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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 quantification 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 models 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 physiologically 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 grassland site in Bontioli Nature Reserve, Burkina Faso (Africa). The results demonstrate that simulations of biogeochemical processes based on soil environmental conditions, 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 models are very similar on the daily scale and hardly depend on spatial soil resolution. [Copyright &y& Elsevier]

West Africa's Sudanian savannas are one of Africa's high-potential "bread baskets", where surplus food (e.g. millet, sorghum, cowpea and livestock) is being produced. These ecosystems currently undergo pronounced land use changes and will likely face shorter rainy seasons with more extreme rainfall and droughts in the future. This could have serious impacts on the vegetation and its carbon dioxide (CO2) exchange with potentially increasing CO2 emissions accelerating climate warming. Understanding how the CO2 fluxes in this area respond to land use changes and environmental fluctuations, in particular rain events, is essential to design sustainable land use strategies. However, this understanding is hampered by a limited data availability. In this study, we monitored net ecosystem exchange (NEE) of CO2, rainfall and other weather and vegetation parameters during four years (2013-2016) at three savanna sites in southern Burkina Faso and northern Ghana. To this end, a monitoring network of three eddy-covariance stations and nine automatic weather stations was established. The three savannas were characterized by different vegetation due to different land use: i) woody and nearly pristine, ii) a mixture of cropland and grassland and iii) grassland under intensive grazing. Other site characteristics such as climate, soil, or topography chosen were as similar as possible. The impact of rain events on CO2 exchange for these three contrasting ecosystems was analyzed for single rain events (short-term) and on a yearly time scale (long-term). We found that the pristine savanna site was a prominent sink of CO2 (-864 to 605 g CO2 m-2 y-1) while the two degraded sites were net CO2 sources (118 to 605 g CO2 m-2 y-1) with a complex relation with annual rainfall amounts. The NEE responses to single rain events revealed that daytime rain systematically decreased the sink strengths at all sites, which might be associated with decreased light availability. At the degraded sites, additional factors increasing CO2 losses were rain duration and dry spell length. The observed patterns of immediate CO2 flux responses to rainfall at differently used savannas indicate strong internal feedbacks between vegetation and land use changes and raise the question whether the CO2 sink potentials might be overestimated with possible implications for global CO2 budgets. Our findings also suggest that carbon sequestration needs to be taken into account when designing sustainable land-use strategies. [ABSTRACT FROM AUTHOR]

Large-scale experimental sites for studying Critical Zone processes are relatively rare for savannas. However, they are essential for a better understanding of environmental changes and their impacts on ecosystem functioning and services. This study gives an overview of a novel field observatory which was established for mesoscale watersheds in the Sudan Savanna near the border of Burkina Faso and Ghana as part of the WASCAL (West African Science Service Centre on Climate Change and Adapted Land Use) programme. The study area is characterized by severe land cover changes due to a strongly increasing demand of agricultural land. The observatory is designed for long-term measurements of selected eco-hydrological processes in fine temporal resolution focusing on land-atmosphere exchange processes. It consists of a network of automatic weather stations, agro-meteorological stations, eddy covariance towers and various hydrological measurement devices, which are complemented by long-term historical measurements from national meteorological and hydrological networks. A unique component of the observatory is a micrometeorological experiment using the eddy covariance towers which is conducted since the initiation of the observatory in 2013. The experiment was implemented at three contrasting sites with different vegetation cover and contrasting land use practices (pristine savanna, mixture of cropland, degraded grassland) to investigate the impact of land use changes on water, energy, and CO2 fluxes. Moreover, the datasets of the new observatory are the basis of various field, data analysis and modelling studies conducted by other disciplines (i.e. agriculture, biology, ecology, remote sensing) in this region and are made available via a new database, the WASCAL Data Discovery Portal. The new large-scale field experiment also forms an excellent platform for future field campaigns and long-term experimental sites and can be used as foundation for an observatory that addresses key processes and components of the Critical Zone. In this presentation we give a detailed overview about the new field observatory with its different components and its advantages and drawbacks in comparison to other large-scale experimental sites for savanna ecosystems (e.g. AMMA catch observatory). In addition, recent outcomes from the long-term micro-meteorological field experiment are shown. Finally, the current challenges and future perspectives are addressed for operating this type of observatory in this region. [ABSTRACT FROM AUTHOR]