Soil Physics 2010
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:TemperatureSolar irradianceWind speedVapor pressure deficit
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?
Local / secondary ones:Local topographyHumidity of the fetchAtmospheric pressureHeat conduction through pan
Available energy
Maintaining vapor pressure gradient
Soil Physics 2010
Quiz, question 2: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 time0
Tem
per
atu
re, °
C
Soil Physics 2010
Quiz, question 2: explanationBox 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.
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 time0
Tem
per
atu
re, °
C
Soil Physics 2010
Main limitations on actual evaporation from soil
time
e, m
m/d
ay
Stage I
Stage II
Stage III
• 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• 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
Measuring evapotranspiration
Meteorology magic:
Eddy Covariancetime
e, m
m/d
ay Stage I
Stage II
Stage III
• 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
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• ?
time
e, m
m/d
ay Stage I
Stage II
Stage III
2 approaches to Stages II & III: Conservation of mass Conservation of energy
Soil Physics 2010
Conservation of Mass
Input - Output = Change in Storage
PrecipitationIrrigation
Condensation (dew)Water table rise
Lateral flowRun-on
EvaporationTranspirationInterceptionAbstractionWater table fallLateral flowRunoff
over soil profileStorage in plants
Too many variablesErrors are too big
E = ET – TPrecipitation
Soil Physics 2010
Conservation of Mass: Lysimeter
Input - Output = Change in Storage
PrecipitationIrrigation
Condensation (dew)
EvaporationTranspirationInterceptionAbstractionRunoffDeep drainage
over soil profileStorage in plants
Soil Physics 2010
Lysimeters
Soil Physics 2010
Lysimeters
Soil Physics 2010
Conservation of Mass: Lysimeter
Soil Physics 2010
Lysimeter summary
Input - Output = Change in Storage
PrecipitationIrrigation
Condensation (dew)
over soil profileStorage in plants
Really good for ETFor Evaporation only:
need 2 lysimeters?CroppedBare soil
EvaporationTranspirationInterceptionAbstractionRunoffDeep drainage
2nd approach: Conservation of Energy
RN = A + LE + S + M
A(heating
the air)
LE(latent heat)
RN (net
radiation)
ET
MM (other)
SS (heating the soil)
Other: mainly photosynthesisSoil Physics 2010
How to measure SS?
SS (heating the soil)
Soil Physics 2010
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
Heitman’s soil E method
LELE (evaporation from the soil)
Soil Physics 2010
Key concept #1: = 0.01 is small relative to measurement error, but LE for = 0.01 is big Key concept #2:
LE in the soil is about E, not ET
SS (heating the soil)