COOL TREES AND 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.