Outline
• Announcements
• Measuring evaporation
Soil Physics 2010
Announcements
• Vote your preferences on doodle
for review sessions next week
• Last homework is posted, due
Wednesday, April 28th
• Quiz?
YES!
Soil Physics 2010
Quiz, question 1:
What factor(s) might limit the actual rate of evaporation from a pan of water
(like, for example, the evaporation pan that John Stinn showed)?
Big ones:
Temperature
Solar irradiance Available energy
Wind speed
Maintaining vapor
pressure gradient
Vapor pressure deficit
Local / secondary ones:
Local topography
Humidity of the fetch
Atmospheric pressure
Heat conduction through pan
Soil Physics 2010
Obnoxious ones ☺ :
Is there water in the pan?
Any solutes in the water?
Is the pan covered?
What color is the pan?
Heater in the pan?
Quiz, question 2:
Temperature, °C
Three almost-identical metal boxes, colored black both inside and outside, are sitting
in the sun in the Arizona desert. For your convenience, the sun doesn’t move for
several days: it is always noon! Each box contains 1 kg of H2O at 0 °C, but there are
some important differences:
Box A is tightly sealed, and the H2O is in the form of ice.
Box B is tightly sealed, and the H2O is in the form of liquid water
Box C has no top, and the H2O is in the form of liquid water
Sketch the expected average H2O temperature of each box.
0
0
Soil Physics 2010
time
Quiz, question 2: explanation
Box A is tightly sealed, and the H2O is in the form of ice. Temperature will remain near 0 °C
until all the ice has melted; after that it will behave like B.
Box B is tightly sealed, and the H2O is in the form of liquid water. The water will get hotter
until the energy coming in equals the energy going out. In other words, it will approach an
equilibrium temperature or plateau.
Temperature, °C
Box C has no top, and the H2O is in the form of liquid water. Because the top is open, water
can evaporate and cool the water, so the plateau will be at a lower temperature. Eventually,
once all the water has evaporated, the box will get hotter (but the water won’t, because it’s all
gone).
0
0
Soil Physics 2010
time
Main limitations on actual evaporation from soil
• Stage I: evaporation from the surface
• Energy available at surface
• Vapor pressure deficit in air near the surface
• Transport of vapor away from the surface
• Stage II: evaporation from a retreating drying front
• Flow of liquid water to the drying front
• Stage III: evaporation from a stationary drying front
e, mm/day
• Diffusion of water vapor from the drying front to air above
the soil surface, which equals…
• Flow of liquid water to the drying front
Soil Physics 2010
Stage
I
Stage
II
time
Stage
III
Meteorology magic:
Eddy Covariance
e, mm/day
Measuring evapotranspiration
Stage
I
Stage
II
time
• Measure air mass fluxes in 3
orthogonal directions, many
times per second.
• Simultaneously measure
concentration(s) of gas(es)
of interest, e.g. H2O
• Make lots of assumptions
and do some fancy math
Soil Physics 2010
Stage
III
Measuring Evaporation (from the soil)?
• Stage I: evaporation from the surface
• Actual ≈ potential ≈ pan
• Stage II: evaporation from a retreating drying front
•?
• Stage III: evaporation from a stationary drying front
•?
Conservation of mass
Conservation of energy
Soil Physics 2010
e, mm/day
2 approaches to Stages II & III:
Stage
I
Stage
II
time
Stage
III
Conservation of Mass
Input - Output = Change in Storage
Precipitation
Irrigation
Condensation (dew)
Water table rise
Lateral flow
Run-on
Evaporation
Transpiration
Interception
Abstraction
Water table fall
Lateral flow
Runoff
Too many variables
Errors are too big
E = ET – T
Precipitation
Soil Physics 2010
q
q over soil profile
Storage in plants
Conservation of Mass: Lysimeter
Input - Output = Change in Storage
Precipitation
Irrigation
Condensation (dew)
Soil Physics 2010
Evaporation
Transpiration
Interception
Abstraction
Runoff
Deep drainage
q over soil profile
Storage in plants
Lysimeters
Soil Physics 2010
Lysimeters
Soil Physics 2010
Conservation of Mass: Lysimeter
Soil Physics 2010
Lysimeter summary
Input - Output = Change in Storage
Precipitation
Irrigation
Condensation (dew)
Evaporation
Transpiration
Interception
Abstraction
Runoff
Deep drainage
Really good for ET
For Evaporation only:
need 2 lysimeters?
Cropped
Bare soil
Soil Physics 2010
q over soil profile
Storage in plants
2nd approach: Conservation of Energy
RN = A + LE + S + M
RN (net
A
radiation)
(heating
the air)
ET
LE
(latent heat)
M (other)
S (heating the soil)
Soil Physics 2010
Other: mainly
photosynthesis
How to measure S?
How much does the soil
warm up (or cool off)?
Need to know temperature
and heat capacity…
… or the sensible
heat flux in and out
S (heating the soil)
Soil Physics 2010
Heitman’s soil E method
Key concept #1:
q = 0.01 is small relative
to measurement error,
but LE for q = 0.01 is big
S (heating the soil)
Soil Physics 2010
Key concept #2:
LE in the soil is
about E, not ET
LE (evaporation
from the soil)