1. ET Theory 101 USCID Workshop http://biomet.ucdavis.edu
PMhr, PMday, PMmon
CUP, SIMETAW (DWR link)
R.L. Snyder, Biometeorology Specialist Copyright © Regents of the University of California

2. Methods of Heat Transfer
Conduction- from molecule to molecule
Heat Source
Metal bar
Convection - by movement of heated air
Radiation - energy passing from one object to another without a connecting medium
Long wave loss from Earth
Short wave gained from the sun
Earth

3. Questions 1 and 2
How many molecules of air are in a cubic meter?
If six billion people count 1000 molecules per hour, it would take 511 million years to count the number of molecules in 1 m3
2.69 x 1025
26,900,000,000,000,000,000,000,000 molecules m-3
The air is mostly nitrogen? (T/F)?
Air is mostly empty space. Less than 0.1% of the volume is occupied by molecules
False
Horstmeyer, S Weatherwise 54:20-27.

4. Questions 3 and 4
Which molecules move faster? A. nitrogen B. oxygen C. carbon dioxide D. water vapor
At 30oC, velocities N Km h-1 O Km h-1 CO Km h-1 H2O Km h-1
When saturated, the air cannot hold more water vapor (T/F)?
False! Less than 0.1% of volume is occupied. Air can hold more water.
Horstmeyer, S Weatherwise 54:20-27.

5. Questions 5
How many molecule collisions per second occur in a m3 of air?
There are about 595 trillion collisions per second between air molecules in a m3 of air.
595,000,000,000,000 collisions/sec
Occupy < 0.1% volume
26,900,000,000,000,000,000,000,000 molecules m-3
Horstmeyer, S Weatherwise 54:20-27.

6. Sensible Heat & Temperature
Lower temperature Less sensible heat
Higher temperature More sensible heat
When the sensible heat content is lows, the temperature is low. When the sensible heat content is high, the temperature is high.

7. Water Molecule

8. Methods of Heat Transfer
Latent Heat - Chemical Heat

9. More Water Vapor More Latent Heat
I am a free water vapor molecule H O
Energy breaks our hydrogen bonds and sets us free H O H O H O H O H O
Water molecules form strong hydrogen bonds. To break off individual molecules from liquid water, energy must be expended to break the hydrogen bonds. H O H O O H
Latent heat

10. Questions 6
False, there is more total heat in humid than in dry air to melt the ice cream
Your ice cream will melt faster at 35% RH than at 75% RH (T/F)?

11. Which wet towel dries fastest?
30 % RH
70 % RH
Ta = 30 oC
Adiabatic Process

12. Questions 7 Diabatic Process
Yes! But both the temperature and vapor pressure must increase following the saturation vapor pressure curve.
Is there evaporation in a greenhouse with 100% relative humidity?
Diabatic Process

13. Radiation e = 1.0 for a black body e < 1.0 for a gray body
= 5.67  10-8 W m-2 K-1
T = absolute temperature = oC

14. 73,483,200 W m-2
390 W m-2

15. Direct Radiation (Q) Radiation that comes directly from the sun
The amount of energy received per unit area is called “Irradiance” and the units are commonly W m-2 = J s-1 m-2.
The amount of energy received depends on the angle of incidence of the radiation

16. Direct beam radiation interception (W m-2)Qs
500
433
250
Qs = Q cos(a)

17. Diffuse Radiation (q)
Solar radiation that is scattered by the sky and comes from all directions.
During much of the day, about 10 to 15% of the radiation received on a flat surface is diffuse radiation.
The diffuse radiation is nearly the same regardless of the surface orientation.
The percentage of diffuse radiation is higher near sunrise and sunset.

18. Direct and diffuse radiation (W m-2)Qs
Qs+ q
500
535
433
468
250
285
Rs = Qs + q
q  0.15 Qs on a horizontal sfc

19. Albedo 30 20
Reflection (%) 10
0.1 1 10
100
Vegetation height (m)

20. Net solar radiation (W m-2)Qs
Qs+ q
= 0.25
500
535
401
433
468
351
250
285
214

21. Net Radiation Rn=(1-a)Rs+Lu+Ld
Rs=Qs+q Rs Rs Lu Ld
cloudy clear
Lu + Ld = -10 W m to Lu + Ld = -100 W m-2

22. Net radiation (W m-2) Qs Qs+ q = 0.25 RLn =-100 500 535 401 301 433
468
351
251
250
285
214
114

23. Net Radiometer
This is an example of a net radiometer for measuring net radiation. Net radiation is calculated as the sum of the downward solar radiation (Rsd) minus the product of the albedo () and Rsd, the longwave downward (RLd) and longwave upward (RLu). Recall that radiation is positive to the surface and negative away from the surface, so RLd is a negative and RLu is a positive number.

24. Soil (Ground) Heat Flux
The soil heat flux density is a function of the temperature gradient with depth in the soil and the thermal conductivity (Ks) is the thermal conductivity of the soil, which mainly depends on soil water content.

25. Soil Heat Flux Density (G )
Heat Plate
Thermocouples d G G2
CV – volumetric heat capacity
This slide shows the equation used to estimate soil heat flux density at the surface from a heat flux plate measurement (G2), the change in the soil volume temperature per unit time (K s-1), the depth to the heat flux plate (d) in m, and the volume specific heat.

26. SURFACE ENERGY BUDGETS+ + - + - - + - + -

27. Sensible Heat Flux Density

28. Latent Heat Flux Density

29. Microclimate & ETo Fetch Requirements Shading Wind blocking
Marine effects

30. Fetch Requirements (Indio)

31. Fetch (98 m Vs 181 m)

32. Empirical ETo – El Dorado Country Club
Adjustments for Regional effects
Wind Blockage
Sunrise-Sunset
Advection
Empirical Equations
CIMIS
Regional effects

33. El Dorado Country Club

34. Torrey Pines Vs Mira Mar

35. Temperature Model

36. Wind Correction

37. Torrey Pines ETo Model

38. Landscape Coefficient
ETL - measured ETo - estimated

39. Dense Canopy Light Interception

40. Sparse Canopy Light Interception
Some light reflected. Increases sensible heat near the surface.

41. Density Correction CG Ground Cover (%) 0.00 1.20 0.80 0.40
Fraction of Maximum Kc 20 40 60 80
CG Ground Cover (%)

43. Citrus Energy Balance - Lindsay 2001

44. Local Advection Transpiration (mm h-1) Net Radiation (W m-2)
Transpiration 23 July mm
Transpiration 26 July mm
0.0
0.4
0.8
1.2
1.6
Time (h)
Transpiration (mm h-1)
-200
200
600
1400
1000 12 6 24 18
Net Radiation (W m-2)

45. Fog Contribution

46. Water Table Contribution
ETc
ETc
No Water Table
With Water Table

47. The End
Thanks