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

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

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. 2001. Weatherwise 54:20-27.

Questions 3 and 4 Which molecules move faster? A. nitrogen B. oxygen C. carbon dioxide D. water vapor At 30oC, velocities N2 - 1709 Km h-1 O2 - 1603 Km h-1 CO2 - 1366 Km h-1 H2O - 2136 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. 2001. Weatherwise 54:20-27.

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. 2001. Weatherwise 54:20-27.

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.

Water Molecule

Methods of Heat Transfer Latent Heat - Chemical Heat

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

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)?

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

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

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+273.15

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

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

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

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.

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

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

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

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

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

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.

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.

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.

SURFACE ENERGY BUDGETS + + - + - - + - + -

Sensible Heat Flux Density

Latent Heat Flux Density

Microclimate & ETo Fetch Requirements Shading Wind blocking Marine effects

Fetch Requirements (Indio)

Fetch (98 m Vs 181 m)

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

El Dorado Country Club

Torrey Pines Vs Mira Mar

Temperature Model

Wind Correction

Torrey Pines ETo Model

Landscape Coefficient ETL - measured ETo - estimated

Dense Canopy Light Interception

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

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

Citrus Energy Balance - Lindsay 2001

Local Advection Transpiration (mm h-1) Net Radiation (W m-2) Transpiration 23 July 9.76 mm Transpiration 26 July 14.66 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)

Fog Contribution

Water Table Contribution ETc ETc No Water Table With Water Table

The End Thanks