Evapotranspiration

Interception

• a portion of the precipitation is stored by vegetation (interception), where it is subject to evapotranspiration (Fig2.8)

Evapotranspiration

• evapotranspiration summarizes all processes that return liquid water back into water vapor
• water needed and solar energy
• of the water taken up by plants, ~95% is returned to the atmosphere through their stomata (Fig)
•  potential evaporation (PE), i.e. the evaporation rate given an unrestricted water supply - different from actual evaporation
• how can the actual evapotranspiratio be measured?
• water balance
• energy balance

Estimation of ET from the water balance

• this approach may suffer from the uncertainties in the numbers, example:

p = 10⁷+/- 5 *10⁵ m³/y
r_si = 10⁹ +/- 1.5*10⁸ m³/y
r_so = 9.95*10⁸ +/- 1.5*10⁸ m³/y
ET = p + r_si - r_so

Estimation of ET from the energy balance

• A control volume for energy conservation (Fig2.9)

Dalton's law: evaporation rate, E, controlled by two factors, the windspeed and the saturation deficit:

for hydrological purposes, it is importnat to know te average precipitation over an area.


E = u ( e_s-e_a )
u: function of windspeed, e_a : current vapor pressure, e_s : saturation pressure at that temperature

• this formula is useful particularly over the oceans, over land the situation is more complicated

• Evaporation, global patterns (Fig)
• E higher over oceans than land (except for areas covered by sea ice)
• E increases with temperature
• oasis effect
• The Atlantic Ocean is loosing water by evaporation (Fig), this loss is balanced by transport of ocean water from the Pacific ocean

Evapotranspiration

• on land Dalton's law only describes the potential evaporation (PE), i.e. the evaporation rate given an unrestricted water supply, more sophisticated approaches:
• based on temperature, mass transfer, energy balance, or combinations of the latter (see Jones (1997), table 3.2)
• Penman equation or combination equation (Fig) has developed to a standard method in hydrology to estimate free water evaporation
• actual evaporation (AE) over land is lower, it depends on:
• depth to water table
• soil characteristic
• local heat budget
• precipitation patterns, etc
• transpiration
• occurs through openings on leaf surfaces (stomata, Fig)
• very complicated dependence from many parameters, difficult to measure
• lumped together with evaporation to the term evapotranspiration
• the Penman equation can be modified to describe ET: Penman-Monteith model
• this model includes the integrated canopy conductance and the atmospheric conductance of water vapor (see resources below for more details), it describes reality reasonably well (Fig D 7-19)
• potential evapotranspiration (PET): "evapotranspiration from an extended surface of short green crop, actively growing, completely shading the ground, of uniform height and not short of water"
• interception
• process by which precipitation falls on vegetative surfaces (the canopy), where it is subject to evaporation
• intercepted water that is lost (Interception loss) contributes significantly to total ET, it has a high surface/volume ratio and is held higher in the air, where windspeeds are higher
• interception losses range from 10%-40% of the precipitation (Fig J 3.4)
• afforestation could increase evaporation losses by up to 50 to 100%, clearcutting increases runoff
• stemflow (flow along the stem), throughfall (the precip that has avoided interception or dripped off the canopy)

interception has also an influence on water quality

Runoff

• 1/3 of global runoff is not gauged
• climatological method of estimating global runoff: RO = P -E
• estimates are very uncertain

Resources

Bohren, C.F. (1987) Clouds in a glass of beer.
NCDC Climate Visualization (CLIMVIS)