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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
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!