table and are able to absorb water for plant growth when the surface soil is dry. Gum tree roots are often over 10 metres deep while the annual lawn grass Poa may only be 0.15 m deep. Poa lawns need to be regularly watered in the summer while gum trees surive for long periods between rain.

Gardeners should aim to harvest all the rain falling on the home block. Deep porous soils are needed to store large volumes of water and vigorous plant growth to transpire water into the atmosphere. A garden equal in area to the house is needed to achieve this plan. Water tanks will help to store water and tankwater can be used in toilets and for other suitable household needs.

All rainwater harvested on the home block reduces flooding and water pollution which is a serious problem in flood waters.