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TT4: EOP Integrative studies on the Niger meso-scale site
TT-leaders: Luc Descroix, Bernard Cappelaere, Guillaume Favreau, Thierry Lebel
Detailed information: TT4 document: EOP Integrative studies on the Niger meso-scale site (latest update: 03.04.2006, pdf, 0.8MB)
Scientific justification and objectives
The lasting drought of the 1970's and 1980's over West-Africa was one motivation for setting up the AMMA project. This drought was especially severe in the Sahel, remaining unabated until 1997. At the same time the population growth remains strong (and in Niger, the growth rate is increasing), causing a severe land use change and in most parts, a generalized soil degradation.
One consequence of the drought was a significant reduction of the large river discharges: the Senegal river annual runoff decreases by 60% (1971-90 vs 1951-70) and the Niger river's by 55%, to be compared with an average rainfall deficit of 25-30%. Dust transport and aeolian erosion also increased significantly. On the other hand, it is now well known that at smaller scales, the water cycle does not necessarily reflect changes witnessed at the regional scale. The number and duration of temporary pools for instance has significantly increased over the past 30 years. Larger runoff leading to this new hydrological state of Sahelian areas is a natural consequence of the degradation of the vegetation. However there is no well established scheme allowing relating this increase in local runoff to the decrease of the large river discharges. The degradation of the vegetation cover has also resulted in a general stage of active erosion involving soil degradation, and a strong silting up of valley bottoms. This causes significant modifications of the through the modification of the hydrological environment: ponds migration, appearance of spreading areas, changes in stream course and location.
In this respect,
AMMA is an unique opportunity to address the scale problems inherent to the
understanding of the complex relationships between climate, environment and
the water cycle. The Niamey area has a rich history of hydrological observations
carried out since the Hapex-Sahel experiment established in 1990 and prolonged
by the CATCH observing system. Given this long history of observations and scientific
results on rainfall distribution, runoff processes, aquifer replenishment and
coupled hydrological/vegetation modelling, the HAPEX-Sahel site has naturally
become one of the three meso-scale sites of the AMMA observing system. Located
between 13°N and 14°N, this Niger "meso-site" is central between
the Ouémé Sudanian site (9.5°N-10.5°N) and the dry-sahelian
site of Hombori, in Mali (15°N-17.5°N).