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.
Annual Rainfall and Evaporation in major Australian Cities
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.