Evapotranspiration

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

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:

• Frequent spray irrigation onto green roofs, or any landscape from harvested rainwater,
• Extended detention on blue roofs

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. 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 CO₂ concentration is reducing transpiration rates. In the short term this is attributed to stomata remaining partially closed whilst still permitting gas exchange, and in the longer term some plants are being found to have lower density of stomata. 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:

• Anything with trees, e.g. soil cells
• stormwater planters
• green roofs, when well irrigated from harvested rainwater
• Bioretention

Calculation

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

Thornthwaite Mather method

The Thornthwaite Mather method of calculating water balance 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.

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): $i=\left({\frac {T}{5}}\right)^{1.514}$ where: T = mean monthly temperature (°C).

Step 2. Calculate the annual heat index (I): $I=\sum _{i=1}^{12}i$

Step 3. Calculate an uncorrected Potential Evapotranspiration (PET_uncorrected, mm): $PET_{uncorrected}=16\cdot \left({\frac {10\cdot T}{I}}\right)^{\alpha }$

where: $\alpha =6.75\times 10^{-7}I^{3}-7.71\times 10^{-5}I^{2}+0.0179I+0.49$

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): $PET=PET_{uncorrected}\cdot {\frac {N}{12}}\cdot {\frac {daylight\ hours}{30}}$

Example

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

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Abiotic transfer of water vapour to the atmosphere.

Abiotic transfer of water vapour to the atmosphere.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

A stormwater design features that provides for the gradual release of a volume of water in order to increase settling of pollutants and protect downstream channels from frequent storm events.

Any form of rain or snow.

The quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/day

The combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

The quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/day

The combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.