Transcript Evaporation and Transpiration

Evaporation and Transpiration
• Evaporation- change of water from liquid to vapor phase
• Potential Evaporation - climatically controlled evaporation from
a surface when the supply water to the surface is unlimited
• Transpiration - evaporation occurring from plant’s leaves
through stomatal openings. Function of stomata is to provide a
place where CO2 can dissolve into water and enter plant tissue.
Evaporation unavoidable in this process - driven by same
process as evaporation.
• Potential Transpiration - Transpiration which would occur if
water supply to plant roots and through vascular system to
stomata was unlimited. Controlled by climate and plant
physiology.
Evaporation
• Two main forces influencing evaporation rate are:
– Supply of solar energy to provide the latent heat of
evaporation.
– Ability to transport evaporated water away from surface
 affected by wind velocity and vapor gradient.
• Transpiration affected by above plus ability of plant to
extract and transmit water from soil to stomata.
Methods of Estimating Evaporation
• energy balances methods
• mass transfer or aerodynamic methods
• combination of energy and mass transfer (Penman
equation)
• pan evaporation data
All these methods were developed to estimate evaporation
from free water surfaces (or completely saturated soil)
Energy Balance Method
• Assumes energy supply the limiting factor.
sensible heat
transfer to air
Hs
net
radiation
energy used in
evaporation
Rn
Qe
heat stored
in system
G
heat conducted to ground
(typically neglected)
• Consider energy balance on a small lake with no
water inputs (or evaporation pan)
Energy Balance Method
• Steady state conservation of energy equation. (assume
water temperature does not change, no flow into or out of
lake)
energy inflows = energy outflows
Rn  Qe  H s  G
•
Rn  Rs (1  As )  Rl (1  Al )  Rb
• Qe  E  w Le
• Hs - sensible heat flux to atmosphere (by convection)
• G - heat conducted to ground are typically small and difficult to
measure.
Energy Balance Method
• If neglect sensible heat transfer to atmosphere (Hs)
and ground (G )
• Substitute equation for Q into energy balance
Rn  Qe  E  w Le
Rn
E
 w Le
• Recall
Le  597.3  0.57T
cal / g
 2.5 x106  2370T J / kg
Energy Balance Method assumes
• no water inflow/outflow to lake
• no change in water temperature of lake
• neglects sensible heat transfer to ground and
atmosphere
• neglects heat energy lost with water which
leaves system as vapor
• calculates evaporation on a daily time
interval
Mass Transfer (Aerodynamic) Method
• based on the concept that rate of turbulent mass transfer of
water vapor from evaporating surface to atmosphere is
limiting factor
z
u
z
T
qv
• Mass transfer is controlled by (1) vapor gradient and (2)
wind velocity which determines rate at which vapor is
carried away.
Mass Transfer (Aerodynamic)
Method
E  B (u )(es  e( z ))
B (u ) 
0.102u
ln z 2 
  z o 
B (u )  0.0027(1 
2
u
)
100
Combination Method (Penman)
• Evaporation can be computed by aerodynamic method when energy
supply not limiting and energy method when vapor transport not
limiting  Typically both factors limiting so use combination of above
methods
E

g
Er 
Ea
 g
 g
• Weighting factors sum to 1. Deviation of weighting factors is based on
physical processes,
4098es

•  = vapor pressure deficit
237.3  T 2
 g = psychrometric constant
g  66.8Pa / 0 C
Combination Method (Penman)
• Combination method is most accurate and most commonly
used method if meteorological information is available.
Particularly good for small, well-monitored areas.
• Need: net radiation, air temperature, humidity, wind speed
• If all this information is not available can use PriestlyTaylor approximation:
E 

Er
 g
• Based on observations that second term in Penman
equation typically  30% of first. This is better for large
areas.
Evaporation Pan
• Since expensive to maintain weather stations required to
use Penman equation, evaporation pans are often used to
directly measure evaporation.
• Standard (Class A) Evaporative Pans are built of unpainted
galvanized iron. 4 ft. diameter, 10 inches deep, set on a
platform 12 inches above ground.
• Water level in pan recorded daily with high precision
micrometer. Evaporation determined by mass balance.
Evaporation Pan
• Mass balance equation
S  I  0
H 2  H1  P  E
 E p  P  ( H 2  H1 )
• Pans measure more evaporation than natural water bodies
because:
– 1) less heat storage capacity (because smaller volume water)
– 2) heat transfer through pan sides
– 3) wind effects caused by pan itself
• Typically estimate
E  K pEp
Evapotranspiration
• Same factors which govern water evaporation from water
surfaces govern evapotranspiration because essentially
transpiration is mainly due to evaporation from stomata.
• In addition plant physiology (plants can control size of
stomata and resistance to flow through roots and vascular
systems) and soil moisture conditions (resistance of flow to
roots) play a role.
• Estimate Evaportranspiration using
Et  K s K c E p
Evapotranspiration
• Alternative empirical equation- Blaney-Criddle equation
PET  Kf
• K= monthly crop coefficient
–
–
–
–
alfalfa
beans
corn
pasture
0.85
0.65
0.75
0.75
• f= monthly consumptive use factor
f 
tp
100