Climate context of the
watershed
Annual precipitation
Precipitation decreases towards the SE where the station of Anoceur is
located. It has an average of 504 mm, lower than that of Sefrou (517.2
mm), and that of Allal El Fassi dam (335.3 mm). This can be explained by
the orographic effect of the sequences of high ridges. The annual
average is 361 mm in Azzaba, 396.2 mm in Fez, 407.4 mm in Ain Timedrine.
Thus, from the results obtained for the 5 stations, it can be deduced
that the average rainfall for the sector studied is 417.8 mm, for the
period between 1957/58 and 2009/10.
Average monthly
precipitation
In the Sefrou watershed, there was a maximum average monthly rain
(figure 4) of 54.92 mm in December, which makes this month the wettest
of all the stations, while July is the driest. with an average rain of
3.86 mm, this will directly influence the runoff flows.
Assessment of the blade of water fallen on the
watershed
The fallen layer of water can be estimated by several methods, making it
possible to integrate into point data relating to the different
stations, a surface result, which makes it possible to estimate spatial
precipitation. ‘Thiessen polygon’ is the applied methodology, as a
statistical method, which amounts to carrying out the weighted average
of the precipitation measured in the rainfall stations of the watershed.
The influence area of each station measurement “called the Thiessen
polygon” is the weighting factor. These polygons are obtained
graphically (figure 5), by plotting the perpendicular bisectors of the
segments connecting two neighboring rainfall stations, on a geographic
or topographic map.
Isohyet method
To make interpretation of precipitation easier, the map of isohyets
shows that there is a progressive precipitation gradient between the
different stations. From this figure, we notice that the rainfall
gradient increases from North to South in the direction of high
altitudes (figure 6).
Thus, precipitation becomes significant at the stations located at high
altitudes. We note that some summer months are characterized by the
predominance of rain mainly linked to the frequency of local
thunderstorms. These are due to the strong insolation on the ground
which sometimes gives rise to updrafts which causes the formation of
stormy precipitation.
Estimated mean of the water slide calculated by the three
methods
In order to reconcile between the three different values found by each
method (table 1), and to quantify the volume of the blade of water
fallen on the basin, it is recommended to calculate an average between
them. Annual volume of precipitated water (in m3), for
the Sefrou watershed, can be estimated as follows:
V = 183, 96\(.10^{6}m^{3}\)/year
Evaporation, Evapotranspiration and flow
deficit
Real Evapotranspiration
(ETR)
Actual evapotranspiration (ETR) is the sum of the quantities of water
evaporated by the soil and by plants when the soil is at its current
specific humidity and the plants at a stage of real physiological and
sanitary development under real conditions and taking into account the
water available.
It turns out that the actual evapotranspiration is very important, it is
calculated from the average of the three methods (Thornthwaite, 1948;
Coutagne,1954; Turc, 1955) is 389.22 mm for the entire watershed, whose
annual average precipitation represents a value of 454.22mm. The
percentage of loss represents 85.69% of inflows from the water system
having a volume close to 161.28 million m3, while the
rest will represent the volume of water drained and the amount of
underground infiltration, the latter reaching 14, 31% with a value of
22.68 million m3 /year.