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3 Land Resources


3.1 Climate

Climate constitutes one of the most important natural resources that strongly influence any sustainable development action; in Senegal as in the other sahelian countries, this natural resource is a limiting factor. Indeed, the climate changes and variability, expressed essentially in rainfall regime fluctuations and temperatures increase, lead to a continuous degradation of the other natural resources. In this chapter, a general description of the Senegal’s climate will be given, with an overview on the climate of the study area followed by its classification. Unfortunately, the data used for this last task correspond to an old climatological period, which is not very suitable to give the most recent classification. Because of the difficulties in obtaining recent climatic data, old available data was used. Two main data sets were collected from two different sources. The first one was downloaded from the National Climatic Data Centre (NCDC) of NOAA web site (www.ncdc.noaa.gov/oa/ncdc) and it comprises the monthly means rainfall of 21 stations for 30 years (1951-1980). The second one was extracted from agroclimatological data of Africa published by the FAO in 1984. It contains monthly means rainfall, evapotranspiration, water vapour pressure, total radiation and monthly mean of the three temperature types (minimum, maximum and average). However, the climatologic period for those data was not specified, but they are already corrected. Those from the NCDC climate data set are raw data and contained some missing data that were estimated by applying the cumulative frequency method on the original data set. After data correction, the mean data was used with Surfer software to draw the isohyets and isotherms maps of the overall country. With those collected data, the climate of the study area was classified. Unfortunately, only one station (Thiès station), with the complete parameters needed for the classification, was present in this area. Some final products were also collected from the Sénégal Républic official web site (www.gouv.sn/meteo).

3.1.1 General description

Senegal belongs to the sahelian part of Africa, where the climate is generally characterised by drought and hot temperatures. Nevertheless, in the case of this country, the drought is slightly mitigated by the presence of a coastal façade (700km). This climate has two main seasons:

The principal climatic features are derived from both geographic and aerological factors. The influence of the geographic factors is expressed on one hand, by the latitude that gives to the climate its tropical characteristics and on the other hand, by the West Africa geometric shape which determines different climatic conditions from the littoral towards the continent. The aerological factors involve three kinds of flux (monsoon, harmattan and the local maritime trade wind from Açores anticyclone), that move easily because of the almost flat terrain.

Monsoon is the wet south-west trade wind from Sainte-Helène anticyclone that blows during the rainy season. Harmattan is a dry and hot trade wind from Sahara that blows during the dry season. The maritime trade wind from Açores anticyclone, permanently wet, blows on the coastal part of the country all the year.

The rainfall regime is characterised by a decreasing gradient from south to north that varies from 1250 mm at Ziguinchor, in the southern part, to 220mm at Podor, in the northern part (Roux and Sagna, 2000). An inter-annual irregularity of this rainfall regime was observed during these last years and this variability is shown by the progressive motion of the isohyets towards the south, about 120km between 1971 and 1990 (Centre de Suivi Ecologique - CSE, 2000). The annual rainfall compared to the average one (1961-1993) shows an irregular variation, which is expressed by prolongation of the dry periods since 1979 (Figure 5).

Figure 5 - Seasonal rainfall anomalies in Senegal (1961-1993).

The precipitation spatial repartition allows dividing the country in two main climatic regions on both sides of the isohyets 500mm (Figure 6):

Figure 6 - Mean rainfall in Senegal (1951-1980).

Temperatures follow the seasonal rhythm. The minimal temperatures occur in January, whereas the maximal temperatures are recorded during the rainy season. The thermal gradient grows from the coast towards inland (Figure 7). Indeed, for Saint-Louis and Matam stations, the maximum average temperature varies from 30.4ēC to 37.1ēC whereas the minimum average temperature ranges from 20.0ēC to 21.8ēC (Table 6).

3.1.2 The climate of the study area

Marine currents are mainly influencing the climate of Thiès; the area is situated in a transitional zone that is subjected to the influence of the maritime trade winds and harmattan. This climate is also characterised by an average temperature of 32°C and a rainy season with a total rainfall ranging from 400 to 700mm. To classify such a climate, we resorted to Gaussen-Bagnouls climatic classification and Thornthwaite model.

Gaussen-Bagnouls classification method

This classification type is a simple one based on the combination of average monthly temperature and total rainfall. It gives more precise climatic classification and its rationality allows also an easily climatic identification by determining separately the numbers of dry and wet months. The Gaussen common aridity index is defined in the way as the dry, or arid month, corresponds to the month having the ratio between precipitation (P) and temperature (T) less than two. These two parameters are plotted as an ombrothermic chart on the same graph doubling the values on the scale of precipitation.

The months, in which the mean temperature curve is higher than the precipitation one, are considered as dry. In the case of the study area, the available data are from Thiès station, the only available in the area. The ombrothermic chart (Figure 8) analysis gives eight dry months that corresponds to a semi-arid climate in the Gaussen-Bagnouls classification system.

Figure 7 - Average temperature in Senegal (1951-1980).

Stations 

Mean of annual total (mm) 

Temperature

(ēC) 

Sunshine Duration (%) 

Total radiation (W/cm2) 

Total length of rainy season (days) 

Rainfall 

Eto 

Tmax 

Tmin 

Tave 

Bambey 

672.0 

2041.0 

34.7 

18.4 

26.6 

66 

959 

97 

Dakar Yoff 

578.0 

1737.0 

27.6 

20.9 

24.3 

69 

468 

86 

Diourbel 

700.0 

1651.0 

36.1 

19.8 

30.9 

66 

459 

106 

Guede 

311.0 

2116.0 

35.2 

19.2 

27.3 

76 

493 

49 

Kaolack 

798.0 

1876.0 

36.0 

20.3 

28.2 

70 

475 

114 

Kédougou 

1269.0 

1663.0 

35.0 

22.5 

28.8 

71 

478 

156 

Kolda 

1256.0 

1779.0 

34.7 

20.1 

27.4 

72 

483 

148 

Linguere 

536.0 

1690.0 

36.6 

20.5 

28.6 

67 

460 

90 

Matam 

535.0 

1675.0 

37.1 

21.8 

29.5 

75 

491 

94 

Podor 

336.0 

1859.0 

36.4 

20.9 

28.7 

75 

486 

57 

Saint-Louis 

346.0 

1724.0 

30.4 

20.0 

25.2 

64 

448 

63 

Tambakounda 

883.0 

2117.0 

35.5 

21.3 

28.1 

61 

441 

129 

Thiès 

694.0 

1828.0 

32.1 

19.2 

25.7 

71 

478 

103 

Ziguinchor 

1547.0 

1896.0 

33.8 

19.9 

26.9 

55 

421 

146 

Table 6 - Climatic parameters in some stations of Senegal (FAO, 1984).

Figure 8 - Gaussen-Bagnouls diagram of Thiès.

Thornthwaite classification model

Directed mainly towards agricultural zones delimitation, Thornthwaite classification (FAO, 1984) is based on the rainfall efficiency index which compares directly the entering water (P) in the climatic region with the exiting water (i.e., the atmosphere evaporative demand called ETP or Eto). Two basic variables are derived from this comparison: the deficit (D) when P<Eto, and the excess (W) when P>Eto. Therefore this model gives more information about the aridity (or humidity) of a given climate according to the class in which its aridity (or humidity) index is ranged. The aridity index (Id = (D/Eto) ´ 100) values range between 0 to 100 while the humidity index (Iw = (W/Eto) ´ 100) values go beyond 100. These two indices are combined in a short one called moisture index: Im = ((W-D)/Eto) ´ 100). In the case study, Thornthwaite moisture index (Im) is equal to –59.84 that belongs to the interval from -66.0 to -33.3 and which corresponds to a semi-arid climate according to Thornthwaite climate classification system (Table 7, Table 8).

Climatic classes 

Climatic description 

Im ranges 

   

Rainfall (mm) 

Eto (mm) 

W

(mm) 

D

(mm) 

Hyperhumid 

>100 

 

January 

163 

 

163 

B4 

Humid 

80-100 

 

February 

162 

 

160 

B3 

Humid 

60-80 

 

March 

219 

 

219 

B2 

Humid 

40-60 

 

April 

202 

 

202 

B1 

Humid 

20-40 

 

May 

181 

 

179 

C2 

Humid/subhumid 

0-20 

 

June 

24 

152 

 

128 

C1 

Subhumid/subarid 

-33.3-0 

 

July 

122 

143 

 

21 

Semi-arid 

-66.0-(-33.3) 

 

August 

273 

120 

153 

 

Arid 

-100-(-66.610

 

September 

206 

123 

83 

 
       

October 

57 

125 

 

68 

       

November 

119 

 

116 

       

December 

119 

 

114 

       

Total 

694 

1728 

236 

1270 

Table 7 (left) - Major climatic classes according to the moisture index of Thornthwaite.

Table 8 (right) - Thiès climate description by Thornthwaite classification model.


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