Agriculture, Ecosystems and Environment 139 (2010) 419–453 Contents lists available at ScienceDirect Agriculture, Ecosystems and Environment journal homepage: www.elsevier.com/locate/agee The carbon balance of European croplands: A cross-site comparison of simulation models Martin Wattenbach a,∗ , Oliver Sus b , Nicolas Vuichard c , Simon Lehuger d , Pia Gottschalk a , Longhui Li c , Adrian Leip e , Mathew Williams b , Enrico Tomelleri f , Werner Leo Kutsch f , Nina Buchmann g , Werner Eugster g , Dominique Dietiker g , Marc Aubinet k , Eric Ceschia h , Pierre Béziat h , Thomas Grünwald i , Astley Hastings a , Bruce Osborne j , Philippe Ciais c , Pierre Cellier d , Pete Smith a a University of Aberdeen, Institute of Biological and Environmental Sciences 23 St. Machar Drive, Aberdeen AB24 3UU, UK b The University of Edinburgh, School of GeoSciences and SAGES, The King’s Buildings, West Mains Road, Edinburgh EH9 3JN, UK c Laboratoire des Sciences du Climat et de l’Environnement (LSCE/IPSL) CEA-CNRS-UVSQ (UMR 1572) CE Saclay L’Orme des merisiers, Bât 712 91191 Gif sur Yvette Cedex, France d Agroscope Reckenholz-Tänikon Research Station ART, Air Pollution/Climate Group, Reckenholzstrasse, 8046 Zurich, Switzerland e Joint Research Centre, Institute for Environment and Sustainability, Climate Change Unit (TP 050), I-21020 Ispra (VA), Italy f Johann Heinrich von Thünen-Institut (vTI), Federal Research Institute for Rural Areas, Forestry and Fisheries, Bundesallee 50, 38116 Braunschweig, Germany g ETH Zurich, Institute of Plant, Animal and Agroecosystem, Universitaetsstrasse 2, 8092 Zurich, Switzerland h CESBIO - Université de Toulouse; UPS, CNES, CNRS, IRD; UMR 5126, 18 Avenue Edouard Belin, 31401 Toulouse Cedex 9, France i Technical University Dresden, Institute of Hydrology and Meteorology, Department of Meteorology, Pienner Strasse 23, D-01737 Tharandt, Germany j UCD School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland k Université de Liège, Physique des bio-systèmes, Avenue de la Faculté 8, 5030 Gembloux, Belgique, UK article info Article history: Received 4 August 2009 Received in revised form 4 August 2010 Accepted 4 August 2010 Available online 25 September 2010 Keywords: Cropland Crop Eddy flux Carbon CO2 Modelling abstract Croplands cover approximately 45% of Europe and play an important role in the overall carbon budget of the continent. However, the estimation of their carbon balance remains uncertain due to the diversity of crops and cropping systems together with the strong influence of human management. Here, we present a multi-site model comparison for four cropland ecosystem models namely the DNDC, ORCHIDEE- STICS, CERES-EGC and SPA models. We compare the accuracy of the models in predicting net ecosystem exchange (NEE), gross primary production (GPP), ecosystem respiration (R eco ) as well as actual evapo- transpiration (ET a ) for winter wheat (Triticum aestivum L.) and maize (Zea mays L.) derived from eddy covariance measurements on five sites along a gradient of climatic conditions from eastern to south- westerly Europe. The models are all able to simulate daily GPP. The simulation results for daily ET a and R eco are, however, less accurate. The resulting simulation of daily NEE is adequate except in some cases where models fail due to a lack in phase and amplitude alignment. ORCHIDEE-STICS and SPA show the best performance. Nevertheless, they are not able to simulate full crop rotations or the multiple management practices used. CERES-EGC, and especially DNDC, although exhibiting a lower level of model accuracy, are able to simulate such conditions, resulting in more accurate simulation of annual cumulative NEE. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Croplands are an important component of the European car- bon balance (Janssens et al., 2003, 2005; Schulze et al., 2009; Kutsch et al., 2010).They cover a large area between 1.10 (EPA- Corine2000) and 1.24 Mkm -2 (Gervois et al., 2008), within the EU27 plus Switzerland, and there have been a number of integrated stud- ies that attempt to quantify, at the continental scale, their carbon ∗ Corresponding author. Present address: Freie Universität Berlin, Institute of Meteorology, Carl-Heinrich-Becker Weg 6-10, D-12165 Berlin, Germany. Tel.: +49 3641 576140; fax: +49 3641 577100. E-mail address: m.wattenbach@abdn.ac.uk (M. Wattenbach). balance. Each previous study has used different approaches, from extrapolation of regional statistics, through remote sensing to mod- elling (Vleeshouwers and Verghagen, 2002; Janssens et al., 2003; Smith, 2004; Smith et al., 2005a; Bondeau et al., 2007; Gervois et al., 2008; Ciais et al., 2009). However, CO 2 emissions are largely determined by the temporal and spatial sequence of human activ- ity and there remains a considerable degree of uncertainty (Smith et al., 2005b; Osborne et al., 2010). Regional- or continental-scale statistics are not consistently available for the entire area of Europe (Ramankutty et al., 2008) and available experimental data are scarce, and come from heterogeneous sources. Remote sensing products lack the accuracy and precision to reflect the degree of temporal and/or spatial heterogeneity of croplands (Reeves et al., 2005; Osborne et al., 2010). In the case of modelling, the data now 0167-8809/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2010.08.004