Modelling and observation of biosphere–atmosphere interactions in natural savannah in Burkina Faso, West Africa Rüdiger Grote a, * , Emmanuel Lehmann a , Christian Brümmer a , Nicolas Brüggemann a , Jörg Szarzynski b , Harald Kunstmann a a Forschungszentrum Karlsruhe, Institute for Meteorology and Climate Research, Kreuzeckbahnstraße 19, 82467 Garmisch-Partenkirchen, Germany b Center for Development Research (ZEF), Department of Ecology and Resource Management, University Bonn, Walter-Flex-Straße 3, 53113 Bonn, Germany article info Article history: Received 19 November 2007 Received in revised form 12 April 2008 Accepted 9 May 2008 Available online 22 May 2008 Keywords: West Africa Savannah Soil water content Soil temperature Energy balance N 2 O emission abstract Savannahs are highly dynamic ecosystems but many of their properties and the related balances of energy, carbon, nitrogen, and water are still poorly understood. A particular scientific issue is the quan- tification of trace gases emitted from the soil of savannah ecosystems and their interaction with regional and global climate and air chemistry. Therefore it is important to develop and evaluate land-surface mod- els that on the one hand represent vegetation and soil dynamics and on the other hand provide energy and water fluxes in a temporal resolution suitable for the application in climate/air chemistry models. In this paper, we present a consistent coupling between a common land-surface model (OSU) and a widely used biogeochemical model (DNDC) that is a first step for a full coupling of climate/air chemistry and biogeochemical processes. For consistency reasons, both models are linked to a general physiologi- cally based plant model to provide the physical boundary conditions as well as the carbon and nitrogen in- and output variables. Evaluation is carried out with measurements of soil temperature, latent heat flux, soil water content, and soil emission data from two vegetation periods collected at a natural grass- land site in Bontioli Nature Reserve, Burkina Faso (Africa). The results demonstrate that simulations of biogeochemical processes based on soil environmental con- ditions, calculated either with the land-surface model or with the unchanged biogeochemical model, do not differ significantly from each other. The OSU model simulates more realistic day-to-day variation of soil temperature as DNDC but the sensitivity of the biogeochemical simulation to this variation is small. In contrast, the sensitivity to differences in soil water content is high, but simulation results of both mod- els are very similar on the daily scale and hardly depend on spatial soil resolution. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction and objectives Savannah is a woodland ecosystem characterised by the coexis- tence of grass and trees competing for water and light. Tropical savannahs cover an area of 17 Â 10 6 km 2 worldwide, which is approximately 20% of the continental surfaces (Scholes and Hall, 1996). The properties of this system are highly dynamic and influ- ence the regional as well as the global climate. For regional climate simulations, this means that the exchange of water and energy be- tween land and atmosphere has to be represented. In addition, glo- bal climate and air chemistry modelling is influenced by net carbon exchange and nitrous oxide emission (Mosier, 1998; Betts, 2007). Both have been shown to be of considerable amount and highly dy- namic in savannah ecosystems (Castaldi et al., 2006). It is already known that these emission strongly depend on land cover, land use, and climate (Elberling et al., 2003; Tao and Jain, 2005; Tews et al., 2006). All of these impacts are currently subjected to fast changes in western Africa i.e. because of increasing human pres- sure on land use and regional hydrology (e.g. Ouattara et al., 2006; Kunstmann and Jung, 2007; Kunstmann et al., 2007) as well as accelerating climate change (Caminade et al., 2006). Thus, it is of utmost importance to understand and represent these mecha- nisms in regional models (see also Foley et al., 1998). However, the dynamics of physical properties as well as carbon-, nitrogen-, and water exchange with the atmosphere in savannah ecosystems are still poorly understood. A major difficulty develops from the fact that hardly any field study captures all of the major processes influencing energy and matter exchange be- tween biosphere and atmosphere. In contrast, process based models include a multitude of dependencies between physical conditions, water fluxes, plant development, and eventually carbon and nitrogen dynamics (Scholes and Archer, 1997). This informa- tion is hardly available for regional simulations which have to rely on the appropriate estimation of state variables. Therefore, extensive evaluations and sensitivity tests are required and 1474-7065/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2008.05.003 * Corresponding author. Tel.: +49 08821 183124; fax: +49 08821 183294. E-mail address: ruediger.grote@imk.fzk.de (R. Grote). Physics and Chemistry of the Earth 34 (2009) 251–260 Contents lists available at ScienceDirect Physics and Chemistry of the Earth journal homepage: www.elsevier.com/locate/pce