Despite the presence of clear interannual and interdecadal variability throughout
the discharge records, the most extreme flood events all occurred before the 1960s.
Only in 1910, 1924, 1945 and 1955 did the maximum flow rates at Pont d’Austerlitz
reach over 2,000 m
3 s
À1 (Fig. 2a). This is partly due to the creation and commissioning, since the mid-1960s, of large-capacity reservoirs upstream from the highly
urbanised and populated Paris region [1]. The three largest reservoirs were built in
derivation of the Seine in 1966 (220 Mm
3 capacity), the Marne in 1974 (360 Mm
3 )
and the Aube in 1990 (180 Mm
3 ). A smaller reservoir (80 Mm
3 ) was created in 1949,
but since it is located far upstream of the Yonne watershed, its impact on extreme
events observed in Paris is less pronounced than the impact of the others. Until now,
these reservoirs have successfully limited the impact of potentially damaging flood
events by storing up to 840 Mm
3 of water. Alternatively, in the summer and during
particularly dry periods, they release water to maintain a minimal target flow rate of
60 m
3 s
À1 in the Seine through Paris. This human-induced perturbation of the Seine’s
hydrodynamic functioning is noticeable in discharge data in at least two ways. First,
the variance of mean and maximum annual discharges has decreased since the
creation of the reservoirs (Figs. 1b and 2a). This demonstrates the ability to reduce
the impact of climatic extremes on river flow by smoothing the flow signal, that is to
say, by distributing rainfall inputs over time more efficiently than in natural
(undisturbed) conditions. Second, the VCN30 values, the yearly minima of 30-day
running average windows [61], have never dropped below 55 m
3 s
À1 since the
beginning of the 1960s. The only exception occurred in 1976 during an extreme
drought (VCN30, 40 m
3 s
À1 , Fig. 2b) concomitantly with an extreme dry year
(annual discharge, 120 m
3 s
À1
, Fig. 1b). Furthermore, the VCN30 has only rarely
dropped below 80 m
3 s
À1 since the beginning of the 1980s (Fig. 2b). This shows that
the reservoirs considerably helped to mitigate the risk of extended low-flow periods,
which may alter the integrity of river ecological habitats, as well as human activities.
The long discharge records at Pont d’Austerlitz hold valuable information on the
overall dynamical response of the Seine hydrosystem to climatic inputs. Like any
other hydrosystem on Earth, the Seine acts as a low-pass filter of effective precipitation, meaning that the highest frequencies in the climatic input signal are
transformed by the various flow processes, whereas the lowest frequencies (typically, the interdecadal fluctuations) are generally transcribed exactly as they are into
the discharge signal. From spectral analysis of effective precipitation and discharge
data, it is possible to obtain a first-order estimate of the mean flow response time of
the three fundamental compartments of a hydrosystem: the surface, the unsaturated
layer and the aquifer [62]. When applied to the Austerlitz data since 1885, we
estimate that the mean response times to climatic events are 1.7 Æ 0.2 days,
5.0 Æ 1.3 days and 2.5 Æ 0.2 years for surface runoff, vadose zone flow and aquifer
flow, respectively. Moreover, from the same analysis, we estimate that 81% (Æ2) of
the long-term river flow is sustained by groundwater. Unsurprisingly, this preliminary analysis shows that the Seine is largely dependent on groundwater stocks,
which are very capacitive and transmissive given the sedimentary nature of the
basin. It also shows that the Seine is highly vulnerable to extreme events, especially
floods, given that the response time of the runoff component is short. Nevertheless,
64
N. Flipo et al.
the discharge records, the most extreme flood events all occurred before the 1960s.
Only in 1910, 1924, 1945 and 1955 did the maximum flow rates at Pont d’Austerlitz
reach over 2,000 m
3 s
À1 (Fig. 2a). This is partly due to the creation and commissioning, since the mid-1960s, of large-capacity reservoirs upstream from the highly
urbanised and populated Paris region [1]. The three largest reservoirs were built in
derivation of the Seine in 1966 (220 Mm
3 capacity), the Marne in 1974 (360 Mm
3 )
and the Aube in 1990 (180 Mm
3 ). A smaller reservoir (80 Mm
3 ) was created in 1949,
but since it is located far upstream of the Yonne watershed, its impact on extreme
events observed in Paris is less pronounced than the impact of the others. Until now,
these reservoirs have successfully limited the impact of potentially damaging flood
events by storing up to 840 Mm
3 of water. Alternatively, in the summer and during
particularly dry periods, they release water to maintain a minimal target flow rate of
60 m
3 s
À1 in the Seine through Paris. This human-induced perturbation of the Seine’s
hydrodynamic functioning is noticeable in discharge data in at least two ways. First,
the variance of mean and maximum annual discharges has decreased since the
creation of the reservoirs (Figs. 1b and 2a). This demonstrates the ability to reduce
the impact of climatic extremes on river flow by smoothing the flow signal, that is to
say, by distributing rainfall inputs over time more efficiently than in natural
(undisturbed) conditions. Second, the VCN30 values, the yearly minima of 30-day
running average windows [61], have never dropped below 55 m
3 s
À1 since the
beginning of the 1960s. The only exception occurred in 1976 during an extreme
drought (VCN30, 40 m
3 s
À1 , Fig. 2b) concomitantly with an extreme dry year
(annual discharge, 120 m
3 s
À1
, Fig. 1b). Furthermore, the VCN30 has only rarely
dropped below 80 m
3 s
À1 since the beginning of the 1980s (Fig. 2b). This shows that
the reservoirs considerably helped to mitigate the risk of extended low-flow periods,
which may alter the integrity of river ecological habitats, as well as human activities.
The long discharge records at Pont d’Austerlitz hold valuable information on the
overall dynamical response of the Seine hydrosystem to climatic inputs. Like any
other hydrosystem on Earth, the Seine acts as a low-pass filter of effective precipitation, meaning that the highest frequencies in the climatic input signal are
transformed by the various flow processes, whereas the lowest frequencies (typically, the interdecadal fluctuations) are generally transcribed exactly as they are into
the discharge signal. From spectral analysis of effective precipitation and discharge
data, it is possible to obtain a first-order estimate of the mean flow response time of
the three fundamental compartments of a hydrosystem: the surface, the unsaturated
layer and the aquifer [62]. When applied to the Austerlitz data since 1885, we
estimate that the mean response times to climatic events are 1.7 Æ 0.2 days,
5.0 Æ 1.3 days and 2.5 Æ 0.2 years for surface runoff, vadose zone flow and aquifer
flow, respectively. Moreover, from the same analysis, we estimate that 81% (Æ2) of
the long-term river flow is sustained by groundwater. Unsurprisingly, this preliminary analysis shows that the Seine is largely dependent on groundwater stocks,
which are very capacitive and transmissive given the sedimentary nature of the
basin. It also shows that the Seine is highly vulnerable to extreme events, especially
floods, given that the response time of the runoff component is short. Nevertheless,
64
N. Flipo et al.