In: Wagner W., Székely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B 
and seasonal distribution of annual rainfall in the catchment 
areas and the resulting water supply contributed by the Niger- 
Bani system. As it takes some time for the water to run off 
from the catchment areas in the Fouta Julon Mountains 
(Guinea) towards the Niger Inland Delta, the inundation 
occurs with a temporal delay of some months, compared with 
the rainy season. Flooding starts in mid October at the 
southern entry of the Delta and lasts until end of December / 
mid January. 
Figure 1. Niger Inland Delta (RAMSAR site) scale 1:200.000 
http://www.wetlands.org/Reports/Country_maps/ 
Mali/lMLOOl/lMLOOl map .jpg 
From this relation result 2 seasonal variations, a rainy <-> dry 
phase and a flooding <-> drainage phase, as illustrated in Fig. 
2. They appear with a temporal delay of about 3 to 4 months 
and are superimposed by a 3 rd undulation that counts for 
several years (period between dry years and years with 
sufficient precipitation). This latter variation is dominantly 
affected by the 2 seasonal ones, but high spatial variability of 
precipitation does not permit a causal linkage. In particular, 
low amount of rainfall in the delta may profit from extended 
rainfall in the head-waters, thus inducing reasonable extent of 
flooding. 
The availability of water represents the main restricting factor 
for vegetation growth in the Sahel. Vegetation follows the 
above described water cycles with a temporal delay, which 
varies from few days (germination of grasses) up to several 
months (death of trees caused by lack of water). A 
development of (annual) grasslands with sparsely distributed 
patches of shrubby vegetation (dominantly composed of 
Combretaceae sp.) is characteristic for the Sahelian landscape 
(Breman and DeRidder 1991). According to (LeHouerou 
1989), these vegetation pattern can be categorised into the 
following 3 layers (see Fig. 2 for a scheme): 
(a) grass layer with annual grasses and herbs (height 40 cm - 
80 cm) 
(b) shrub layer (height 50 cm - 300 cm) 
(c) tree layer, sparsely distributed single trees (height 3 m to 
6 m) 
Ligneous layers of shrubs and trees cover only small parts (up 
to 25 %) of the surface, while the grass layer extend over up 
to 80%, (KuBerow 1995). Annual grasses are withering 
during dry season, thus grassy layers are affected and/or 
destructed by bush fires and strong winds. Pat-terns of bare 
soil appear as a result, that extend during the mid- and late dry 
season. The generally low vegetation cover therefore 
disappears periodically completely. 
non-flooded ridges 
flooded areas seasonaly flooded 
mayos 
c: Bourgoutiére 
d: Vetiveraie 
e: shrubs and bushes 
- Plllostigma reticulatum 
- Guiera senegalensls 
- Salvadora pérsica 
Figure 2. Subsection of the Niger Inland Delta - landscape 
profile and vegetation pattern, adapted from 
(Diallo 2000) 
3. DATA AND METHODS 
3.1 GIMMS 15-day NDVI composite data 
The GIMMS (Global Inventory Monitoring and Modelling 
Study) NDVI data record combine measurements from several 
satellite sensors. To ensure consistency between the multi 
temporal data, several corrections for a wide range of factors 
that affect the calculation of NDVI values have to be applied. 
According to (Pinzon et. al.,. 2005) GIMMS data are corrected 
for sensor degradation and intercalibration differences, global 
cloud cover contamination, viewing angle effects due to 
satellite drift, volcanic aerosols, and low signal-to-noise ratios 
due to sub-pixel cloud contamination and water vapour. A 
well known fact are the shortcomings of the AVHRR sensor 
design for a vegetation monitoring, as for instance the 
AVHRR channel 2 (nIR) overlaps a wavelength interval in 
which considerable absorption by atmospheric water vapour 
occurs (Steven et al., 2003, Cihlar et. al., 2001). 
This global dataset, known as the GIMMS NDVIg, is the only 
publicly available AVHRR dataset to extend from 1981 to 
2006 (Tucker et. al., 2005). Due to the correction scheme it is 
a dynamic data set, that must be recalculated every time a new 
period of data is added. The Niger Inland Delta and a small