as reservoir lakes have altered the natural functioning of the basin since the
mid-1970s, the apparent aquifer response time increases to 7.3 Æ 0.2 years, and
the fraction of groundwater supply to fluvial systems appears to be about 90% when
the spectral analysis of discharge [62] is applied to data between 1974 and 2018. It
makes sense that, somehow, the capacitive component of the system is perceived by
simple interpretation tools to have a higher dampening effect, and a slightly higher
contribution to total fluxes, in the presence of reservoirs. However, it is not possible
to attribute this effect to subsurface property changes at this time scale, as it is
conceptualised by these tools [62]. This demonstrates once more the sensitivity of
river flow dynamics to territory developments, but it also shows the necessity of
adopting more complex theoretical and modelling schemes to precisely understand,
quantify and predict flow paths and processes throughout the basin, which is the
exact purpose of this chapter.
3 Development of the Seine Basin Model
3.1 CaWaQS Model
The functioning of the Seine hydrosystem is nowadays simulated with CaWaQS
(CAtchment WAter Quality Simulator) [43–45, 62, 63]. It is a spatially distributed
model which simulates the water balance and dynamics of water flow in all compartments of a hydrosystem based on the blueprint published by de Marsily et al.
[64] and first implemented as the MODCOU model [65–67]. A first attempt of
reprogramming in Fortran90 was achieved with the EauDyssée project [34, 35, 68–
71], which mostly accounted for river stage fluctuations [34, 35, 69]. Following the
extension by Flipo et al. [72] of the nested groundwater flow concept [73] in the
stream–aquifer interface, CaWaQS 2.x was recoded [45] combining multiple C
libraries. Running at a daily time step, calculations of surface, subsurface and
groundwater flows are structured around five main components [45, 68]:
• A surface module, mainly conditioned by land use, climate and parent soil
materials, which calculates the surface water balance via a conceptual reservoirbased approach [74, 75] using rainfall and potential evapotranspiration (PET)
data to estimate actual evapotranspiration (AET), runoff and infiltration fluxes on
each surface layer cell.
• An unsaturated module, which transfers water infiltration from the subsurface to
the water table using a set of reservoirs so that the infiltration is diffused in time to
form the aquifer recharge [76, 77].
• A saturated module that solves the pseudo 3D-diffusivity equation [78] in a
multilayer aquifer system with a finite volume numerical scheme that uses
water recharge as well as water withdrawals as forcings.
• A conductance model that represents surface–subsurface water exchanges
[72, 79, 80].
Pluri-annual Water Budget on the Seine Basin: Past, Current and Future Trends
65
mid-1970s, the apparent aquifer response time increases to 7.3 Æ 0.2 years, and
the fraction of groundwater supply to fluvial systems appears to be about 90% when
the spectral analysis of discharge [62] is applied to data between 1974 and 2018. It
makes sense that, somehow, the capacitive component of the system is perceived by
simple interpretation tools to have a higher dampening effect, and a slightly higher
contribution to total fluxes, in the presence of reservoirs. However, it is not possible
to attribute this effect to subsurface property changes at this time scale, as it is
conceptualised by these tools [62]. This demonstrates once more the sensitivity of
river flow dynamics to territory developments, but it also shows the necessity of
adopting more complex theoretical and modelling schemes to precisely understand,
quantify and predict flow paths and processes throughout the basin, which is the
exact purpose of this chapter.
3 Development of the Seine Basin Model
3.1 CaWaQS Model
The functioning of the Seine hydrosystem is nowadays simulated with CaWaQS
(CAtchment WAter Quality Simulator) [43–45, 62, 63]. It is a spatially distributed
model which simulates the water balance and dynamics of water flow in all compartments of a hydrosystem based on the blueprint published by de Marsily et al.
[64] and first implemented as the MODCOU model [65–67]. A first attempt of
reprogramming in Fortran90 was achieved with the EauDyssée project [34, 35, 68–
71], which mostly accounted for river stage fluctuations [34, 35, 69]. Following the
extension by Flipo et al. [72] of the nested groundwater flow concept [73] in the
stream–aquifer interface, CaWaQS 2.x was recoded [45] combining multiple C
libraries. Running at a daily time step, calculations of surface, subsurface and
groundwater flows are structured around five main components [45, 68]:
• A surface module, mainly conditioned by land use, climate and parent soil
materials, which calculates the surface water balance via a conceptual reservoirbased approach [74, 75] using rainfall and potential evapotranspiration (PET)
data to estimate actual evapotranspiration (AET), runoff and infiltration fluxes on
each surface layer cell.
• An unsaturated module, which transfers water infiltration from the subsurface to
the water table using a set of reservoirs so that the infiltration is diffused in time to
form the aquifer recharge [76, 77].
• A saturated module that solves the pseudo 3D-diffusivity equation [78] in a
multilayer aquifer system with a finite volume numerical scheme that uses
water recharge as well as water withdrawals as forcings.
• A conductance model that represents surface–subsurface water exchanges
[72, 79, 80].
Pluri-annual Water Budget on the Seine Basin: Past, Current and Future Trends
65