Evapotranspiration

Eric Peterson

GEO 361 - Hydrology

Evaporation – process by which water is transferred from the land and water masses of the earth to the atmosphere.

Transpiration – transfer of water from plants to the atmosphere, soil moisture taken up by vegetation is eventually evaporated as it exits plant pores.

Evapotranspiration – combination of evaporation and transpiration.

Within the United States ~ 4,200 bgd of precipitation falls

2/3 is returned to the atmosphere Evaporation rates vary spatially and

temporally.

Where would you expect the highest evaporation rates?

Why? Where would you expect the lowest

evaporation rates?

Globally, where would you expect the highest evaporation rates?

Globally, where would you expect the lowest evaporation rates?

Why?

June 1998 Mean Surface Temperature

Where Will Precipitation and Evaporation Occur?

Temporal variation Will evaporation occur at the same rate all

year long? Why

Evaporation Occurs when water changes state from a liquid to

a gas Function of:

Solar radiation Differences in vapor pressure between the surface

and the overlying air Temperature Wind Atmospheric pressure

Net Evaporation A continuous exchange of water occurs

between the atmosphere and the Earth’s surface

When more water evaporates than returns there is net evaporation

Net evaporation ceases when the air is saturated (water vapor pressure = saturated vapor pressure)

Hornsberger et al., 1998

Factors needed for Evaporation For water to change states – energy is

needed to supply latent heat of vaporization (the quantity of energy that is added when a unit mass of substance vaporizes

Concentration gradient in the water vapor, this is enhanced by air circulation.

Evaporation and Fick’s First Law Evaporation is a diffusive process that follows Fick’s first

law that states molecules of a substance will move from a region of high concentration to a region of low concentration:

Where Jj is the flux of species j crossing a certain area per unit time

Dj is the diffusion coefficient of species j

is the concentration gradient of species j, and is the driving force for the movement

x

cDJ j

jj

x

c j

Fick’s law applied to Evaporation Applying Fick’s law

Where E is the evaporation rate (L/t) es and ea are the vapor pressures of the evaporating

surface and the overlying air (M/(Lt2)) va is the wind speed (L/t)

KE is a coefficient that represents the efficiency of vertical transport of water vapor by turbulent eddies of wind (Lt2/M)

)( asaE eevKE

KE is a function of the wind and is found using:

Where a is the density of air (M/L3) P is the atmospheric pressure [M/(L*T2)] zm is the height at which the wind speed and air

vapor pressure are measured (L) zd & z0 are the zero-plane displacement and the

roughness height of the surface (L)

2

0

ln25.6

1622.0

z

zzPK

dmw

aE

zd (zero-plane displacement) = 0.7 zveg represents a height where wind velocity is minimal.

z0 (roughness height) = 0.1 zveg represents a height where wind is being influenced by surface features