From the doroughts in the 1960s - 80s onwards, a vegeation greening trend was observed in the Sahel zone by remote sensing studies (Hickler et al., 2005; Herrmann et al., 2005; Anyamba et al., 2005). Hickler et al. (2005) find precipitation to be the controlling factor of this greening trend. However, recent studies find a stagnation of the greening . While Jung et al. (2010) The limiting factor seems to be soil moisture (Jung et al., 2010; Yan et al., 2013). This trend has been linked to global climate effects (Yan et al., 2013). (citation not found: doi:10.1016/j.cosust.2012.06.006.)
Is the greening trend still stagnating in the Sahel zone? Can different effects be identified in different regions?
What is the relation between vegetation greening and soil moisture?
Is the relation of soil moisture and vegetation activity changing, and is this causing shifts in vegetation activity?
The analysis will be based on time series of different datasets: remote sensed NDVI, tree cover and soil moisture and interpolated evapotranspiration, rainfall and soil type data. Breaks in trends will be detected following the methodology of de Jong et al. (2013).
Explore the relationships of NDVI and soil moisture, evapotranspiration and soil moisture and NDVI and evapotranspiration.
Find temporal breaks / trend in this relation. Identify regions with different dynamics.
(Optional: Look for spatial correlation in the observed breaks.)
Tree cover and soil type grids could be used for stratification. Climate variables could be used to confirm the trends visible in the soil moisture data, and NDVI data could be validated by using different satellite products. Field measured data from the TROBIT project could be used to validate some of the findings / datasets. Climate data combined with evapotranspiration data can be used to identify regions with demand vs. supply limited evapotranspiration. This can be used to judge the importance of soil moisture.
GIMMS NDVI3g time series (1981-2011)
Gridded soil moisture time series merged from active/passive microwave observations 1978-2010 Liu et al. (2012).
Gridded terrestrial evapotranspiration time series from interpolated fluxtower measurements and regression of remote sensed radiation 1982 - 2011 (Jung et al., 2010).
Ground data for validation: TROBIT field data.
Further: Soil maps (ISRIC), Precentage of tree cover (MODIS), Climate data (CRU), SST indices
What does the relation SM - NDVI mean physically. To what degree SM limits vegetation?
what is the advantage of using SM over e.g. precepitation.
Thomas Hickler, Lars Eklundh, Jonathan W. Seaquist, Benjamin Smith, Jonas Ardö, Lennart Olsson, Martin T. Sykes, Martin Sjöström. Precipitation controls Sahel greening trend. Geophysical Research Letters 32, L21415 (2005). Link
Stefanie M. Herrmann, Assaf Anyamba, Compton J. Tucker. Recent trends in vegetation dynamics in the African Sahel and their relationship to climate. Global Environmental Change 15, 394–404 (2005). Link
A. Anyamba, C. J. Tucker. Analysis of Sahelian vegetation dynamics using NOAA-AVHRR NDVI data from 1981–2003. Journal of Arid Environments 63, 596–614 (2005). Link
Martin Jung, Markus Reichstein, Philippe Ciais, Sonia I. Seneviratne, Justin Sheffield, Michael L. Goulden, Gordon Bonan, Alessandro Cescatti, Jiquan Chen, Richard de Jeu, A. Johannes Dolman, Werner Eugster, Dieter Gerten, Damiano Gianelle, Nadine Gobron, Jens Heinke, John Kimball, Beverly E. Law, Leonardo Montagnani, Qiaozhen Mu, Brigitte Mueller, Keith Oleson, Dario Papale, Andrew D. Richardson, Olivier Roupsard, Steve Running, Enrico Tomelleri, Nicolas Viovy, Ulrich Weber, Christopher Williams, Eric Wood, Sönke Zaehle, Ke Zhang. Recent decline in the global land evapotranspiration trend due to limited moisture supply. Nature 467, 951–954 (2010). Link
Hao Yan, Qin Yu, Zai-Chun Zhu, Ranga B. Myneni, Hui-Min Yan, Shao-Qiang Wang, Herman H. Shugart. Diagnostic analysis of interannual variation of global land evapotranspiration over 1982–2011: Assessing the impact of ENSO. Journal of Geophysical Research: Atmospheres n/a–n/a (2013). Link
Rogier de Jong, Michael E. Schaepman, Reinhard Furrer, Sytze de Bruin, Peter H. Verburg. Spatial relationship between climatologies and changes in global vegetation activity. Global Change Biology 19, 1953–1964 (2013). Link
Y. Y. Liu, W. A. Dorigo, R. M. Parinussa, R. A. M. De Jeu, W. Wagner, M. F. McCabe, J. P. Evans, AIJM Van Dijk. Trend-preserving blending of passive and active microwave soil moisture retrievals. Remote Sensing of Environment 123, 280–297 (2012). Link