Observed and modelled ecosystem respiration and gross primary production of a grassland in southwestern France.

Abstract. In this work, the rich dataset acquired at the SMOSREX experimental site is used to enhance the A-gs version of the Interactions between Soil, Biosphere and Atmosphere (ISBA) model. A simple representation of the soil moisture effect on the ecosystem respiration is implemented in the ISBA-A-gs model. It results in an improvement of the modelled CO2 flux over a grassland in southwestern France. The former temperature-only dependent respiration formulation used in ISBA-A-gs is not able to model the limitation of the respiration under dry conditions. In addition to soil moisture and soil temperature, the only parameter required in this formulation is the ecosystem respiration parameter Re25. It can be estimated by means of eddy covariance measurements of turbulent nighttime CO2 flux (i.e. ecosystem respiration). The resulting correlation between observed and modelled net ecosystem exchange is r2=0.63 with a bias of −2.18 μmol m−2 s−1. It is shown that when CO2 observations are not available, it is possible to use a more complex model, able to represent the heterotrophic respiration and all the components of the autotrophic respiration, to estimate Re25 with similar results. The modelled ecosystem respiration estimates are provided by the Carbon Cycle (CC) version of ISBA (ISBA-CC). ISBA-CC is a version of ISBA able to simulate all the respiration components, whereas ISBA-A-gs uses a single equation for ecosystem respiration. ISBA-A-gs is easier to handle and more convenient than ISBA-CC for the practical use in atmospheric or hydrological models. Surface water and energy flux observations, as well as Gross Primary Production (GPP) estimates, are compared with model outputs. The dependence of GPP to air temperature is investigated. The observed GPP is less sensitive to temperature than the modelled GPP. Finally, the simulations of the ISBA-A-gs model are analysed over a seven year period (2001–2007). Modelled soil moisture and Leaf Area Index (LAI) are confronted with the observed surface and root-zone soil moisture content (m3 m−3), and with LAI estimates derived from surface reflectance measurements.

[1]  Riccardo Valentini,et al.  Trace gas exchange over terrestrial ecosystems: methods and perspectives in micrometeorology , 1997 .

[2]  Jean-Christophe Calvet,et al.  Modelling energy and CO2 fluxes with an interactive vegetation land surface model-Evaluation at high and middle latitudes , 2008 .

[3]  Joost C. B. Hoedjes,et al.  SMOSREX: A long term field campaign experiment for soil moisture and land surface processes remote sensing , 2006 .

[4]  R. Lacaze,et al.  Global mapping of vegetation parameters from POLDER multiangular measurements for studies of surface-atmosphere interactions: A pragmatic method and its validation , 2002 .

[5]  Yann Kerr,et al.  Joint assimilation of surface soil moisture and LAI observations into a land surface model , 2008 .

[6]  Jean-Christophe Calvet,et al.  Modelling CO2-enrichment effects using an interactive vegetation SVAT scheme , 2001 .

[7]  Jean-Louis Roujean,et al.  Ability of the land surface model ISBA‐A‐gs to simulate leaf area index at the global scale: Comparison with satellites products , 2006 .

[8]  Ü. Rannik,et al.  Estimates of the annual net carbon and water exchange of forests: the EUROFLUX methodology , 2000 .

[9]  E. Rastetter,et al.  Aspects of spatial and temporal aggregation in estimating regional carbon dioxide fluxes from temperate forest soils , 1994 .

[10]  W. Parton,et al.  Analysis of factors controlling soil organic matter levels in Great Plains grasslands , 1987 .

[11]  S. Planton,et al.  A Simple Parameterization of Land Surface Processes for Meteorological Models , 1989 .

[12]  J. William Munger,et al.  Measurements of carbon sequestration by long‐term eddy covariance: methods and a critical evaluation of accuracy , 1996 .

[13]  D. Lamb,et al.  Soil respiration in the seasonally dry tropics near Townsville, North-Queensland , 1990 .

[14]  S. Rambal,et al.  Local variations of ecosystem functions in Mediterranean evergreen oak woodland , 1996 .

[15]  John W. Doran,et al.  Steady‐State Aerobic Microbial Activity as a Function of Soil Water Content , 1990 .

[16]  J. Mahfouf,et al.  The ISBA land surface parameterisation scheme , 1996 .

[17]  W. Oechel,et al.  FLUXNET: A New Tool to Study the Temporal and Spatial Variability of Ecosystem-Scale Carbon Dioxide, Water Vapor, and Energy Flux Densities , 2001 .

[18]  Jeffrey A. Andrews,et al.  Soil respiration and the global carbon cycle , 2000 .

[19]  Jean-Pierre Wigneron,et al.  Monitoring of water and carbon fluxes using a land data assimilation system: a case study for southwestern France , 2010 .

[20]  Jean-Pierre Wigneron,et al.  Modelling LAI at a regional scale with ISBA-A-gs: comparison with satellite-derived LAI over southwestern France , 2009 .

[21]  M. Reichstein,et al.  Temperature dependence of organic matter decomposition: a critical review using literature data analyzed with different models , 1998, Biology and Fertility of Soils.

[22]  J. Calvet,et al.  Heat and water exchanges of fallow land covered with a plant-residue mulch layer: a modelling study using the three year MUREX data set , 2001 .

[23]  A. Holtslag,et al.  Effect of open-path gas analyzer wetness on eddy covariance flux measurements: A proposed solution , 2008 .

[24]  J. Deardorff.  Efficient prediction of ground surface temperature and moisture, with inclusion of a layer of vegetation , 1978 .

[25]  J. Noilhan,et al.  MUREX: a land-surface field experiment to study the annual cycle of the energy and water budgets , 1999 .

[26]  Vincent Rivalland,et al.  Transpiration and CO 2 fluxes of a pine forest: modelling the undergrowth effect , 2005 .

[27]  D. Epron,et al.  Soil CO2 efflux in a beech forest: dependence on soil temperature and soil water content , 1999 .

[28]  Jean-Pierre Wigneron,et al.  An interactive vegetation SVAT model tested against data from six contrasting sites , 1998 .

[29]  J. Singh,et al.  Plant decomposition and soil respiration in terrestrial ecosystems , 1977, The Botanical Review.

[30]  C. Potter,et al.  Interannual variability in global soil respiration, 1980–94 , 2002 .

[31]  D. Perrin,et al.  Modelisation de la reponse des flux de respiration d'un sol forestier selon les principales variables climatiques. , 2004 .

[32]  Clément Albergel,et al.  Evaluation of the observation operator Jacobian for leaf area index data assimilation with an extended Kalman filter , 2010 .

[33]  Ü. Rannik,et al.  Productivity overshadows temperature in determining soil and ecosystem respiration across European forests , 2001 .

[34]  M. Kirschbaum,et al.  The temperature dependence of soil organic matter decomposition, and the effect of global warming on soil organic C storage , 1995 .

[35]  J. Lloyd,et al.  On the temperature dependence of soil respiration , 1994 .

[36]  Jean-Christophe Calvet,et al.  Investigating soil and atmospheric plant water stress using physiological and micrometeorological data , 2000 .

[37]  J. Mahfouf,et al.  Inclusion of Gravitational Drainage in a Land Surface Scheme Based on the Force-Restore Method. , 1996 .