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)