76
M. D. Alba-Rodríguez et al.
continuous. The average duration of a series is more than 50 years and the most
complete ones exceed 200 years.
Next, we must distinguish the part of that precipitation that has generated direct
runoff, which we call effective precipitation, from the part that does not generate
runoff and is not captured by the sanitation system, which we will call abstractions.
The abstractions for the moment do not interest us because they are lost water for
direct runoff, but we will return to them later since they acquire importance in the
integral balance of the system since one part evaporates, another infiltrates soaking
the soil and is used by the plants, and the rest remains on the surface forming puddles.
The runoff produced in the specific case of an urban basin, due to its specific
characteristics, is very different from that originating in rural hydrology. On the one
hand, due to the urbanization process, the surface is altered, making it impermeable
and modifying the natural network of runoff and drainage. On the other hand, these are
small basins with minimum infiltration surfaces, which produce short concentration
times and important flows. It can be simplified by calculating the runoff coefficient
(C) that represents the relationship between the flow of water that runs through the
surface as a consequence of a rainfall event (PEf) and the flow precipitated on it
(PTo), that is, the water of the total rainfall that actually generates surface runoff
once the soil has been completely saturated (see Fig. 6, Eq. 6).
This factor C is not constant, so its determination is approximate, since it varies
according to the magnitude of the rain and the topographic, soil and land use conditions. A sloping area does not respond to the same amount of rain as a plain, nor does
a garden area in front of a road. The runoff coefficient can take values between zero
and one (0 ≤ C ≤ 1). They are determined from the annual values of precipitation
and flow rate and are tabulated in the literature on the subject of surface hydrology.
The following table presents the most common C values used for the calculation of
urban basins (see Table 2).
In any case, the determination of an average run-off coefficient for a uniform area
can be considered. And when an area is made up of different types of land, different
surfaces are calculated as the average run-off coefficient by making a weighted
average of the different run-off coefficients of each of the sub-areas into which the
area under consideration can be divided. In this way, we arrive at the expression of
the weighted runoff coefficient (see Fig. 6, Eq. 7) for an area formed by different
sub-areas with different runoff coefficients.
Based on the fact that the unit of litres/m
2 is equivalent to the precipitation in mm,
the volumes of water are deducted. In other words, one litre spread over a surface
area of one square metre produces a sheet of water one millimetre thick. On the
one hand, if we apply the surfaces considered to PTo, we obtain the total volumes
of precipitation (VTo), and on the other hand, if we apply them to the effective
precipitation (PEf), we obtain the volumes of runoff (VEf). The difference is the
volume of abstractions or losses (Vab) (see Fig. 6, Eqs. 8–10).
The infiltrations
The permeability of soil is its capacity to allow the flow of a fluid through it. This
mechanical property of soils, which is measured by the coefficient of permeability
M. D. Alba-Rodríguez et al.
continuous. The average duration of a series is more than 50 years and the most
complete ones exceed 200 years.
Next, we must distinguish the part of that precipitation that has generated direct
runoff, which we call effective precipitation, from the part that does not generate
runoff and is not captured by the sanitation system, which we will call abstractions.
The abstractions for the moment do not interest us because they are lost water for
direct runoff, but we will return to them later since they acquire importance in the
integral balance of the system since one part evaporates, another infiltrates soaking
the soil and is used by the plants, and the rest remains on the surface forming puddles.
The runoff produced in the specific case of an urban basin, due to its specific
characteristics, is very different from that originating in rural hydrology. On the one
hand, due to the urbanization process, the surface is altered, making it impermeable
and modifying the natural network of runoff and drainage. On the other hand, these are
small basins with minimum infiltration surfaces, which produce short concentration
times and important flows. It can be simplified by calculating the runoff coefficient
(C) that represents the relationship between the flow of water that runs through the
surface as a consequence of a rainfall event (PEf) and the flow precipitated on it
(PTo), that is, the water of the total rainfall that actually generates surface runoff
once the soil has been completely saturated (see Fig. 6, Eq. 6).
This factor C is not constant, so its determination is approximate, since it varies
according to the magnitude of the rain and the topographic, soil and land use conditions. A sloping area does not respond to the same amount of rain as a plain, nor does
a garden area in front of a road. The runoff coefficient can take values between zero
and one (0 ≤ C ≤ 1). They are determined from the annual values of precipitation
and flow rate and are tabulated in the literature on the subject of surface hydrology.
The following table presents the most common C values used for the calculation of
urban basins (see Table 2).
In any case, the determination of an average run-off coefficient for a uniform area
can be considered. And when an area is made up of different types of land, different
surfaces are calculated as the average run-off coefficient by making a weighted
average of the different run-off coefficients of each of the sub-areas into which the
area under consideration can be divided. In this way, we arrive at the expression of
the weighted runoff coefficient (see Fig. 6, Eq. 7) for an area formed by different
sub-areas with different runoff coefficients.
Based on the fact that the unit of litres/m
2 is equivalent to the precipitation in mm,
the volumes of water are deducted. In other words, one litre spread over a surface
area of one square metre produces a sheet of water one millimetre thick. On the
one hand, if we apply the surfaces considered to PTo, we obtain the total volumes
of precipitation (VTo), and on the other hand, if we apply them to the effective
precipitation (PEf), we obtain the volumes of runoff (VEf). The difference is the
volume of abstractions or losses (Vab) (see Fig. 6, Eqs. 8–10).
The infiltrations
The permeability of soil is its capacity to allow the flow of a fluid through it. This
mechanical property of soils, which is measured by the coefficient of permeability