SIMULATING ENERGY, WATER AND CO2 FLUXES AT REPRESENTATIVE DESERT ECOSYSTEMS OVER CENTRAL ASIA Longhui Li Examining Committee: Prof.dr.ir. A. Veldkamp University of Twente Prof.dr.ing. W. Verhoef University of Twente Prof.dr. H-J Hendricks-Franssen Forschungszentrum Juelich Prof.dr. B.J.J.M. van den Hurk KNMU/Free University Amsterdam Prof.dr. Xin Li CAREERI, Chine Academy of Sciences ITC dissertation number 271 ITC, P.O. Box 217, 7500 AE Enschede, The Netherlands ISBN 978-90-365-3883-1 DOI 10.3990/1.9789036538831 Cover designed by Longhui Li Printed by ITC Printing Department Copyright © 2015 by Longhui Li SIMULATING ENERGY, WATER AND CO2 FLUXES AT REPRESENTATIVE DESERT ECOSYSTEMS OVER CENTRAL ASIA DISSERTATION To obtain the degree of doctor at the University of Twente, on the authority of the rector magnificus, prof.dr. H. Brinksma, on account of the decision of the graduation committee, to be publicly defended on Thursday 23 April 2015 at 15.45 hrs by Longhui Li born on 16/08/1978 in Shangnan, Shaanxi, China This thesis is approved by Prof.dr. Zhongbo Su, promotor Prof.dr. Qiang Yu, promotor Dr. Christiaan van der Tol, co-promotor Dedicated to my parents and family Table of Content Acknowledgements ............................................................................................ v Chapter 1 General Introduction ..................................................................... 1 1.1 Scientific Background ........................................................................... 2 1.1.1 History of Land Surface Models (LSMs) ...................................... 2 1.1.2 Global FLUXNET ......................................................................... 6 1.1.3 Improving LSMs Using EC data ................................................... 8 1.2 Statement of Research Problem ............................................................ 9 1.2.1 Lack of EC data over Central Asia ................................................ 9 1.2.2 Abiotic carbon process in alkaline soil desert ecosystems over Central Asia ......................................................................... 10 1.3 Statement of Objective ........................................................................ 12 1.3.1 Conducting/compiling EC measurements in Central Asian desert ecosystems [OBJ1] ............................................................ 12 1.3.2 Investigating net ecosystem CO2 exchange over Central Asian desert ecosystems [OBJ2] ..................................... 12 1.3.3 Determining the key ecological parameters of desert shrubs for the use in LSM [OBJ3] ............................................... 12 1.3.4 Developing an effective root water uptake function .......... (RWUF) for CLM and comparing with other RWUFs [OBJ4] .................. 13 1.3.5 Evaluation of CLM in simulating carbon fluxes over Central Asian desert ecosystems [OBJ5] ..................................... 13 1.4 Outline of the Thesis ........................................................................... 14 Chapter 2 Determination of Key Ecological Parameters of Desert Shrubs for the Use in the Land Surface Model .......................................................... 17 2.1 Introduction ......................................................................................... 19 2.2 Materials and Methods ........................................................................ 22 2.2.1 Site characteristics and climate data ............................................ 22 2.2.2 Response of photosynthesis to light ............................................ 23 2.2.3 Leaf transpiration ......................................................................... 24 2.2.4 Canopy evapotranspiration and soil evaporation ......................... 26 2.3 Model .................................................................................................. 27 2.3.1 Photosynthesis model .................................................................. 30 2.3.2 Stomatal conductance model ....................................................... 32 2.3.3 The coupling of photosynthesis and stomatal conductance ......... 32 i 2.3.4 Evapotranspiration model ............................................................ 33 2.3.5 Determination of photosynthetic parameters ............................... 33 2.3.6 Priestley-Taylor model ................................................................ 34 2.4 Results ................................................................................................. 34 2.4.1 Response of transpiration to meteorological variables ................ 34 2.4.2 Initial quantum efficiency and maximum carboxylation rate ...... 37 2.4.3 Diurnal patterns of the measured and simulated transpirations ............................................................................... 38 2.4.4 Comparisons between the measured and simulated daily canopy evapotranspiration and soil evaporation .......................... 40 2.4.5 Comparison between Shuttleworth-Wallace and Priestley Taylor models .............................................................................. 41 2.5 Discussion and Conclusion ................................................................. 42 Chapter 3 Growing Season Net Ecosystem CO2 Exchange over Central Asian Desert Ecosystems ................................................................................. 47 3.1 Introduction ......................................................................................... 49 3.2 Materials and Methods ........................................................................ 50 3.2.1 Site descriptions ........................................................................... 50 3.2.2 Eddy covariance and ancillary measurements ............................. 52 3.2.3 Data processing and gap-filling ................................................... 53 3.3 Results ................................................................................................. 54 3.3.1 Climatical and meteorological conditions ................................... 54 3.3.2 Effects of friction velocity on nighttime NEE ............................. 55 3.3.3 Diurnal variations of NEE ........................................................... 57 3.3.4 Dependency of daytime NEE on PAR ......................................... 58 3.3.5 Responses of nighttime NEE to soil temperature and soil moisture ....................................................................................... 60 3.3.6 Daily NEE dynamics during growing season .............................. 62 3.4 Discussion ........................................................................................... 63 3.5 Conclusions ......................................................................................... 66 Chapter 4 Representing the Root Water Uptake Process in the Common Land Model for Better Simulating the Energy and Water Vapour Fluxes in a Central Asian Desert Ecosystem ......................................................... 69 4.1 Introduction ......................................................................................... 71 4.2 Material and Methods ......................................................................... 73 4.2.1 Site description ............................................................................ 73 ii 4.2.2 Common Land Model (CLM) ..................................................... 75 4.2.3 Sensitivity analysis ...................................................................... 78 4.2.4 Statistical analysis ........................................................................ 79 4.3 Results ................................................................................................. 81 4.3.1 Meteorological conditions ........................................................... 81 4.3.2 Energy balance closure and footprint area ................................... 82 4.3.3 Performance of model simulations using default RWUF ............ 83 4.3.4 Impact of RWUF on the model’s performance ........................... 89 4.3.5 Sensitivity of CLM to the parameter m ....................................... 90 4.4. Discussion ........................................................................................... 92 4.5 Conclusion .......................................................................................... 94 Chapter 5 Comparison of Root Water Uptake Functions to Simulate Surface Energy Fluxes within a Deep-rooted Desert Shrub Ecosystem . 95 5.1 Introduction ....................................................................................... 97 5.2 Materials and Methods ..................................................................... 99 5.2.1 Experimental descriptions ........................................................... 99 5.2.2 Model Description ..................................................................... 101 5.2.3 Default RWU function in CLM – RWUF0 ............................... 104 5.2.4 Lai and Katul (2000) RWU function – RWUF1........................ 105 5.2.5 Li et al. (2006) RWU function – RWUF2 ................................. 106 5.2.6 Zheng and Wang (2007) RWU function – RWUF3 .................. 107 5.2.7 Root distribution function .......................................................... 108 5.2.8 Statistical analysis ...................................................................... 108 5.2.9 Uncertainty and sensitivity analysis .......................................... 109 5.3 Results ..............................................................................................