Combined Uses of Supervised Classification and Normalized Difference Vegetation Index Techniques to Monitor Land Degradation in the Saloum Saline Estuary System - The Land/Ocean Interactions in the Coastal Zone of West and Central Africa - page 59

Fig. 7 Interannual variation in
salinity and rainfall at Kaolack
locality from 1927 to 2012

salinity

Precipitation

120

1400

1200

100

1000

80

800

60

600

40

400

20

200

0

0

salinity level in estuary system. This is the case for the inverse
estuary of the Sine Saloum where river waters with salinities
much higher than seawater salinity occur (Fig. 7 ). In this
region, the climate is characterized by an extended dry season,
cool from November to March and warm from April to June,
and by a short wet and warm season from July to October.
Since the 1920s, the annual rainfall has decreased in this
region with variable magnitude and drought period is much
pronounced in recent decades (Pagès and Citeau 1990 ). The
combined effects of reduced freshwater inputs, intense
evaporation, and a low slope in the lower estuary have resulted
in an overall high salinity and an inversion of the salinity
gradient upstream the Saloum River course. This character-
istic occurs to a lesser extent in the Diomboss branch.
The intermittent and the reverse flows of the Saloum
River due to the runoff deficit caused saltwater advance up
to 60 km upstream, contaminating surface water, ground-
water, and large areas of agricultural soils located in these
zones. Salinity in the Saloum River showed an upstream
gradual increase from 36.7 % at the mouth to higher than
90 % at Kaolack (Pagès and Citeau 1990 ).
The chemical and isotopic data of water sampled in the
Saloum River estuary (Faye et al. 2003 , 2004 , 2009 ; Dieng
2012 unpublished) revealed that high salinity is induced by
seawater advance through tide dynamic. During the dry
season (December to May) where maximum air temperatures
and evaporation occur, seawater advance may reach 90 km
inland (Kaolack locality) and salinity as high as 60 g/l (Dieng
2012 unpublished). Consequences of this high salinity are
contamination of the shallow groundwater resources, large
areas of arable land with formation of saline barren soils at
the vicinity of the estuary system and the economically
valuable mangrove ecosystem which plays a vital role to the
majority of this rural population. The saltwater contamina-
tion constitutes a serious problem in this region.
Higher salinity content in the upstream estuary has also
consequences on vegetation and soil resources in that the
mudflat has been affected by these hydroclimatic variations.
The morphopedological modifications induced by a high

y ¼ 1 : 47x 0 : 43
R 2 ¼ 0 : 98 for NDVI image pair 1992 ð y Þ 1984 ð x Þ

ð 7 Þ

y ¼ 0 : 73x þ 0 : 29
R 2 ¼ 0 : 98 for NDVI image pair 1999 ðÞ 1992 ð x Þ

ð 8 Þ

y ¼ 1 : 7x þ 0 : 02
R 2 ¼ 0 : 98 for NDVI image pair 2010 ð y Þ 1999 :

ð 9 Þ

For the NDVI image pairs 1992 (y) - 1984 (x) and 2010
(y) - 1999 (x), the slope coefficients, respectively, 1.4 and
1.7 reflect that the NDVI for the unchanged pixels increases.
For the NDVI image pair 1999 (y) - 1992 (x), the NDVI for
the unchanged pixels decreases due to precipitation increases
in 1999, which is reflected in the 0.73 slope coefficient. These
equations were used to predict the 1992 NDVI, the 1999
NDVI, and the 2010 NDVI images, respectively, from the
1984, 1992, and 1999 NDVI images. The three NDVI dif-
ference images were generated by subtracting the predicted
NDVI from the actual NDVI.
In the period 1984-1992, 73.1 % of the study area
showed an increasing NDVI, in 1992-1999 81 %, and in
1999-2010 79.5 %. Crossing these results with precipitation
values for the same periods (Fig. 6 ), we can see that the
increase in NDVI is caused by the increase in rainfall levels.
Although the rainfall levels increased in those periods, the
location of the areas that show decreasing NDVI is located in
zones occupied by tan around the Saloum River due to the
high salinity of the water river (Figs. 3 and 6 ). In fact, the
land-cover maps show that the majority of the transitions
that show NDVI are transition to decreasing tan.

Naturals Factors of Estuary's Dynamic

Climate change has been particularly evident in west Africa in
the last 30 years. Increased drought has led to a significant
decrease in freshwater flow as well as an increase in the