Partitioning plant transpiration and soil

evaporation with eddy covariance and

stable isotope method in North China Plain

Prof. Mei Xurong, Theme Leader Scientist

Director General, IEDA, CAAS

Director, Key Laboratory of Agro-environment, MOA

Director, Consultant Group for Agricultural Disaster Mitigation, MOA

meixr@ieda.org.cn

Introduction

Total Area:

303,585 km2

Population:

300Million

Water resource:

13.5 BCM(5%)

Rainfall:

550-650 mm

Cropping System:

Winter wheat-

Summer Maize

Cereal Production:

70% of wheat

30% of Maize

Water & Food in the North China Plain

0

200

400

600

800

1000

1982 1986 1990 1994 1998 2002 2006

年代

年蒸散量、年降水量(mm）

降水 蒸散Evapotranspiration Precipitation

year

An

nu

al

P &

ET

Water Scarcity in the North China Plain

0

20

40

60

80

100

120

140

160

180

Oct Nov Dec Jan Feb Mar Apr May June July August Sept

Month

水量（

mm

）

亏缺水量

作物需水量

降水量

Winter Wheat Summer Maize

Water Deficit

ET0

Rainfall

-30% ETc!

Ground Water Table 2001.12

Water Scarcity in the North China Plain

Improve Water Productivity in NCP

)(m nConsumptioWater

(kg) Yield Crop(WP) tyProductiviWater

3

ionTranspiratnEvaporatio

IndexHarvest Biomass

Option1: Increase crop yield by using same quantity of water

Option2: Reduce water consumption while maintain the yield

ETa = Tr + Es = P + I – D + dW

Soil evaporation (Es) is a physically controlled flux, but

plant transpiration (Tr) is strongly influenced by plant

physiology and can also be affected by abiotic

environmental condition

Es can be a major flux in sparse vegetation (low LAI) or

wetting soil surface after irrigation/precipitation

Es is ‘non productive’ water use and accounts 1/3 ~ 1/2 of

actual evapotranspiration (ETa ) which depend upon the

water management practices

Need to understand the Es regime and means to control

Improve Water Management in NCP

Conventional approaches for partitioning ETa

Combination of soil lysimeters for evaporation (Es) and sap flow

sensors/chambers for plant transpiration (Tr) - poor spatial

representation

Eddy covariance system /Bowen ratio – energy balance system /

weighting macro-lysimeter for evapotranspiration (ETa) and soil

lysimeters for evaporation (Es) or sap flow sensors/ chamber system

for plant transpiration (Tr) - scale transformation, fetch length

Theatrical methods such as Shuttleworth-Wallace model, dual crop

coefficients method and time series analysis method – parameters

uncertainty

Incorporating measurements of isotopic

concentration of water in soil and plant and air

vapor as a tracer can overcome the limitations

of those conventional methods

• ET partitioning from isotopes of canopy vapor

• δET using the Keeling plot approach

• δE estimated by Craig-Gordon model

ET partitioning from isotopes of canopy vapor

ETa = Tr + Es

1=Tr/ETa+Es/ETa

δT= δT Tr /ETa + δTEs /ETa

δETETa = δTTr + δEEs

δET= δTTr /ETa + δEEs /ETa

Es δE

Tr δT

ETa δET

δET – δT Fs = =Es/ETa δE – δT

δ = (Rsample/Rstandard – 1) * 1000

δET using the Keeling plot approach

Keeling relationship for water vapor

Slope (m) Intercept (b)

Δξ is an isotopic diffuse coefficient;

δS is the isotopic composition of liquid water at the evaporating front;

δV the isotopic composition of the background atmospheric water vapor;

εL-V the temperature dependent equilibrium fraction factor

αL-V is (1- εL-V )X1000;

h the relative humidity normalized to the temperature of the soil surface

h

1 /1000 1

L V S V L V S V L VE

h

h h

δE estimated by Craig-Gordon model

Shortcomings……

Traditional cold-trap method is time consuming and

labor-intensive, and has limited most studies to short

period (several days), small scale (chamber scale),

and low time precision (daily)

There were some differences between the

measurements and real values because of sampling

pollution, isotope fraction from condensation, and

assumption for isotopic steady state in soil –plant-

atmosphere

Water isotope analyzer

with liquid water injector

Field deployable

water vapor isotope analyzer

Real-time and continuous measurements of 18O and D in air vapor

and liquid water by tunable diode laser absorption spectroscopy

provide an opportunity to perform in situ and continuous

evapotranspiration partitioning on diurnal timescale

Objectives: Assess the accuracy of isotopic

method in partitioning ET over irrigated wheat

field in North China Pain to guide the Es

management

Partitioning E and ET by using mini-Lysimeters and

eddy covariance system

Partitioning E and T using in situ measurements by

air vapor isotopic analyzer

Comparing the results estimated by conventional

methods (EC + mini-Lysimeter) with the ones done

by isotopic methods

昌平

Materials and Methods

75 mm 45mm

Sprinkler irrigation

Rainfall

Soil moisture

dynamics and

irrigation

EC System

BR System

Picarro System

EC/BR

Ta Rn

Ra Vw

P RH

Ts G

Soil water content profile

Soil evaporation-MLS

5cm

30cm

80cm

100cm

160cm

2H / 18O air vapor isotope analyzer

Extracting liquid water Soil & plant sampling

Purifying liquid water Isotopic composition analysis

Isotopic composition analysis of water in soil and plant samples

Canopy cover variation and fitted curve during

experimental period

Results

18O/16O hour scale dynamics with rainfall and irrigation and weather parameters

The results showed that δ18O composition of air vapor at two heights

correlated significantly with VPD and Rn with mean correlation

coefficients about 0.696 (n=1250，α<0.001) and 0.704 (n=1250，α<0.001).

P+I

Jointing

Filling Maturing

Booting

The relationship between δ18O and 1/vapor H2O content

18O/16O

D/H

Sprinkler Irrigation

2012/7/26 25

y = -48006x - 13.22

R² = 0.405 n=75 <0.001

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

0.00E+00 4.00E-05 8.00E-05 1.20E-04 1.60E-04

O1

8/O

16

dis

crim

inat

ion

(‰

)

1/Vapor H2O

y = -52591x - 12.273

R² = 0.418 n=75 <0.001

-22.0

-18.0

-14.0

-10.0

-6.0

0.00E+00 2.00E-05 4.00E-05 6.00E-05 8.00E-05 1.00E-04 1.20E-04

O1

8/O

16

dis

crim

inat

ion

(‰

)

1/Vapor H2O

9-May

y = -42308x - 15.72

R² = 0.653 n=85 <0.001

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

2.00E-05 6.00E-05 1.00E-04 1.40E-04 1.80E-04

O1

8/O

16

dis

crim

inat

ion

(‰

)

1/ Vapor H2O

3-Jun

y = -87843x - 10.7

R² = 0.813 n=80 <0.001

-30.0

-26.0

-22.0

-18.0

-14.0

-10.0

3.00E-05 4.00E-05 5.00E-05 6.00E-05 7.00E-05 8.00E-05 9.00E-05

O1

8/O

16

dis

crim

inat

ion

(‰

)

1/ Vapor H2O

12-Jun

δET-The ‘Keeling plot’

ET_EC + Es_Micro-lysimeter

y = 1.1967x

R2 = 0.8468

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Fs-EC_MLS

Fs-

SI

The relative contribution of soil evaporation to

evapotranspiration (Fs) estimated by eddy covariance –

micro-lysimeters method and its comparison with

isotopic method during the experimental period

0.50

0.60

0.70

0.80

0.90

1.00

17/Apr 27/Apr 7/May 17/May 27/May 6/Jun 16/Jun

ET

/ E

T F

T

Date

ISS法 MLS法

The relationship between Es/ET and canopy cover (Cc)

Crop sowing-

tillering

tillering-

jointing

tillering-

heading

flowering-

filling

Filling-

mature Total

Wheat 0.58 0.32 0.76 0.85 0.78 0.71

Maize 0.33 0.72 0.82 0.80 0.68

The ratio of plant transpiration to evapotranspiation (Tr/ET)

30% of Evaporation!

There was a good consistent between the estimated

Es/ET by the stable isotopic method and ones by the

conventional method, indicating that combination of

Keeling plot method with in situ continuous

measurements of water vapor stable isotope

composition can accurately partition

evapotranspiration in wheat field of NCP

Managing evaporation in the low LAI period is the

effective solutions to reduce the non-productive water

use while improve water productivity, eg., mulching,

insufficient irrigation, etc.

Conclusions

2012/7/26 31

Thank You!