Lecture 5 & 6-Evaporation

Question 1

What is evapotranspiration (evaporation)?

Answer 1

Water in liquid or solid phase becomes vapour.
Includes:
-evaporation of liquid from water or land surfaces
-transpiration from leaves
-sublimation from ice and snow (solid to gas)

Question 2

Evaporation in hydrological terms:

Answer 2

• loss from a system
• precip is input, first loss is evaporation, depends on energy, moisture gradient, clean air
• generally a large component of our water balance than runoff (globally)

Question 3

Practical importance of understanding evaporative processes

Answer 3

• major component of energy and water vapour exchange (climate predictions)
• water availability
• plant growth & ecosystem response to change
• irrigation efficiency
• reservoir supply
• implications for groundwater and overland flow (flood response)

Question 4

Types of evapotranspiration

Answer 4

• evaporation
• transpiration (later)
• interception (later)
• sublimation (not in this class)

Question 5

What is evaporation from a physical and hydrology persepctive?

Answer 5

• liquid to gas conversion (physical)

- loss of water from a wet surface to the air through conversion into vapour (hydrology)

Question 6

Ficks first law of diffusion

Answer 6

“a diffusing substance moves from where its concentration is larger to where its concentration is smaller at a rate proportional to the spatial gradient of concentration”

Essentially: opposite process to condensation (dry to wet), evaporation is balancing from wet to dry (balance water vapour gradients, high to low pressure)

Question 7

Conditions for evaporation to occur:

Answer 7

• energy (sun/heat)
• method of exchanging air (wind)
• low humidity (dry air)

temp below dew pt temp but warm so it can hold more water
vapour pressure difference between air and surface

Question 8

Ke (coefficient) in mass-transfer approach

Answer 8

• derived from the vertical wind speed profile
• dependent on surface roughness (changes the intensity of eddies)
• how easily is wet air replaced? (easier with smooth surface)

Question 9

Vapour pressure difference

Answer 9

vapour pressure at surface (theoretical maximum at saturation) minus vapour pressure of the air (kPa)
es - ea
where both are dependent on temperature (C) and ea is also dependent on the relative humidity

Question 10

Evaporation rate formula - Mass transfer approach

Answer 10

E=(Ke)(va)(es-ea)

where:
E = evaporation rate
va = wind speed (@2m height)
es = vapour pressure at the surface
ea = vapour pressure in the air
Ke = coefficient reflecting the efficiency of vertical transport of water vapour by turbulent eddies of wind

Question 11

Information needed to calculate evaporation rate (mass-transfer approach)

Answer 11

wind speed
turbulence (surface roughness)
temperature or air and surface
relative humidity of air

Question 12

Mass-transfer approach assumptions and uses:

Answer 12

• only assumes that evaporation is affected vertically (only looks at surface and air above), doesn’t account for energy/mass from the sides
• doesn’t take into consideration fluctuations of energy/temperature within the moment
• doesn’t look at all the types of energy that influence heat transfer and vapour transfer
• good for looking at what is happening in a single instant
• good for body of water (way more complicated for land or soil)

Question 13

How we calculate evaporation rates depends on:

Answer 13

• surface type
• water availability
• stored energy use
• water-advected energy use

Question 14

stored energy use

Answer 14

evaporation needs energy which will lower the temperature of the thing that is being evaporated from

Question 15

water advected energy use / exchange

Answer 15

with significant inputs/outputs of water that are at a different temperature than the surface the temperature of that surface will be affected by the input/outputs
so if looking at a large reservoir with small input this might not make a difference but with a small water supply and large input this could make a big difference

Question 16

other considerations for calculating evaporation rates

Answer 16

• purpose of study
• available data (and cost of study)
• time period of interest

Question 17

Evaporation from water calculation issues:

Answer 17

• water advected heat
• changes in heat storage (small water bodies may change temp over the season/day, large bodies need more energy to change temp)

Question 18

Issues/uncertainties when calculating evaporation will depend on:

Answer 18

• Area
• Volume
• Water residence time relative to time period of interest

Question 19

Mass-transfer approach description

Answer 19

calculate free-water evaporation (theoretical maximum)

doesn’t consider advection or heat storage changes

Question 20

Methods for calculating evaporation

Answer 20

• water balance approach
• mass transfer approach
• eddy correlation approach
• energy balance approach
• penman or combination approach
• pan evaporation approach

Question 21

Water balance approach

Answer 21

apply water balance equation to water body of interest over a time period and solve for evaporation
if we know the change in volume, the inputs and outputs, the difference will be evaporation

E= W + SWin + GWin - GWout - SWout - change in volume
E=evaporation
W = precipitation
SW = surface water
GW = ground water

Question 22

Water balance approach practical considerations and applicability

Answer 22

-concept is simple but application is very complex
-don’t know true quantities
large uncertainties in final estimate
-measurement problems generally preclude the use of this method
-problems if evaporation is only small fraction
-can provide check on other approaches
-good for gross estimates
-okay for ‘long’ time periods

Question 23

Mass-transfer approach issues

Answer 23

• measurements required: wind speed, surface temp, air temp, relative humidity
• need to know measurement height

Question 24

Mass-transfer approach applicability

Answer 24

• gives instantaneous rate
• only average up to 1 day (not monthly)
• good for artificially heated water bodies with known surface temp
• Ts not normally measured on naturally heated water bodies

Question 25

Energy balance approach

Answer 25

net latent heat flux is dependent on net shortwave (solar) radiation, net longwave (emitted from earth/clouds) radiation, geothermal heat flux, sensible heat flux, water-advection energy (horizontal energy movement), change in stored energy in water body of time period of study

convert latent heat flux into evaporation by dividing above by density of water and latent heat of vaporisation (amount of energy to go from liquid to gas)

Question 26

Energy balance approach practical considerations

Answer 26

• requires knowledge of all components (good measurements at short timeframes)
• error transfer
• point vs distributed

Question 27

Energy balance applicability

Answer 27

• more usable with really short time periods and in environments that are really dry or really cold
• data intensive, computationally intensive, mostly used in academic / scientific setting
• often gets closest to reality
• considers energy fluxes

Question 28

Penman or combination approach

Answer 28

combination of mass-transfer and energy balance approaches

Question 29

Pan evaporation approach

Answer 29

E = W - [V2 - V1]
W is precipitation
V1 volume at the start
V2 volume at the end

Question 30

Pan evaporation approach practical considerations

Answer 30

• not the same as a lake - smaller heat capacity and advective processes different
• no surface water inputs - changes input
• heat transfer through walls - enhances evaporation
• type of pan used
• splash effects

Question 31

Pan evaporation approach applicability

Answer 31

• useful baseline
• cheap, easy, maintenance
• good for long term studies