Evaporation from Flux Towers

S = P – D - ET

Change in water content of volume of soil precipitation

drainage

By Dr Marcy LitvakDept of Biological Sciences

University of Texas at Austin(now at the University of New Mexico)

Energy budgeting approach

Can directly measure each of these

variables

How do you partition H and E??

SensibleHeat flux

LatentHeat flux

Eddy Covariance

Directly measure how much CO2

or H2O vapor blows in or out of a site in wind gusts.

Net Ecosystem Production

Integrated measure of ecosystem fluxes

Link changes in [CO2] or [H2O] in the air above a canopy with the upward or downward movement of that air

Net Ecosystem Exchange

Flux CO2 = w ’ CO2’

30 minute timescale

Updraft [CO2] > downdraft [CO2]

Flux >0 carbon source

Updraft [CO2] < downdraft [CO2]

Flux < 0 carbon sink

02

00

40

06

00

80

01

00

0

146.0 146.5 147.0 147.5 148.0

May 26, 2000 May 27, 2000

Sunlight

CO2 Exchange

CO

2 E

xch

ang

e (

mo

l m-2 s

-1)

Su

nlig

ht

(Wm

-2)

• The net CO2 flux is calculated for each half hour from the measurements of vertical wind and CO2 concentration.

• A positive flux indicates a net loss of CO2 from the surface (respiration) and a negative flux indicates the net uptake of CO2 (photosynthesis)

-20

-15

-10

-50

5

12 AM 12PM 12AM 12PM 12AM

CO

2 E

xcha

nge

(m

ol m

-2 s

-1)

Ann

ual C

acc

umul

atio

n (

Ton

s C

ha-1

)

1999 2000

• A years worth of half-hour data can be summed to determine how much Carbon the ecosystem gained or lost

0

1

2

3

4

5

ET -Eddy covariance method

• Measurement of vertical transfer of water vapor driven by convective motion

• Directly measure flux by sensing properties of eddies as they pass through a measurement level on an instantaneous basis

• Statistical tool

Basic Theory

Mean

Fluctuation

Instantaneous signal

InstantaneousPerturbation from

The mean

All atmospheric entities show short-period fluctuations about their long term mean value

Time averaged property

Turbulent mixing

Propterties carried by eddies:Mass, density ρVertical velocity w

Volumetric content

)'( = )'( )'(

1) Expand2) Simplify: a) remove all terms with single primed entity b) remove terms with fluctuations c) remove terms containing mean vertical velocity

Eddy Covariance

Eddy covariance

Velocity of air being moved upwards or downwardsm s-1

Fluctuation of entity about it’s meang kg air-1

Density of air kg air m-3

F = ρw’ x’

Average vertical flux of entity over 30 minute period

At any given instant, multiply velocity of airbeing moved upwards or downwards at a speed of m s-1, by the fluctuation of the entitiyabout its mean

= g m-2 s-1m g s kg

kgm3

Result:vertical speed of transfer of entity measured in m s-1 and at a concentration of g per kg of air

Eddy covariance

g of entity transferred vertically, per square meter of surface area per second

Fluctuation about the mean of

vertical wind speed

Fluctuation about the mean of

density of water vapor in air

Mean density of air

Latent heat of vaporization(J kg-1 ˚C-1)

m kgs m2

kg m3

J kg

J m2s

= W m2

=

QE = ρ w’ρv’Lv

Latent Heat

Fluctuation about the mean of

vertical wind speed

Fluctuation about the mean of

air temperature

Mean density of air

Specific heat of air at constant pressure(J kg-1 ˚C-1)

m ◦Cs

kg m3

J kg ˚C

J m2s

= W m2

=

QH = ρ w’ T’Cp

Sensible Heat

Instrumentation Requirements

IRGA

3-D Sonic anemometer

Net radiometerPyrronometer

Quantumsensor

Instrumentation Requirements

Challenges of operating eddy flux systems in remote locations!

Advantages of eddy covariance

• Inherently averages small-scale variability of fluxes over a surface area that increaes with measurement height

• Measurements are continuous and in high temporal resolution

• Fluxes are determined without disturbing the surface being monitored

• Great tool to look at ecosystem physiology

Disadvantages

• Need turbulence!• Gap filling issues• Relatively Expensive• Stationarity issues• Open-path IRGA issues

• The eddy covariance method is most accurate when the atmospheric conditions (wind, temperature, humidity, CO2) are steady, the underlying vegetation is homogeneous and it is situated on flat terrain for an extended distance upwind.

Stationiarity

AdvectionHorizontal concentration gradients may also lead to perturbation calculation errors

H +

Le

(W m

-2)

-200

0

200

400

600

800

1000

y = 0.93x - 4.24r2 = 0.85n = 4304

a)

Rnet - G (W m-2)

-200 0 200 400 600 800 1000

H +

Le

(W m

-2)

-200

0

200

400

600

800

1000

y = 0.94x - 7.09r2 = 0.86n = 3310

b)

Issue of energy balance closure

Impact of encroachment of Ashe juniper and Honey mesquite on carbon and water cycling in central Texas savannas

Collaboration with:James Heilman, Kevin McInnes, James Kjelgaard, Texas A&MMelba Crawford, Roberto Gutierrez, Amy Neuenschwander, UTFreeman Ranch - Texas State University

Marcy LitvakSection of Integrative Biology

University of Texas, Austin

Figure 1. Location and geographical extent of Edwards Plateau

Extensive areas of Edwards Plateau historically were dominated by fairly open live-oak savannas

Due to overgrazing and fire suppression policies….grasslands are disappearing as woody species increase

Ashe juniperHoney mesquite

Worst-case scenario:

Research Objectives• Determine sink strength for carbon associated with woody

encroachment and analyze the variables that determine gains/losses of carbon from key central Texas ecosystems

• Determine change in ET, energy balance and potential groundwater recharge associated with woody encroachment

• Provide objective data for validation of land surface process models (CLM2 – Liang Yang, UT) related to growth, primary production, water cycling, hydrology

• Aid in regional scale modeling efforts

Carbon/water tradeoff

GrasslandTAMU

WoodlandTAMU

TransitionUT

Study site

3 stages of woody encroachmentOpen grassland, transition site, closed canopy woodland

-NEE carbon, water, energy: open-path eddy covariance(net radiation, solar radiation (incoming, upwelling), PAR, air temperature, relative humidity, precipitation)

-physiological measures of ecosystem component fluxesleaf-level gas exchange, sap-flow, bole-respiration rates, herbaceous NEE

-soil carbon, soil microclimate, soil respiration rates

-Ecosystem structure biomass, LAI, species composition

Experimental design

open grasslandMay 2004

(TAMU) Transition site – July 200415-20 year old juniper,mesquite

Live Oak-Ashe juniper woodland – July 2004

(TAMU)

Cumulative NEE for three land covers - Freeman Ranch

-100

0

100

200

300

400

500

600

700

200 250 300 350 400

Day of Year - 2004

Cum

ulat

ive

NE

E (g

C m

-2)

grassland

transition

forest

Cumulative ET for three land covers at Freeman Ranch

0

50

100

150

200

250

300

350

200 250 300 350 400

Day of Year - 2004

Cum

ulat

ive

ET (m

m d

ay-1

) grassland

transition

forest

H/ (E)

Bowen Ratio

Energy balance approach to estimating convective fluxesSeeks to partition energy available into sensible and latent heat terms

Typical values:

0.1- 0.3 tropical rainforests; soil wet year-round0.4 – 0.8 temperate forests and grasslands2-6 semi-arid regions; extremely dry soils> 10 deserts

Bowen RatioBowen (1926)

B can be approximated as a function of vertical

differences of temperature and vapor pressure in the air,

or ,B = g (t2- t1 ) / ( e2 –e1 )

PsychrometerConstant

F(T,P)

air temperatures measured at two points at different

heights above the land surface

vapor pressures measured at the same two points

Average values of the air-temperature differences (t2 - t1) and vapor-pressure differences (e2 - e1),

taken every 30 seconds for a 30-minute period

are used to determine .

Bowen RatioBowen Ratio

= QH

QE

= Tρv

Ca

Lv

Specific heat capacity

Latent heatOf vaporization

Bowen Ratio

The energy budget can then be solved for LE: LE = ( Rn –G – W) / ( 1+ )

Uses gradients of heat and water to partition available energy into SH and LE

Assumptions:•One-dimensional heat and vapor flow, only vertical

•No transfer to/from measurement area from adjacent area•No significant heat storage in plant canopy

•2 fluxes originate from same point on land surface•Atmosphere equally able to transfer heat and water vapor,

so turbulence need not be considered

Needs large tract of uniform vegetation

Sensors to measure air temperature and humidity

Determine average differentials for 15-minutes, then switch sensors, and determine average differentials

for another 15 minutes to avoid sensor bias