3.1.1 Climatic parameters

According to the two above-mentioned parameters that influence the Moroccan climate, the region of Coastal meseta has been divided in three altimetric zones: the Atlantic coast (0-150m), the lower plateau (150-550m) and the higher plateau (550-1,300m).

All the available data were collected by the Ministere des Eaux et des Forets-Division de Recherche et des Experimentations Forestièrs-Section de Climatologie and they present different characteristics both for the period of observation and for the type of climatic parameters. In Table 10 the available climatic data, divided for the three different zones, are shown.

Regarding the period of observations, two different data sets were available, one corresponding to a historic period and one to a more recent period.

For the historic period (1897-1954) the mean monthly and annual precipitation were available. The data set is very variable in fact they pass from data that cover a one year interval (Casablanca) to those that cover a four years one (Bled Denddoun and Afourer). All the data illustrate a situation that date back to the half first of the last century.

The most recent period data (1933-1963) consist in minimum temperature, maximum temperature and mean annual precipitation for the same stations located in a large area near Ben Slimane.

The stations of the two different periods are not always the same ones.

Table 10 - Available climatic data for the region of Ben Slimane

The wet season starts in November and finishes in January; the precipitation reaches in December the highest value of 80mm on the Atlantic coast and on the lower plateau and the value of 65mm on the higher plateau. In general the mean annual precipitation ranges from 400 to 570mm on the Atlantic coast, from 420 to 580mm on the lower plateau and from 320 to 790mm on the higher plateau (Afourer and Oulmes areas respectively). This last greater range is mainly due to the different altitudes in the higher plateau, in fact Oulmes and Afourer are respectively located at 1259 and 450m. The available precipitation values of the stations were interpolated, by Surfer software using a Krieging algorithm, to obtain a spatial distribution of the precipitation and to have a general overview of Coastal meseta. Figure 11 shows the isohetes of the area, at 50mm distance.

Figure 11 - Distribution of the mean annual precipitation

The historic precipitation data and the data of the more recent temperatures were analysed together using the Bagnoul and Gaussen diagrams. In these diagrams, also called ombrothermic diagrams, the dry period corresponds to the interval of the months in which precipitation values are smaller than twice the temperature values (P<2T). From the analysis of the ombrothermic diagrams (Figure 12) it is evident that the dry period has the same length in all the three analysed areas, more or less from May to October. The intensity of the water deficit period is similar on the lower and on the higher plateau but it is smaller in the Atlantic area.

For plant growth, temperature is one of the principal parameters. Analysing the data of the period 1933-1963 the mean minimum and maximum temperatures for the two six month periods (Table 11) were calculated.

From the comparison of the above-mentioned table with Table 12, that illustrates the cardinal temperatures for two major groups of cultivation, it is possible to deduce that the study area is mostly suitable for cereal crops, like durum wheat, wheat and barley.

Figure 12 - Bagnoul and Gaussen diagrams

Table 11 - Mean maximum and minimum temperatures

Table 12 - Cardinal temperatures of some crops

Climatic parameters of study area

For the study area, beside the data collected by the Ben Slimane station, the available data derive from another two stations: Ain Tizgha and Bir Guettara. The first station is located in the central part of the study area to the East of Ben Slimane, the second one is not far and to the South. For Ain Tizgha station the minimum and the maximum temperatures for the period 1981-1993 and the monthly mean precipitations for the period 1973-1998 were available. For the Bir Guettara station only the monthly mean precipitations for the period 1969-1999 were available, because 41% of monthly mean precipitation were missed for Ain Tizgha and 34% for Bir Guettara. Before analysing the climatic parameters the missing data were estimated by regression from the original data set.

Table 13 shows in detail some of the climatic data collected for the study area: the mean monthly precipitation, the annual precipitation and the rainy days.

From the analysis of the rainy days it is evident that the maximum number of rainy days is in January and February and that Ain Tizgha has more rainy days than Bir Guettara from November to February. The yearly mean precipitation varies from 320mm in Bir Guettara to 480mm in Ben Slimane but this data is not very indicative of the present situation because Ben Slimane data date back to the historic period. The minimum monthly precipitation in Ain Tizgha and Ben Slimane is 1.3 and 4mm respectively in August and June, while the maximum precipitations are 71 and 98mm in December.

The temperature range is shown in Table 14 where it can be seen that January and February are the coldest months, while July and August are the hottest ones. The temperature varies from 12.2°C in January at Ben Slimane to 24.7°C in August at Ain Tizgha with an annual mean of 18°C at Ben Slimane and 19°C at Ain Tizgha.

Table 13 - Climatic parameter of the stations in the study area

Table 14 - Monthly and yearly temperatures

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