TRANSPIRATION INTRODUCTION

 

 

TRANSPIRATION BY TREES

Posted on March 9, 2011 by tedfloyd

 

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.

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.
How Transpiration Works
Transpiration is the flow of water vapour from leaves into the atmosphere.

The driving force of transpiration is radiation from the sun heating the interior of leaves. Evaporation occurs at the surface of cells and water vapour flows through stomates into the drier atmosphere.

The loss of water at the leaf surface creates a suction pulling water up through the xylem tubes in the stem and roots and water is sucked out of the soil through root hairs Water movement by transpiration pull is assisted by osmotic pressure and capillary rise.

 The major factors affecting the rate of transpiration is the strength of solar radiation and the presence of available soil water. Transpiration is greatest in the middle of the day when maximum solar radiation occurs and nearly ceases during the middle of the night.

The rate of transpiration is increased by winds, low relative humid y and low atmospheric pressure.

Photosynthesis requires, carbon dioxide to enter through open stomates and oxygen to flow out. Water vapour also flows out through open stomates into the atmosphere. Stomates are small breathing pores mainly found in the lower leaf surface. Plants regulate water loss through transpiration by opening and closing stomates.

Normally stomates are open in the day and closed in night. Cacti and some desert plants conserve water by closing stomates in day and opening them in the night.

In different plant species the number, size and location of stomates helps to control transpiration rate. Often plants close their stomates during high temperatures and water shortages.

In the winter transpiration is slow and when deciduous trees lose their leaves nearly stops.

Higher leaf area in plants increases transpiration and at the top of a canopy, transpiration is greater than close to ground level. An open canopy facil ates transpiration compared to a dense, closed canopy. A tree standing alone has a higher transpiration rate than a tree in a forest surrounded by many tall trees.

There needs to be sufficient available soil water for transpiration. When a water shortage occurs, leaves will droop and wilting occurs.

An evaporimeter measures evaporation directly from a water surface. In Sydney the annual evaporation is 1800mm and is higher than the rainfall of 1200. The ratio of P/E (precip ation/evaporation) indicates when soil water is available for plant growth. In Sydney the annual P/E is 0.67 and in January is 0.33.

Normally transpiration can not exceed evaporation and under ideal cond ions transpiration may reach 95% of evaporation. In the middle of winter transpiration from deciduous trees when they have lost their leaves is less than 15% of evaporation.

Evapotranspiration is the sum of evaporation from the soil surface plus transpiration from plants.

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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.

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.

 

 

 

About tedfloyd

born 18th Aug 1946 at summer hill, sydney australia school north junee primary yanco agricultural high school leaving certificate 1963 sydney university facultary of agriculture major soil science 1964 to 1968 BSc Agr web. http://www.ramin.com.au/creekcare

 

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COOL TREES

COOL TREES AND TRANSPIRATION

COOL TREES

TRANSPIRATION

When water vapour is transpired from leaves into the atmosphere, the plant and surrounding air is cooled down.

When sitting in the shade of trees the air temperature can be 5 degrees centigrade cooler than in the sun. The shade of the tree will cool you down and transpiration cooling will lower the air temperature even further.

During transpiration liquid water is absorbed by plant roots, flows up xylem tubes in stems and into leaves. In the cells surrounding the substomata chamber, water evaporates from the cell wall into the chamber. When the stomata pores are open water vapour diffuses out into the atmosphere. Transpiration Cooling Energy is needed to evaporate liquid water into water vapour. Energy is absorbed into liquid water and this reduces the temperature of the surrounding plant tissue and nearby atmosphere. To evaporate 1 gram of water 590 calories of energy is required.

A large gum tree growing during the summer in Sydney will transpire up to 200 liters of water a day. The energy required to evaporate 200 liters is 118,000 cal. The energy used to evaporate 200 liters will help to cool the tree and the surrounding air.

Energy absorbed at the surface of the earth from incoming radiation has to be removed from the earths surface to maintain an energy balance. Approximately 25% of incoming radiation energy is absorbed by evapotranspiration (transpiration + evaporation).

Heat Island The air temperature in cities is raised compared to natural forests. The heat island in cities is caused by several factors and reduced transpiration by vegetation is an important part of the heat island. Heat absorbed by buildings heat the surrounding air. Heat absorbed by plants is partially used to evaporate water during transpiration.

In cities the volume of water transpired by trees and vegetation is reduced compared to natural forests. Large areas of forest are cut down and replaced by buildings, paving and roads. Soft surfaces have permeable soils and are covered by vegetation and impermeable roads and buildings are hard surfaces.

Increasing Transpiration Cooling When environmental conditions are right transpiration cooling can be increased. Hot summer temperatures can be decreased and cool spaces produced in small spaces surrounding a house or along the banks of an urban creek. Transpiration cooling is encouraged by nurturing the growth of vegetation.

Transpiration is increased when the leaf area index is increased. The leaf area index is the measurement of the total area of all leaves. The LAI (leaf area index ) of a forest can be 6 or more and at ground level it is dark. (a LAI of 6 means the total leaf area is 6x the ground surface area).

Transpiration cooling only occurs when there is ample soil moisture. During droughts and dry periods there is no soil moisture available for transpiration to occur. In garden soils moisture can be raised by increasing water infiltration.

The water holding capacity of soils can be improved to increase the store of water available for transpiration. The depth plant roots penetrate into soils improves the ability of plants to absorb water from soils. Deep roots increase the volume of water available for transpiration.

Stomata Stomata are found mainly on leaf lower surface. Transpiration occurs when stomata pore is open. Two guard cells surrounding stomata pore control opening and closing of pore. Stomata close at night and during a water shortage. Water vapour pressure is high in substomata chamber facilitating diffusion of water vapour out through open pore into atmosphere.

Evaporation of water in substomata chamber needs 590 cal/g water. This provides the cooling effect of transpiration. In cities the growth of artificial bush can be increased by irrigation using stormwater runoff.

Irrigation will increase the volume of transpiration and will partially cancel the negative effect of increased hard surfaces in urban areas. Irrigation increases plant growth in a smaller area of soft surfaces available in cities. Before the drought during the 1990s parks and gardens were regularly irrigated with town water. It is now considered town water is very valuable used for drinking and should not be extensively used to irrigate parks and gardens.

Using stormwater for irrigation also helps to manage water pollution and flooding problems. Stormwater should not be considered a problem and should be used for useful purposes.

Creek Corridors In urban areas islands of bushland can be connected by green corridors to facilitate the movement of native animals in hostile built up suburbs. Excellent corridors established in creek valleys can be irrigated by stormwater, increasing the volume of transpiration and producing cool spaces. Irrigated creek corridors will become cool spaces valued by city dwellers and give comfort to native animals on hot summer days.

Water Infiltration into Soils When rainwater falls onto soils some of the rainwater infiltrates into the soil and some water flows overland down the hill into drainage lines and creeks. Most of the rainwater entering soils becomes available to plants and is transpired into the atmosphere. Excess water filtrating into soils filters down to the ground water.

Water infiltrating into soils is of great benefit. Rainwater flowing overland often becomes a problem causing flooding and adding to water pollution.

The ability of soils to absorb rainwater can be increased by careful soil husbandry. Home gardeners can increase water infiltration and increase plant growth. The more water absorbed by home gardens the more transpiration and this may reduce temperatures. The use of water and plant growth can produce cool spaces in a home garden.

A higher soil moisture content has a cooling effect on the surrounding environment. One way to increase the ability of soils to hold moisture is to increase soil organic matter. If plant growth is encouraged by irrigation, soil organic matter will be increased and the ability of soils to store water will increase and the surrounding temperature will be cooler. The establishment of gardens surrounding a house will increase transpiration cooling and reduce summer temperatures. Concrete paving increases temperature. Shade cloth is not as good as shade from trees because shade cloth only reflects radiation and has no transpiration cooling.

Urban parks can help to decrease elevated temperatures caused by heat island effect.

Appendix Hydrologic balance Inner Sydney Rainfall 1200mm/yr . Transpiration (10% of 1200) 120mm Stormwater runoff 1180mm . Sydney before settlement Average yearly rainfall 1200mm . Transpiration (60% of 1200) 720mm Runoff to rivers 180mm Interception by plant leaves 180 mm Seepage to groundwater 120mm In the inner suburbs of Sydney only 10% of the total rainfall of 1200mm infiltrates into the soil and then transpired back into the atmosphere by vegetation. In the natural forests surrounding Sydney 70% of rainfall filtrates into the soil and 10% of soil water filtrates down to the watertable and 60% of soil water is transpired back into the atmosphere by dense vegetation. Sydney has a rainfall of 1200mm and in natural conditions it is expected 720mm of rain will filtrate into soils and then transpired by plants back into the atmosphere. In built up Sydney only 120mm of rain infiltrate into soils and then transpired by plants into the atmosphere. Transpiration cooling in build up Sydney is greatly reduced when trees are cleared for the development of suburbs. An estimation of the transpiration rate in Sydney can be made from the hydrological balance of a built up catchment. The area of hard surfaces is high in central Sydney and most rainwater becomes stormwater runoff. High surface runoff and high evaporation will reduce leaching of water down to water table. In Sydney it is assumed the rate of transpiration is approximately equal to rate of water infiltration into soils. This is only a general assumption and values of water transpiration and infiltration will vary greatly for specific sites. The above estimates of transpiration show there is a big difference in transpiration cooling in the built up areas of Sydney and the surrounding bush areas. In Sydney transpiration is 120mm a year and in bush areas surrounding Sydney transpiration is 720 mm a year.

TRANSPIRATION AND URBAN CATCHMENTS

TRANSPIRATION and URBAN CATCHMENTS

Transpiration Benefits For Urban Catchment Management

An analysis by Ted Floyd
First published in the Stormwater Industry Association Bulletin, no 92, Aug 2001

In Sydney a large gum tree transpires about 200 litres of water a day. This volume of transpiration will occur from a tree with 6m diameter of foliage on a hot summer day when the soil is still moist after recent rain.

Urbanisation of a catchment increases the area of impervious surfaces. Roads and buildings seal the surface which prevents water absorption by the soil. Impervious surfaces also prevent evaporation from soils.


Water Balance


 

1. Typical Undisturbed Catchment in Sydney Region

    Interception and evaporation  
    from plant surfaces 15%
    Runoff 15%
    Seepage to groundwater and rivers 10%
    Evapotranspiration 60%

 

2. High Density Urban Catchment

    Runoff up to 90%
    Evapotranspiration and  
    seepage to ground water 10%

Evapotranspiration

Falling rain is intercepted by vegetation. A small proportion of the rain is evaporated directly from the plants surfaces. During rain, water is stored on the surfaces of leaves and stems. When it ceases to rain, water will continue to drip from a tree. This can help to even out a rainstorm and to reduce flood peaks.

Table 1 presents rainfall and evaporation data for Sydney. During the summer months evaporation is greater than rainfall. The difference between evaporation and rainfall is demonstrated by the P/E ratio. Yearly P/E for Sydney is 0.67 which demonstrates that evaporation is greater than rainfall.

During the growth of vegetation, water is removed from the soil by transpiration. The rate of transpiration is proportional to the leaf area. Trees have a large leaf area and deep roots encouraging a high transpiration rate. Transpiration by plants helps to dry out soils. During rain, water will infiltrate more readily into a dry soil. The removal of water by transpiration allows more water to enter soils during rain and this will reduce water runoff and lower flooding.


Table 1 RAINFALL AND EVAPORATION DATA SYDNEY


 

Monthly Rainfall – means (mm)

  (Observatory Hill)
  Jan Feb Mar Apr May June Jul Aug Sep Oct Nov Dec

  102 113 135 124 121 131 100 77 69 78 81 78
  Yearly mean 1213mm

 

Monthly Evaporation (mm/month)

  (Sydney Airport)
  220 176 164 126 90 78 90 115 141 171 192 239
  Yearly mean 1802mm

 

Daily Evaporation (mm/day)

  7 6 5 4 3 3 3 4 5 6 6 8

 

P/E (precipitation/evaporation)

  0.46 0.64 0.82 0.98 1.34 1.68 1.11 0.67 0.49 0.46 0.42 0.33
  Yearly P/E 0.67

The rate of evapotranspiration is regulated by the total evaporation. Different plant species transpire at different rates. The ratio between evapotranspiration/total evaporation is often called the crop factor and can be as high as 0.95 for Lucerne in Jan. Deciduous trees vary from 0.75 in Jan down to 0.1 in June. When deciduous trees lose their leaves in winter they have a very low evapotranspiration rate. Table 2 lists values of crop evaporation factors for several crops.


Table 2 EVAPOTRANSPIRATION RATES OF AGRICULTURAL CROPS


 

Crops

Crop Evaporation Factors


    Jan July

  Lucerne 0.95 0.55
  Citrus 0.55 0.50
  Grapevines 0.60 0.15
  Deciduous orchard 0.75 0.15
  Pasture 0.70 0.40

  Crop factor= Evapotranspiration Total evaporation
  Reid, R. L. Ed. (1981). Manual of Australian Agriculture Heineman, Melbourne

Deep rooting plants have the ability to utilise water at greater depths in the soil. This enables these plants to grow and transpire when shallow rooting plants have wilted and ceased to transpire. Deep rooting plants generally have high crop factors, e.g. Lucerne develops very deep roots and has a high crop factor.

The depth roots penetrate into a soil depends on the availability of water. If no available water is in the subsoil, plants develop a shallow root system. Frequent light watering encourages shallow roots while less frequent heavy watering ensures water penetrates deep into the subsoil and encourages growth of deep roots. Plants with deep roots are more drought tolerant. Some Australian native trees have been observed with roots as deep as 30 metres. Table 3 lists the root depths of several common grasses.


Table 3. ROOT DEPTH OF COMMON TURF GRASSES


 

Species

Root depth in metres


  Kikuyu 2.40  
  Paspalum and Couch 1.50
  Buffalo 1.00
  Kentucky bluegrass 0.40
  Bent grasses 0.35
  Poa annua 0.15
  Lucerne 6.00

Handreck, K.A. and Black, N. D. (1994)
Growing Media-for ornamental plants and turf.
University of NSW Press, Sydney.
  


In certain situations the removal of trees has resulted in the rise of the water table. This has caused great problems when there is salt in the subsoil. A rising water table brings the salt to the surface and this can kill vegetation and in urban areas salt can cause damage to buildings. In areas susceptible to salting, water infiltration into soils should not be encouraged and gardens and lawns should not be over watered.

The vegetation covering the land is a very important component of the water cycle. When vegetation is removed or reduced the cheeks and balances in the water cycle are disrupted. Trees are large, with deep roots having a major role in maintaining a balanced water cycle.