Hydrological
characterization
Data collection
The available mean annual flows are those of the Ain Louali station
(figure 15), with the following geographical coordinates (X=555,650m; Y=
377,000m; Z= 245 m), located downstream of the catchment area towards
the outlet, these data were obtained on the basis of the monthly values
of the flows provided by the Sebou Hydraulic Watershed Agency. The
period runs from 1968 to 1970 and from 1979 to 2005, a period of 29
years.
It should be noted that the converting data for the 1970/79 period are
absent, to resolve this problem and to ensure data continuity, the
values for these nine years have been estimated, following the shape of
the flow curve by statistical correlation methods.
Study of the flow rate
Average annual flow
The series of observations thus taken shows that the average annual flow
is around 454 l/s. The following figure shows the evolution of annual
flows during the period considered(figure 16):
The specific flow or Qsp is a measure of the average flow of
precipitation within a river catchment area. It is defined as the number
of liters of water that flows on average every second per square
kilometer of the basin. Formulation: this is the value of the flow Q
(L/s or m3/s) relative to the surface A of the
watershed (km2):
Qsp = Q / A
The specific flow is also used to express peak flows during floods. In
general, the peak Qsp during floods decreases when the size of the
watershed increases.
Specific flow : Qsp = \(1.12\ l/s{/km}^{2}\)
The highest average interannual flow is 1000 l/s recorded in 1969/1970,
while the lowest is 300 l/s recorded in 1993/1994 and in 2002/2003.
Average monthly flows
The maximum monthly average flow appears in March (390 l/s), on the
other hand the minimum flow is observed in August (325 l/s), the
following hydrogram shows the variation of the average monthly flows in
Ain Louali (figure 17):
This figure shows that the water concentration requires seven months,
from September to March to reach its maximum which represents the peak
in March with a value of 390 l/s. The time of the recession begins from
the end of March to August, a period of five months to arrive at the
minimum quantity of flows. It should be noted that the flood and
recession times are approximate since we work with average monthly
flows.
Specific flows and height of the flow of
water
Many hydrological studies often focus on comparing the hydrological
regimes of different stations or streams. It is therefore advantageous
to give here the specific flows in l/s/Km² for the period from 68/69 to
2003/04. The specific flow formula:
q = Q / S
- q: specific flow in l/s/Km².
- Q: average flow in l/s.
- S: area of the catchment area in Km².
The height of the flow of water in mm is given as a function of q which
is the specific flow rate (Parade, 1974):
- H = 31.536 x q for one year.
- H = 2,419 x q for a month of 28 days.
- H = 2,592 x q for a month of 30 days.
- H = 2,678 x q for a month of 31 days.
The monthly values of the specific flow rate and the height of the flow
of water flow are given in the following table (table 4):
The total volume of water flown is 27.73 mm/year, or approximately 6%
of the average volume of water that fell into the watershed (454.22
mm/year). The H/P runoff coefficient (27.73 / 454.22) is 0.061.
Rainfall-discharge
relationship
The following table (table 5) shows the monthly flows and precipitation
for the period 68/69 to 2004/2005:
When we compare the hydrograph of average monthly flows with that of
average monthly precipitation (figure 18), we see that there is a
synchronism between flows and monthly precipitation. Indeed, the two
curves thus plotted for the same period roughly evolve in the same
meaning and in the same way:
- The decrease in rainfall inputs is accompanied by a late decrease
inflow recorded around the summer months.
- These are maximum precipitation values in December and January
followed by a maximum flow in March, with a delay of one month; time
required for the arrival of significant inputs to the measurement
station which is located near the outlet of the watershed. This
discrepancy can be explained by the lithological nature of the
watershed or by its morphology.
In general, the flow of runoff is one of the most important components
of the outflow from a hydrological regime fed by precipitation.
Final water balance
estimate
The main purpose of any hydrological study is to estimate the water
balance of the region concerned, to establish a database corresponding
to local water reserves, as well as to guarantee integrated management
of these resources. The following table summarizes the results of the
Sefrou watershed water balance assessment (table 6):
According to the table (6), the maximum percentage is that of
evapotranspiration (87.67%), which can be explained by the
high-temperature values during the dry period and by the evolution of
the vegetation in the area.
It is important to underline the fundamental points concerning the study
carried out under this paragraph. The aspects relating to the hydrology
of the Sefrou catchment area bear witness to the Mediterranean
hydrological character where the dry season is strongly pronounced. It
is therefore obvious to deduce that the hydrological drought has been
well marked during the previous 30 years, which has, moreover, been
confirmed by the method of differences in the average interannual flow
rates.
The study of the hydrological regime using the method of determining the
”precipitation-flow” relationship has made it possible to define a
rainfall regime. The mean annual flow is 454 l/s.
The study of the monthly flows shows that there is a similar evolution
with a small delay between the maximum values of the flows and the
monthly flow coefficients show that the Sefrou catchment area is
characterized by an irregular regime. For its part, the study of the
specific flows and the height of the runoff water level made it possible
to estimate the runoff coefficient which is 0.061.
The water balance showed that the runoff estimation represents 6.06% of
total precipitation. As for infiltration, it represents 6.23% of
precipitation.
Sefrou watershed provides real water resources and thus ensures the
supply of drinking water for the inhabitants of the Middle Atlas regions
and more specifically for the habitats of the city of Sefrou. Despite
the usefulness of these important capacities, they were causing risks
represented by winter floods causing human and material damage, which is
solved by the establishment of development projects in the region.