Evapotranspiration Flashcards
What is evaporation
The process where water in liquid or solid state is transformed into a gas
Who showed the driving force for evaporation?
John Dalton- molecules are constantly moving and evaporation occurs when more molecules are changing to the vapour phase then reverting back to a liquid.
What is the latent heat of vaporization?
The amount of heat required to overcome the latent heat barrier and break through the water surface
Latent heat of water?
540 cal/g at 100 degrees of solar radiation
Transpiration?
Evaporation from vegetation
Evapotranspiration?
Net amount of water evaporated from soil and transpired from plants
Potential ET?
-The amount of water which could be evaporated or evapotranspired if moisture was not limiting.
-Evaporation from a free water surface is always at a potential rate
Actual Evapotranspiration
Multiply the potential Evapotranspiration of the reference crop by the crop coefficient and the soil coefficient
Average Evapotranspiration for Ontario
450 mm/year
Reference Evapotranspiration
Rate at which water would be removed from soil and plant surfaces of a specific crop
Open water Evapotranspiration
The rate at which water would be removed from from an open water surface
Pan evaporation
-Rate at which water would be removed from a small container
-multiply by pan coefficient to get actual evaporation
Factors influencing evaportranspiration
-Energy supply
-Meteorological conditions
-Evaporating surface
-Water quality
What are some pros and cons associated with water budget equation?
-simple but not reliable
-complicated when snow is a huge factor
-good for feasibility studies
-good for long periods of time
Field plots
-inventory of all inputs and outputs and change in storage
-Water table deep
-uses water budget equation
Soil moisture depletion budget
-measuring changes in soil moisture over a period of time
-works when multiple measurements are taken to form an average, and depth of water table is greater than depth of root zone
Tanks and Lysimeters
-small tank where crop grown under natural conditions
-uses water budget equation
Energy balance method
-most complex method
-considers incoming energy from solar, geothermal, and other sources
-1 Langley (ly) = 1 cal/cm2
Mass transfer method
-wind has a major effect on Evapotranspiration
-wind removes Vapor laden air by convection
-keeps boundary layer thin
-high rate of water transfer from liquid to vapour state
-wind is turbulent and connective diffusion is larger than molecular diffusion
Transport of water away from surface depending on
-humidy gradient above water
-wind speed across surface
Thornthwaite Method
-requires minimal data
-empirical equation
-predicts monthly Evapotranspiration from temperature and latitude
-only applicable in the central US
Reference surface
-need a reference surface to determine evaporation rates for other surfaces via crop and soil coefficients
-grass and alfalfa are good reference surfaces because the characteristics are well defined
Interception
-rainfall on vegetation and trapped on surface
-eventually lost as ET
-form of depression storage
