Evapotranspiration

==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). 
 
==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
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, and in the longer term some plants are being found to have lower density of stomataE. 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. 
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==
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===
The Thornthwaite Mather method of calculating water balanceThornthwaite, 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. 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 elsewhereSellinger,C.E. Computer Program for Estimating Evapotranspiration using the Thornthwaite Method NOAA Technical Memorandum ERL GLERL-101, Great Lakes Environmental Research Laboratory 1996 Accessed 8 Jan 2018 . 
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 (PETuncorrected, mm):
PET_{uncorrected}=16\cdot \left ( \frac{10\cdot T}{I} \right )^{\alpha }
where: 
\alpha=6.75\times10^{-7}I^{3}-7.71\times10^{-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
==References==
category: Modelling