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==Evaporation==
==Evaporation==
Evaporation is the phase transfer of water into atmospheric vapour. It occurs from all wet surfaces, including the leaves of plants and damp soil. The climatic factors which influence the rate of evaporation include net radiation (i.e. sunshine), wind speed, and vapour pressure or relative humidity.
Evaporation is the abiotic phase transfer of water into atmospheric vapour. It occurs from all wet surfaces, including the leaves of plants and damp soil. The climatic factors which influence the rate of evaporation include:
*net radiation i.e. daylight (positive effect),
*wind speed (positive effect), and
*vapour pressure or relative humidity (negative effect).
==Transpiration==
==Transpiration==
This is process of water being removed from the soil and being vaporised into the atmosphere by plants. Upwards flow is created within the plant tissues via osmotic potential.
This is process of water being removed from the soil and being vaporised into the atmosphere by plants. Upwards flow is created within the plant tissues via osmotic potential.
Water evaporates from each leaf's stomata, so that the sap becomes more concentrated. Water is drawn upwards through the stems to equalise this pressure.
Water evaporates from each leaf's stomata, so that the sap becomes more concentrated. Water is drawn upwards through the stems to equalise this pressure. The factors which influence the rate of transpiration include:
*net radiation i.e. daylight (positive effect),
*wind speed (positive effect),
*vapour pressure or relative humidity (negative effect),
AND
*soil wetness (positive effect), and
*leaf charactersitics e.g. leaf area index, stomatal density (positive effect).
Rising atmospheric CO<sub>2</sub> concentration is reducing transpiration rates. In the short term this is attributed to stomata remaining partially closed whilst still permitting gas exchange<ref>
Engineer, Cawas B. et al. CO<sub>2</sub> Sensing and CO<sub>2</sub> Regulation of Stomatal Conductance: Advances and Open Questions, Trends in Plant Science , Volume 21 , Issue 1 , 16 - 30 DOI: https://doi.org/10.1016/j.tplants.2015.08.014</ref>, and in the longer term some plants are being found to have lower density of stomata<ref>E. I. Lammertsma, H. J. de Boer, S. C. Dekker, D. L. Dilcher, A. F. Lotter, F. Wagner-Cremer. Global CO2 rise leads to reduced maximum stomatal conductance in Florida vegetation. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1100371108</ref>.
Transpiration is typically able to remove water from deeper within the soil profile than evaporation. The depth will depend upon the plants root structure and depth, and its inherent transpiration rate.
Transpiration is typically able to remove water from deeper within the soil profile than evaporation. The depth will depend upon the plants root structure and depth, and its inherent transpiration rate.