Inter-annual memory effects between soil moisture and NDVI in the Sahel

The Sahel lies in the semi-arid zone of North Africa between the Sahara Desert to the north and the savannas and rainforest to the south. In the Sahel, the variability of vegetation growth is predominately controlled by moisture reception in the concurrent rainy season. The inter-annual variability of normalized difference vegetation index (NDVI) during the rainy season (April to August) can be explained very well by soil moisture level within the same time period (with highest r≈0.86). However, once the primary effect of soil moisture for the concurrent period is removed, the residual inter-annual variability in NDVI is hypothesized to be able to highlight a memory effect from previous months or years. The objectives of this thesis is to identify and quantify such a memory effect between soil moisture and NDVI at sub-continental level, as well as to better describe the influence of land cover types as well as soil types on the identified memory effect. The data sets used in this study are as follows: (1) a 27-year NDVI time series from the Global Inventory Modelling and Mapping Studies (GIMMS) group, (2) a 27-year Modelled Soil Moisture data set provided by the Climate Prediction Centre (CPC) of the NOAA National Centres for Environmental Prediction (NCEP). (3) land cover map from Global Land Cover (GLC) 2000 and (4) Global Soil Regions map provided by the Natural Resources Conservation Service (NRCS) of United States Department of Agriculture (USDA). Results indicate a 7-to-9 month time lags exist in western part of the Sahel (among countries i.e. Mauritania, Senegal, Mali, Guinea, and Ghana), as well as in the central to eastern parts of the Sahel (among countries i.e. Niger, Chad, Cameroon, Central African Republic, and Sudan). Other results have shown land covers with crops and open grassland with sparse shrubs are more sensitive to soil moisture changes than shrubs. Soil types with high water-holding capacities, such as Alfisols and Vertisols, if present in the environment, will ensure that shallow-rooted plants (i.e. Crops and grasses) will be intolerant to low soil moisture content 7-to-9 previous, especially in the dry years.