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1-Water molecules enter the roots passively.
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.
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.
Image from the Wikimedia commons, credit - Laurel Jules


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).

LID practices which optimise evaporation include:


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. 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),


  • soil wetness (positive effect), and
  • 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[1], and in the longer term some plants are being found to have lower density of stomata[2]. 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.

LID practices which optimise transpiration include:


In the Toronto Region, an interactive online tool has been developed to estimate localised precipitation and potential evapotranspiration conditions.

Thornthwaite Mather method[edit]

The Thornthwaite Mather method of calculating water balance[3] is recommended within the 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[4].

We are in the process of writing a tool to semi-automate these; please feel free request a draft copy, or to contribute your own How to contribute.

Step 1. Calculate the monthly heat index (i): where: T = mean monthly temperature (°C).

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

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


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):


For the town of Richmond Hill (Lat = 43.87)
For monthly calculations, data can be found in Environment Canada Climate Normals

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
Declination (rad) - 0.372 - 0.232 - 0.035 0.171 0.330 0.407 0.372 0.239 0.052 - 0.157 - 0.325 - 0.407
Day length (hr) 9 10 12 13 15 15 15 14 12 11 9 9
Temperature (°C) - 6.20 - 4.90 - 0.30 6.90 13.30 18.70 21.40 20.30 15.90 9.10 3.10 - 2.70
Monthly heat index, i 0.00 0.00 0.00 1.63 4.40 7.37 9.04 8.34 5.76 2.48 0.48 0.00
PET (mm, uncorrected) 0.00 0.00 0.00 29.91 62.44 91.50 106.44 100.33 76.28 40.80 12.19 0.00
days in month 31 28 31 30 31 30 31 31 30 31 30 31
Corrected PET (mm) 0.0 0.0 0.0 33.1 78.3 116.4 136.9 119.3 78.7 38.1 9.6 0.0

  1. 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
  2. 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
  3. 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. Thornthwaite1957
  4. Sellinger,C.E. Computer Program for Estimating Evapotranspiration using the Thornthwaite Method NOAA Technical Memorandum ERL GLERL-101, Great Lakes Environmental Research Laboratory 1996 [1] Accessed 8 Jan 2018