Evapotranspiration Eric Peterson GEO 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.

Latent heat - amount of heat released or adsorbed during a change in state.

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 J j is the flux of species j crossing a certain area per unit time D j is the diffusion coefficient of species j is the concentration gradient of species j, and is the driving force for the movement

Fick’s law applied to Evaporation Applying Fick’s law Where E is the evaporation rate (L/t) e s and e a are the vapor pressures of the evaporating surface and the overlying air (M/(Lt 2 )) v a is the wind speed (L/t) K E is a coefficient that represents the efficiency of vertical transport of water vapor by turbulent eddies of wind (Lt 2 /M)

K E is a function of the wind and is found using: Where  a is the density of air (M/L 3 ) P is the atmospheric pressure [M/(L*T 2 )] z m is the height at which the wind speed and air vapor pressure are measured (L) z d & z 0 are the zero-plane displacement and the roughness height of the surface (L)

z d (zero-plane displacement) = 0.7 z veg represents a height where wind velocity is minimal. z 0 (roughness height) = 0.1 z veg represents a height where wind is being influenced by surface features