Changes

[[File:Transpiration Overview.svg|thumb|

[[File:Transpiration Overview.svg|thumb|1-Water molecules enter the roots passively. <br> 2-Once the water has entered the xylem, the forces of adhesion and cohesion take effect. Adhesion causes the molecules to adhere to sides of the vessels, and cohesion causes the molecules to adhere to each other, forming a column of water that extends from the roots to the top of the plant. <br> 3-Water from the xylem is let into the spongy mesophyll, where it can come into contact with stomata. When the stomata are open, the cells in this layer are exposed to the outside air, and evaporation occurs. When water molecules evaporate out of the stomata, they create tension in the water column, as the forces of cohesion pull nearby molecules up with them, creating pulling force down the length of the xylem. <br> Image from the Wikimedia commons, credit - Laurel Jules]]

 

==Evaporation==

==Evaporation==

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:  

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:  

*wind speed (positive effect), and  

*wind speed (positive effect), and  

*vapour pressure or relative humidity (negative effect).  

*vapour pressure or relative humidity (negative effect).  

{{textbox 1=

{{textbox|LID practices which optimise evaporation include:  

LID practices which optimise evaporation include:  

*Frequent spray irrigation onto [[green roofs]], or any landscape from [[rainwater harvesting| harvested rainwater]],

*Frequent spray irrigation onto [[green roofs]], or any landscape from [[rainwater harvesting| harvested rainwater]],

*Extended detention on [[blue roofs]].}}  

*Extended detention on [[blue roofs]]}}  

 

==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.  

*soil wetness (positive effect), and

*soil wetness (positive effect), and

*leaf charactersitics e.g. leaf area index, stomatal density (positive effect).  

*leaf charactersitics e.g. leaf area index, stomatal density (positive effect).  

Rising atmospheric CO₂ concentration is reducing transpiration rates. In the short term this is attributed to stomata remaining partially closed whilst still permitting gas exchange<ref>

Rising atmospheric CO₂ 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. CO₂ Sensing and CO₂ 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>.   

Engineer, Cawas B. et al. CO₂ Sensing and CO₂ 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.  

==Calculation==

==Calculation==

[[File: TRCA_water_balance_tool.PNG|thumb|In the Toronto Region, an interactive online tool has been developed to estimate localised precipitation and potential evapotranspiration conditions.|link=https://trca.ca/conservation/drinking-water-source-protection/trspa-water-balance-tool/]]

[[File: TRCA_water_balance_tool.PNG|thumb|In the Toronto Region, an interactive online tool has been developed to estimate localised precipitation and potential evapotranspiration conditions.|link=https://trca.ca/conservation/drinking-water-source-protection/trspa-water-balance-tool/]]

===Thornthwaite Mather method===

===Thornthwaite Mather method===

The Thornthwaite Mather method of calculating water balance<ref>Thornthwaite, C. W., and Mather, J. R., Instructions and Tables for Computing Potential Evapotranspiration and the Water Balance, Drexel Institute of Technology, Laboratory of Climatology, 1957.{{#l:Thornthwaite1957.pdf}}</ref> is recommended within the [https://www.ontario.ca/document/stormwater-management-planning-and-design-manual 2003 Ontario Stormwater Management Planning and Design Manual]. The original paper (linked below) provides the required calculations as a series of lookup tables. The following equations are derived from these, and are widely documented elsewhere<ref>Sellinger,C.E. Computer Program for Estimating Evapotranspiration using the Thornthwaite Method NOAA Technical Memorandum ERL GLERL-101, Great Lakes Environmental Research Laboratory 1996 [https://www.glerl.noaa.gov/pubs/tech_reports/glerl-101/tm-101.pdf] Accessed 8 Jan 2018 </ref>.  

The Thornthwaite Mather method of calculating water balance<ref>Thornthwaite, C. W., and Mather, J. R., Instructions and Tables for Computing Potential Evapotranspiration and the Water Balance, Drexel Institute of Technology, Laboratory of Climatology, 1957.{{#l:Thornthwaite1957.pdf}}</ref> is recommended within the [https://www.ontario.ca/document/stormwater-management-planning-and-design-manual 2003 Ontario Stormwater Management Planning and Design Manual]. The original paper (linked below) provides the required calculations as a series of lookup tables. The following equations are derived from these, and are widely documented elsewhere<ref>Sellinger,C.E. Computer Program for Estimating Evapotranspiration using the Thornthwaite Method NOAA Technical Memorandum ERL GLERL-101, Great Lakes Environmental Research Laboratory 1996 [https://www.glerl.noaa.gov/pubs/tech_reports/glerl-101/tm-101.pdf] Accessed 8 Jan 2018 </ref>.  

<math>i=\left ( \frac{T}{5} \right )^{1.514}</math>

<math>i=\left ( \frac{T}{5} \right )^{1.514}</math>

where:

where:

''T'' = mean monthly temperature.  

''T'' = mean monthly temperature (°C).  

'''Step 2.''' Calculate the annual heat index (''I''):

'''Step 2.''' Calculate the annual heat index (''I''):

<math>I=\sum_{i=1}^{12}i</math>

<math>I=\sum_{i=1}^{12}i</math>

'''Step 3.''' Calculate an uncorrected Potential Evapotranspiration (''PET_uncorrected''):

'''Step 3.''' Calculate an uncorrected Potential Evapotranspiration (''PET_uncorrected'', mm):

<math>PET_{uncorrected}=16\cdot \left ( \frac{10\cdot T}{I} \right )^{\alpha }</math>

<math>PET_{uncorrected}=16\cdot \left ( \frac{10\cdot T}{I} \right )^{\alpha }</math>

<math>\alpha=6.75\times10^{-7}I^{3}-7.71\times10^{-5}I^{2}+0.0179I+0.49</math>

<math>\alpha=6.75\times10^{-7}I^{3}-7.71\times10^{-5}I^{2}+0.0179I+0.49</math>

'''Step 4.''' Apply a monthly correction to account for varying daylight hours and number of days in a month, to find the corrected potential evapotranspiration (''PET''):

'''Step 4.''' Apply a monthly correction to account for varying daylight hours and number of days in a month, to find the corrected potential evapotranspiration (''PET'', mm):

<math>PET=PET_{uncorrected}\cdot\frac{N}{12}\cdot\frac{daylight\ hours}{30}</math>

<math>PET=PET_{uncorrected}\cdot\frac{N}{12}\cdot\frac{daylight\ hours}{30}</math>

| '''0.0''' ||'''0.0''' || '''0.0''' || '''33.1''' ||''' 78.3''' || '''116.4''' || '''136.9''' || '''119.3''' || '''78.7''' || '''38.1''' || '''9.6''' || '''0.0'''

| '''0.0''' ||'''0.0''' || '''0.0''' || '''33.1''' ||''' 78.3''' || '''116.4''' || '''136.9''' || '''119.3''' || '''78.7''' || '''38.1''' || '''9.6''' || '''0.0'''

|}

|}

----

----