Browsing by Author "Mauder, Matthias"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Carbon dioxide fluxes from contrasting ecosystems in the Sudanian Savanna in West Africa
    (Carbon Balance and Management, 2015-01) Quansah, Emmanuel; Mauder, Matthias; Balogun, Ahmed; Amekudzi, Leonard
    Background: The terrestrial land surface in West Africa is made up of several types of savanna ecosystems differing in land use changes which modulate gas exchanges between their vegetation and the overlying atmosphere. This study compares diurnal and seasonal estimates of CO2 fluxes from three contrasting ecosystems, a grassland, a mixture of fallow and cropland, and nature reserve in the Sudanian Savanna and relate them to water availability and land use characteristics. Results: Over the study period, and for the three study sites, low soil moisture availability, high vapour pressure deficit and low ecosystem respiration were prevalent during the dry season (November to March), but the contrary occurred during the rainy season (May to October). Carbon uptake predominantly took place in the rainy season, while net carbon efflux occurred in the dry season as well as the dry to wet and wet to dry transition periods (AM and ND) respectively. Carbon uptake decreased in the order of the nature reserve, a mixture of fallow and cropland, and grassland. Only the nature reserve ecosystem at the Nazinga Park served as a net sink of CO2, mostly by virtue of a several times larger carbon uptake and ecosystem water use efficiency during the rainy season than at the other sites. These differences were influenced by albedo, LAI, EWUE, PPFD and climatology during the period of study. Conclusion: These results suggest that land use characteristics affect plant physiological processes that lead to flux exchanges over the Sudanian Savanna ecosystems. It affects the diurnal, seasonal and annual changes in NEE and its composite signals, GPP and RE. GPP and NEE were generally related as NEE scaled with photosynthesis with higher CO2 assimilation leading to higher GPP. However, CO2 effluxes over the study period suggest that besides biomass regrowth, other processes, most likely from the soil might have also contributed to the enhancement of ecosystem respiration.
  • Thumbnail Image
    Item
    Numerical simulation of surface energy and water balances over a semiarid grassland ecosystem in the West African Savanna
    (Advances in Meteorology, 2017) Quansah, Emmanuel; Katata, Genki; Mauder, Matthias; Annor, Thompson; Amekudzi, Leonard K.; Bliefernicht, Jan; Heinzeller, Dominikus; Balogun, Ahmed A.; Kunstmann, Harald
    To understand surface energy exchange processes over the semiarid regions in West Africa, numerical simulations of surface energy and water balances were carried out using a one-dimensional multilayer atmosphere-SOil-VEGetation (SOLVEG) model for selected days of the dry and rainy seasons over a savanna grassland ecosystem in Sumbrungu in the Upper East region of Ghana. Te measured Bowen ratio was used to partition the residual energy into the observed sensible heat flux (�) and latent heat flux (LE) in order to investigate the impact of the surface energy closure on model performance. Te results showed that the model overall reproduced the diurnal changes in the observed energy fluxes, especially the net radiation (Rn), compared to halfhourly eddy covariance flux measurements, for the study periods. Te performance measure in terms of the correlation coefcient (�), centred root mean square error (RMSE), and normalized standard deviation (�) between the simulated � and LE and their corresponding uncorrected observed values ranged between R = 0.63–0.99 and 0.83–0.94, RMSE = 0.88–1.25 and 0.88–1.92, and � = 0.95–2.23 and 0.13–2.82 for the dry and rainy periods respectively, indicating a moderate to good model performance. Te partitioning of � and LE by SOLVEG was generally in agreement with the observations during the dry period but showed clear discrepancies during the rainy period, particularly afer rainfall events. Further sensitivity tests over longer simulation periods (e.g., 1 year) are required to improve model performance and to investigate seasonal exchanges of surface energy fluxes over the West African Savanna ecosystems in more details.