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1 Comparison of water balance and root water uptake models in simulating CO 2 and H 2 O fluxes and growth of wheat Authors: T. H. guyen a, *, M. Langensiepen a, J. Vanderborght c, H. Hueging a, C. M. Mboh a, F. Ewert a, b a University of Bonn, Institute of Crop Science and Resource Conservation (IRES), Katzenburgweg 5, Bonn, Germany b Leibniz Centre for Agricultural Landscape Research (ZALF), Institute of Landscape Systems Analysis, Eberswalder Strasse 84, Muencheberg, Germany c Agrosphere, Institute of Bio- and Geosciences (IBG-3), Forschungszentrum Jülich GmbH, 52428, Jülich, Germany Background: State of art in crop models and land surface models Plant hydraulic conductance: Explains stomatal behaviors: anisohydric (wheat) and isohydric (maize) Affects crop growth The hydraulic conductance is rarely considered in many crop models and LSMs These models can not simulate specific stomatal behaviors The coupled photosynthesis and stomatal conductance model (A n-g s) Water stress factor Macroscopic root water uptake TR32/Bonn 4th April 2018 Source: Tardieu and Simmoneau, 1998; Couvreur et al., 2014; Vadez, 2014, Kramer and Boyer, 1995; Peterson and Steudle, 1993 Tipping bucket WB model Physically based WB model 2 Objectives The overall aim of the study is to investigate whether consideration of plant hydraulic conductance improves the calculation of gas fluxes exchange and crop processes. To analyze and compare the predictive capacity of three different WB and RWU modelling approaches with and without consideration of plant hydraulic conductance at daily resolution To compare the performance of the same WB and RWU with different temporal resolutions (daily versus hourly) To investigate the modelling constraints, implication, and the application Materials & Methods: model configurations Crop model LITULCC2 (Rodriguez et al., 2000) Root growth model SLIMROOT model (Williams and Izaurralde, 2005) Water balance model Tipping bucket (van Laar et al., 1997) Physical based model (HILLFLOW1D Bronstert and Plate, 1997) Root water uptake model Conceptual (van Laar et al., 1997) Feddes RWU (Feddes et al., 1978) Couvreur RWU (Couvreur et al., 2014) 3 4 Materials & Methods: inclusion of plant hydraulic conductance in crop model Materials & Methods: inclusion of plant hydraulic conductance in crop model CoD FeD TiD FeH 1 θ θ FC fwat = θ θ WP θ θ FC θ WP θ θ FC WP 0 θ θ WP (7) AMAX l,t = VCMAX l,t( Ci l,t Γ ) Ci l,t + KMC 1 + O 2 KMO (5) Source: Couvreur et al., 2014; Cai et al., 2017 fwat = T act T p (8) CoH gs l,t = a 1 + b 1FGR l,t Ci l,t Γ (1 + DS l,t ) D 0 ψ leaf the leaf water potential (m); T p potential transpiration (m d -1 ); T act actual transpiration (m d -1 ) ψ i the soil water potential in ith soil layer (m); ψ sr the effective root zone water potential sensed by the plant (m); K plant is plant hydraulic conductance (m 3 m -2 m -1 d -1 ) RLD i the normalized or relative RLD of i th soil layer (-) K plant,doy root hydraulic conductance at day of the year (day -1 ), K plant, initial : root hydraulic conductance at initial time (at emergency) (day -1 ); TRLD initial: total root length density at start of growing season (at emergency) (cm cm -2 ); TRLD i : total root length density at each day of growing (cm cm -2 ); RLD i : root length density of soil layer i th (cm cm -3 ); z i is the thickness of the i th soil layer (m); ψ threshold a threshold constant leaf water potential for specific plants (m) 5 6 Thuy guyen Uni Bonn 1

2 Materials and Methods: study sites Selhausen (orth Rhine-Westphalia, Germany) Two soil types: Upper part: stony soil (60% stone content in the top layer) Lower part: loamy soil (4% stone content) Three water treatments in each part (irrigated, rainfed and sheltered) Winter wheat in 2016 Canopy gas chamber Soil respiration Leaf water potential Sap flow Soil and root observation Crop growth Phenology data Height Aboveground dry matter LAI C: content Result & Discussion: gross assimilation rate Three models with daily resolution predicted similarly in irrigated and rainfed plots. The physically based models performed better than the tipping bucket Simulations of models with hourly resolution were similar to those with daily resolution 7 8 Results & Discussion: daily actual transpiration Results & Discussion: hourly actual transpiration All models slightly underestimated daily T act Three models with daily resolution predicted transpiration rate similarly Aggregated daily transpiration rate from hourly resolution was about the same as with daily transpiration rate from daily resolution 9 Both models (CoH and FeH) underestimated hourly T act, especially in the upper and lower irrigated and rainfed plots Both models (CoH and FeH) simulated hourly T act well in sheltered plots Hourly Courveur s model simulated best the variability of T act (standard deviation closer to observed point) in the upper and lower and sheltered plots 10 Results & Discussion: crop growth Result & Discussion: leaf water potential from hourly Couvreur s model Three models with daily resolution simulated dry matter and LAI well; tipping bucket approach (TiD) under-predicts dry matter in both sheltered plots Simulated dry matter with daily time step is similar to simulated dry matter with hourly time step with Feddes s approach Simulated leaf water potential is closer to the observed leaf water potential in the main growing season but overestimated in the late season The model captures the anisohydric stomatal behavior of winter wheat Thuy guyen Uni Bonn 2

3 Conclusion Under non- water stress condition, the tipping bucket and the physically based WB and RWU approaches simulated biomass, LAI, water and photosynthesis fluxes, and soil water fluxes similarly Physically based WB and RWU approaches outperformed tipping bucket under water stress for gross assimilation rate Hourly resolution simulated similarly gas fluxes, soil water content and crop growth in comparison with the daily resolution for both Couvreur and Feddes model The inclusion of plant hydraulic conductance using Couvreur s RWU approach into a crop model slightly improved the gas fluxes simulation and crop growth Consideration of plant hydraulic conductance in a crop model allowed to capture the anisohydric stomatal behavior Outlook The newly coupled model (modified LITULCC2 with Couvreur method) with the inclusion of hydraulic conductance should be validated for other years and field conditions The newly coupled model (modified LITULCC2 with Couvreur method) requires further testing for other wheat cultivars or crop types (isohydric) Materials & Methods: inclusion of plant hydraulic conductance in crop model et CoD FeD Source: Couvreur Cai et TiD al., 2017 al., 2014; FeH Thank you very much ame: Thuy Huu guyen tngu@uni-bonn.de Phone: Address: Katzenburgweg 5, Bonn fwat = 1 θ θ FC θ θ WP θ θ FC θ WP θ θ FC WP 0 fwat = T act T p (8) θ θ WP CoH (7) AMAX l,t = VCMAX l,t( Ci l,t Γ ) gs l,t = a 1 + Ci l,t + KMC 1 + O 2 KMO b 1FGR l,t Ci l,t Γ (1 + DS l,t D 0 ) (5) This research was financed by the German Science Foundation (DFG) within framework of Transregional Collaborative Research Center 32 Patterns in Soil-Vegetation-Atmosphere-Systems (TR32, 15 ψ leaf the leaf water potential (m); T p potential transpiration (m d -1 ); T act actual transpiration (m d -1 ) ψ i the soil water potential in ith soil layer (m); ψ sr the effective root zone water potential sensed by the plant (m); K plant is plant hydraulic conductance (m 3 m -2 m -1 d -1 ) RLD i the normalized or relative RLD of i th soil layer (-) K plant,doy root hydraulic conductance at day of the year (day -1 ), K plant, initial : root hydraulic conductance at initial time (at emergency) (day -1 ); TRLD initial: total root length density at start of growing season (at emergency) (cm cm -2 ); TRLD i : total root length density at each day of growing (cm cm -2 ); RLD i : root length density of soil layer i th (cm cm -3 ); z i is the thickness of the i th soil layer (m); ψ threshold a threshold constant leaf water potential for specific plants (m) 16 Results & Discussion: crop growth Root length/biomass Three models with daily resolution simulated dry matter and LAI well; tipping bucket approach (TiD) under-predicts dry matter in both sheltered plots Simulated dry matter with daily time step is similar to simulated dry matter with hourly time step with Feddes s approach Thuy guyen Uni Bonn 3

4 Soil water content Result & Discussion: leaf water potential from hourly Couvreur s model Simulated leaf water potential is closer to the observed leaf water potential in the main growing season but overestimated in the late season The model captures the anisohydric stomatal behavior of winter wheat Water use efficiency Taylor diagram for daily actual transpiration Comparisons in requirement of parameters, weather input, time running, and applications Crop parameters Soil hydraulic parameter Daily Hourly CoD FeD TiD CoH FeH Photosynthesis x x x x x Root growth parameters x x x x x RWU parameters x x x x Soil parameter (PWP, FC) x x x x x van Genuchten parameters x x x x Computation time (second per simulation) Weather input data Daily x x x Hourly x x Results & Discussion: LAI Crop specific type (stomata behavior) x x Thuy guyen Uni Bonn 4

5 Depth (cm) CoD FeD TiD CoH FeH T-test (pf value) to compare means of simulated output to mean of observed data Gas fluxes n sample Treatment CoD FeD TiD CoH FeH U-I 5.02E E E E E-16 U-R 2.24E E E E E-14 U-S 1.53E E E E E-11 Pg 66 L-I 5.56E E E E E-13 L-R 8.71E E E E E-11 L-S 9.94E E E E-14 U-I 8.62E E E E E-06 U-R 2.21E E E U-S 4.77E E E E E-07 Tact 44 L-I 2.49E E E E E-07 L-R 6.84E E E E E-09 L-S 4.54E E RMSE of soil water content versus observed SWC U-I U-R U-S L-I L-R L-S A A A A A A A A A A A A A A A A A A Crop parameters Soil parameters Modelling approach: photosynthesis and stomatal conductance model in LITULCC2 AMAX l,t = VCMAXl,t( Cil,t Γ ) Cil,t+KMC 1+ O2 KMO EFF l,t = J 2.1 fwat1 (1) Ci l,t Γ 4.5(Ci l,t + 2Γ ) (2) FGR l,t = AMAX l,t 1 e Il,t EFFl,t AMAXl,t (3) gs l,t = a 1 + fwat = 1 Ci l,t = Ca FGR l,t (4) gs l,t b1fgrl,t fwat2 Cil,t Γ (1+ DS l,t D0 ) (5) 1 θ θ FC θ θ WP θ θ FC θ WP θ θ FC WP 0 θ θ WP AMAX: Light saturated leaf photosynthesis/or Rubisco carboxylation limited rate (μm m -2 s -1 ); VCMAX: Maximum carboxylation rate of Rubisco enzyme (μm m -2 s -1 ); Ci: Intercellular CO2 concentration (μm mol -1 ) ; Ca: Atmospheric CO2 concentration (μm mol -1 ); KMC Michelis-Menten constant for CO2 (μm m -2 s -1 ); KMO: Michelis-Menten constant for O2 (μm m -2 s -1 ); O2:Atmospheric oxygen concentration (μm mol -1 ); Γ*: CO2 compensation point (μm mol -1 ); EFF: Photosynthesis rate when RuBP regeneration is limiting (μm m -2 s -1 ); J: Conversion energy from radiation to mole photon (mole photons MJ -1 ); FGR:et leaf photosynthesis rate (μm m -2 s -1 ); I: The total absorbed flux of radiation (MJ m -2 s -1 ); gs: Bulk stomatal conductance (mol m -2 s -1 1); a1: Residual stomatal conductance when FGR = 0 (mol m -2 s -1 ); b1: Fitting parameter [-]; DS the vapor pressure deficit at the leaf surface (Pa); D0: empirical coefficient reflecting the sensitivity of the stomata to VPD (Pa); l: Sub-indices indicates canopy layer (sunlit and shaded leaf) [-]; t: Sub-indices indicates time of the day [-]; fwat: Water stress factor for stomatal conductance and photosynthesis [-]; θ, θ WP, θ FC: Current soil water content, soil water content at wilting point and field capacity, respectively (cm 3 cm -3 ). 29 Modelling approach : Couvreur s RWU model (Couvreur et al. 2014) RWU = ψ sr = RLD i = RLD iδz i/ i=1 ψ irld i Δz i/( i=1 i=1 RLD i Δz i (1) RLD iδz i ) (2) T water stress = K plant ψ sr ψ threshold (3) ψ leaf = ψ sr T p K plant If ψ leaf < ψ (4) threshold then ψ leaf = ψ threshold T plant = max(0, min(t p, T stress)) (5) i=1 T prld i + i=1 K comp(ψ i ψ sr)rld i T act = max(0, min (T plant, RWU)) (7) RLD i the normalized or relative RLD of i th soil layer (-); RLD i is root length density of soil hydraulic conductance [cm cm - 3 ]; ψ threshold a threshold constant leaf water potential for specific plants (m); ψ leaf the leaf water potential (m); T p potential transpiration (m d -1 ); T plant is plant transpiration rate specific for crop type (m d -1 ); T act actual transpiration [m d -1 ] after consideration of both root capacity, soil available water and crop capacity; RWU is the water uptake rate by root system (m d -1 ); K comp the compensatory RWU conductance of the plant [m 3 m -2 m -1 d -1 ]; ψ i the soil water potential in ith soil layer [m]; z i is the thickness of the ith soil layer (m); the number of soil layers (-); ψ sr the effective root zone water potential sensed by the plant (m); T water stress is plant transpiration rate under water stress (m d -1 ), K plant is plant hydraulic conductance (m 3 m -2 m -1 d -1 ) 30 Thuy guyen Uni Bonn 5

6 Modelling approach: Feddes s RWU model (Feddes et al., 1978) Irrigation and sheltered days Instantaneous gross primary products Thuy guyen Uni Bonn 6