Rebuilding a past trajectory involves the reconstruction of both past climate and
past land use. Data assimilation in global circulation models was proposed in the
1990s and allowed the reanalysis of meteorological data over 40 years [14], which
has led, in the 2010s, to multiple decade-long climate reanalysis [15–17] and also
century-long reanalysis [18, 19]. Bonnet et al. [20] leveraged on those century-long
reanalyses by downscaling them at the Seine basin scale. In the continuity of this
work, a first reconstruction of the past hydrological trajectory of the Seine basin was
proposed by Bonnet [21]. However, the Anthropocene era [22, 23] also introduced
major land use changes over the past century [24–26], which we will account for in
our attempt to rebuild the hydrosystem trajectory since the early 1900s.
This work is the accomplishment of 30 years of hydrogeological studies on the
Seine basin within the PIREN-Seine programme. Starting from local studies at the
river–aquifer interface [27, 28] and the knowledge on deep long-term water circulations in the Paris basin aquifer system [29–32], a first coupled hydrological–
hydrogeological model of the Seine basin was proposed by Gomez et al. [33] and
further enhanced for the simulation of surface–subsurface interactions [34, 35]. This
modelling tool spread in the hydrometeorological community inspiriting global
circulation models [36] and initiating combinations with soil-vegetation-atmosphere-transfer models [9, 37, 38]. It was also used for various combined applications to study the trajectory of nitrate due to agricultural practices [39–41] as well as
the impact of climate change on water quality [42]. All these studies led to the
distributed process-based hydrological–hydrogeological model CaWaQS [43–45]
that is used in this anniversary chapter on the trajectory of the Seine basin water
resources.
2 Historical Records of the Seine River Discharge in Paris
The Pont d’Austerlitz gauging station offers the longest and most viable mean daily
discharge time series for the Seine River. The first estimates of discharge go back to
1885. At this station, the Seine River drains an area of 43,800 km
2 , which corresponds to 60% of the total basin. Hence, this data set provides a valuable glimpse
into the large-scale hydrological functioning of a significant part of the basin. It also
bears remarkable traces of the long-term evolution of environmental factors that
influence flow dynamics, whether natural or human-induced. Furthermore, the
Austerlitz discharge data provide the opportunity to analyse the response of the
basin to historical extreme events (floods and droughts).
Overall, the Seine reaches high-flow conditions in mid-February with a multiannual average flow rate of 583 m
3 s
À1 and smoothly arrives at low-flow periods in
August with an average rate of 125 m
3 s
À1 (Fig. 1a). Yet, the interannual variability
of flow is remarkably high, especially between wet periods where the difference
between the 0.95 and 0.05 quantiles is on the order of 1,000 m
3 s
À1 . This is also
reflected in historical extrema: discharge records range from a minimum 20 m
3 s
À1
(historical drought of 1921) to over a maximum 2,600 m
3 s
À1 (historical flood of
Pluri-annual Water Budget on the Seine Basin: Past, Current and Future Trends
61
past land use. Data assimilation in global circulation models was proposed in the
1990s and allowed the reanalysis of meteorological data over 40 years [14], which
has led, in the 2010s, to multiple decade-long climate reanalysis [15–17] and also
century-long reanalysis [18, 19]. Bonnet et al. [20] leveraged on those century-long
reanalyses by downscaling them at the Seine basin scale. In the continuity of this
work, a first reconstruction of the past hydrological trajectory of the Seine basin was
proposed by Bonnet [21]. However, the Anthropocene era [22, 23] also introduced
major land use changes over the past century [24–26], which we will account for in
our attempt to rebuild the hydrosystem trajectory since the early 1900s.
This work is the accomplishment of 30 years of hydrogeological studies on the
Seine basin within the PIREN-Seine programme. Starting from local studies at the
river–aquifer interface [27, 28] and the knowledge on deep long-term water circulations in the Paris basin aquifer system [29–32], a first coupled hydrological–
hydrogeological model of the Seine basin was proposed by Gomez et al. [33] and
further enhanced for the simulation of surface–subsurface interactions [34, 35]. This
modelling tool spread in the hydrometeorological community inspiriting global
circulation models [36] and initiating combinations with soil-vegetation-atmosphere-transfer models [9, 37, 38]. It was also used for various combined applications to study the trajectory of nitrate due to agricultural practices [39–41] as well as
the impact of climate change on water quality [42]. All these studies led to the
distributed process-based hydrological–hydrogeological model CaWaQS [43–45]
that is used in this anniversary chapter on the trajectory of the Seine basin water
resources.
2 Historical Records of the Seine River Discharge in Paris
The Pont d’Austerlitz gauging station offers the longest and most viable mean daily
discharge time series for the Seine River. The first estimates of discharge go back to
1885. At this station, the Seine River drains an area of 43,800 km
2 , which corresponds to 60% of the total basin. Hence, this data set provides a valuable glimpse
into the large-scale hydrological functioning of a significant part of the basin. It also
bears remarkable traces of the long-term evolution of environmental factors that
influence flow dynamics, whether natural or human-induced. Furthermore, the
Austerlitz discharge data provide the opportunity to analyse the response of the
basin to historical extreme events (floods and droughts).
Overall, the Seine reaches high-flow conditions in mid-February with a multiannual average flow rate of 583 m
3 s
À1 and smoothly arrives at low-flow periods in
August with an average rate of 125 m
3 s
À1 (Fig. 1a). Yet, the interannual variability
of flow is remarkably high, especially between wet periods where the difference
between the 0.95 and 0.05 quantiles is on the order of 1,000 m
3 s
À1 . This is also
reflected in historical extrema: discharge records range from a minimum 20 m
3 s
À1
(historical drought of 1921) to over a maximum 2,600 m
3 s
À1 (historical flood of
Pluri-annual Water Budget on the Seine Basin: Past, Current and Future Trends
61