Partitioning plant transpiration and soil evaporation with eddy covariance and stable isotope method in North China Plain

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1 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

2 Introduction Water & Food in the North China Plain Total Area: 303,585 km 2 Population: 300Million Water resource: 13.5 BCM(5%) Rainfall: mm Cropping System: Winter wheat- Summer Maize Cereal Production: 70% of wheat 30% of Maize

3 Annual P & ET 年蒸散量 年降水量 (mm) 水量 (mm ) Water Scarcity in the North China Plain 亏缺水量 Water Deficit 作物需水量 ET 0 降水量 Rainfall Oct Nov Dec Jan Feb Mar Apr May June July August Sept Month Winter Wheat Summer Maize Precipitation 降水 蒸散 Evapotranspiration -30% ET c! year 年代

4 Water Scarcity in the North China Plain Ground Water Table

5 Improve Water Productivity in NCP Crop Yield(kg) Water Productivi ty (WP) 3 Water Consumption (m ) Biomass Harvest Index Evaporatio n Transpiration Option1: Increase crop yield by using same quantity of water Option2: Reduce water consumption while maintain the yield

6 Improve Water Management in NCP ET a = T r + E s = P + I D + dw Soil evaporation (E s ) is a physically controlled flux, but plant transpiration (T r ) is strongly influenced by plant physiology and can also be affected by abiotic environmental condition E s can be a major flux in sparse vegetation (low LAI) or wetting soil surface after irrigation/precipitation E s is non productive water use and accounts 1/3 ~ 1/2 of actual evapotranspiration (ET a ) which depend upon the water management practices Need to understand the E s regime and means to control

7 Conventional approaches for partitioning ET a Combination of soil lysimeters for evaporation (E s ) and sap flow sensors/chambers for plant transpiration (T r ) - poor spatial representation Eddy covariance system /Bowen ratio energy balance system / weighting macro-lysimeter for evapotranspiration (ET a ) and soil lysimeters for evaporation (E s ) or sap flow sensors/ chamber system for plant transpiration (T r ) - scale transformation, fetch length Theatrical methods such as Shuttleworth-Wallace model, dual crop coefficients method and time series analysis method parameters uncertainty

8 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 δ ET using the Keeling plot approach δ E estimated by Craig-Gordon model

9 ET partitioning from isotopes of canopy vapor ET a δ ET ET a = T r + E s 1=T r /ET a +E s /ET a T r δ T δ T = δ T T r /ET a + δ T E s /ET a δ ET ET a = δ T T r + δ E E s δ ET = δ T T r /ET a + δ E E s /ET a E s δ E F s = δ ET δ T δ E δ T =E s /ET a δ = (R sample /R standard 1) * 1000

10 δ ET using the Keeling plot approach Keeling relationship for water vapor Slope (m) Intercept (b)

11 δ E estimated by Craig-Gordon model E L V S h V L V S h V L V 1 h / h Δξ is an isotopic diffuse coefficient; δ S δ V is the isotopic composition of liquid water at the evaporating front; the isotopic composition of the background atmospheric water vapor; ε L-V the temperature dependent equilibrium fraction factor α L-V is (1- ε L-V )X1000; h the relative humidity normalized to the temperature of the soil surface

12 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 plantatmosphere

13 Water isotope analyzer with liquid water injector Field deployable water vapor isotope analyzer Real-time and continuous measurements of 18 O 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

14 Objectives: Assess the accuracy of isotopic method in partitioning ET over irrigated wheat field in North China Pain to guide the E s 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

15 Materials and Methods 昌平

16 Sprinkler irrigation Rainfall 75 mm 45mm Soil moisture dynamics and irrigation

17 EC System Picarro System BR System EC/BR T a R n R a V w P RH T s G

18 Soil evaporation-mls Soil water content profile

19 160cm 100cm 80cm 30cm 5cm 2 H / 18 O air vapor isotope analyzer

20 Isotopic composition analysis of water in soil and plant samples Soil & plant sampling Extracting liquid water Purifying liquid water Isotopic composition analysis

21 Results Canopy cover variation and fitted curve during experimental period

22 18 O/ 16 O hour scale dynamics with rainfall and irrigation and weather parameters P+I The results showed that δ 18 O composition of air vapor at two heights correlated significantly with VPD and R n with mean correlation coefficients about (n=1250,α<0.001) and (n=1250,α<0.001).

23 The relationship between δ 18 O and 1/vapor H 2 O content Jointing Booting Filling Maturing

24 Sprinkler Irrigation 18 O/ 16 O D/H

25 O18/O16 discrimination ( ) O18/O16 discrimination ( ) O18/O16 discrimination ( ) O18/O16 discrimination ( ) δ ET -The Keeling plot 1/Vapor H2O 0.00E E E E E /Vapor H 2 O 0.00E E E E E E E y = x R² = n=75 < May y = x R² = n=75 < / Vapor H2O 2.00E E E E E / Vapor H2O 3.00E E E E E E E y = x R² = n=85 < Jun y = x R² = n=80 < Jun /7/

26 ET_EC + Es_Micro-lysimeter

27 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 E T / ET F T ISS 法 MLS 法 y = x R 2 = /Apr 27/Apr 7/May 17/May 27/May 6/Jun 16/Jun Date Fs-EC_MLS

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

29 The ratio of plant transpiration to evapotranspiation (Tr/ET) Crop sowingtillering tilleringjointing tilleringheading floweringfilling Fillingmature Total Wheat Maize % of Evaporation!

30 Conclusions 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.

31 Thank You! 2012/7/26 31