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J. Verbesselt, R. De Jong, W. Dorigo, A. Zeileis, J. Ardö, M. Herold:
"Structural Change in Soil Moisture and Vegetation Activity Trends in the African Sahel";
Talk: GV2M: Global Vegetation Monitoring and Modeling, Avignon; 2014-02-03 - 2014-02-07.

Soil moisture is one of the main drivers of the exchange of water, energy, and carbon between the land surface and the atmosphere. Vegetation plays a large role in this interaction and its response to variations in soil moisture (among other factors) can be measured using satellite sensors. Trends, anomalies and associated ecosystem impacts can now be studied with a combination of long-term soil moisture (1978-2011) and vegetation activity records (1982- 2011). However, the integration of consecutive satellite sensors to create these time series, together with residual cloud and atmosphere effects, can introduce data artifacts and affect trend analysis. As such, the long records require that we consider structural trend shifts and not only monotonic trends. Here, we revisit techniques for the detection of such shifts in vegetation activity and soil moisture observations for the overlapping time period from 1982- 2011. Subsequently, we apply a classification scheme to describe ecologically meaningful change types. The analysis uses satellite-based ESA Climate Change Initiative (CCI) soil moisture and GIMMS NDVI3g time series. Our focus is on the African Sahel, where soil moisture and vegetation growth are tightly linked. The fact that soil moisture drives vegetation growth in semi-arid ecosystems helps to disentangle potential data artifacts from real changes driven by climate and human influences. Monotonic decreasing soil moisture trends have been reported, while rainfall has been increasing since the Sahel drought of the 1970s and early 1980s. By accounting for structural changes in soil moisture and vegetation activity trends, we aim to find answers for the increasing rainfall and decreasing soil moisture ambiguity. Do local trends in vegetation growth, data artifacts, or land degradation explain the negative soil moisture trends?