Regional coupled surface-subsurface hydrological model fitting based on a spatially distributed minimalist reduction of frequency-domain discharge data

Abstract

Abstract. Although integrated water resources models are indispensable tools for water management at various scales, it is of primary importance to ensure their proper fitting on hydrological variables, avoiding flaws related to equifinality. An innovative step-wise fitting methodology is therefore proposed, which can be applied for any river basin model, from catchment to continental scale as far as hydrological models or land surface models are concerned. The methodology focuses on hydrosystems considering both surface water and groundwater, as well as internal water fluxes such as river baseflow. It is based on the thorough analysis of hydrological signal transformation by various components of a coupled surface--subsurface hydrosystem, in a nested approach, that considers the conditionality of parameter fields on their input forcing fluxes. The methodology is based on the decomposition of hydrological signal in the frequency domain with the HYMIT (HYdrological MInimalist Transfer function) method (Schuite et al., 2019). Parameters derived from HYMIT are used to fit the coupled surface–subsurface hydrological model CaWaQS3.03 using a step-wise methodology, which relies on successive Markov chain Monte Carlo optimizations, related to various objective functions representing the dependency of the hydrological parameters fields on forcing input fluxes. This new methodology enables significant progress to be made in terms of the spatial distribution of the model parameters and the water balance components, at the regional scale. The use of many control stations such as discharge gauging stations with HYMIT leads to a coarse parameter distribution that is then refined by the fitting of CaWaQS parameters on its own mesh. The step-wise methodology is exemplified with the Seine River basin (~76,000 km2). In particular, it made it possible to spatially identify fundamental hydrological values, such as rainfall partitioning into actual evapotranspiration, as well as runoff and aquifer recharge through its impluvium, in both the time and frequency domains. Such a fitted model facilitates the analysis of both the overall and detailed territorial functioning of the river basin, including explicitly the aquifer system. A reference piezometric map of the upmost free aquifer units and a water budget of the Seine basin are established, detailing all external and internal fluxes up to the exchanges between the eight simulated aquifer layers. The results showed that the overall contribution of the aquifer system to the river discharge of the river network in the Seine basin varies spatially within a wide range (5–96 %), with an overall contribution at the outlet of the basin of 67 %. The geological substratum greatly influences the contribution of groundwater to the river discharge.

Assessing water and energy fluxes in a regional hydrosystem: case study of the Seine basin

“…However, this potential is yet to be quantified by estimating the energy budget of the aquifer system. Given the large population and food production of the Seine basin, water resources are of high strategic importance, as about 1 km 3 of groundwater is extracted every year over the whole basin [Flipo et al, 2022 ORCHIDEE is a SVAT model, initially designed to be coupled to an atmospheric global circulation model [Krinner et al, 2005], but it can also be used in stand-alone mode, using an input meteorological data set. ORCHIDEE tag 2.2 was developed as part of the IPSL-CM6 model [Boucher et al, 2020] to contribute to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) [Eyring et al, 2016].…”

“…Mostly relying on the former MODCOU-NEWSAM software [Ledoux et al, 1989] based on the blueprints of de Marsily et al [1978], the physically-based CaWaQS3.x [Flipo et al, 2022[Flipo et al, , 2007] computes, at a daily time-step, the main physical processes controlling the water budget and flow dynamics in each compartment of a hydrosystem. Each CaWaQS library deals with a specific compartment of a hydrosystem.…”

“…A pre-existing CaWaQS application over the Seine basin, which considers the main aquifer units in interaction with the Seine river and its main tributaries [Flipo et al, 2022], was used here. From the oldest to the most recent, these aquifers can be grouped according to the following four main ensembles: (i) an ensemble of aquifer and aquitard units, ranging from Lower Jurassic (Hettangian stage) to Lower Cretaceous (Albian stage), (ii) the large Upper Cretaceous chalk aquifer, (iii) a Tertiary aquifer complex including the five main aquifer layers dating from Palaeocene to Miocene stages and (iv) recent alluvial deposits (Pleistocene, Holocene).…”

“…The bias was calculated after the simulated pluriannual mean discharge (estimated by total runoff times basin area) was corrected for yearly groundwater abstraction (annual average equivalent of 37 m 3 •s −1 during 2008-2012, AESN) from the basin. Previous studies [Flipo et al, 2022] indicate that around two-thirds of effective rainfall contributes to aquifer recharge in the basin, and this ratio was also considered as a qualitative target for the calibration.…”

“…CaWaQS parameterizations of river network and groundwater were taken from the pre-calibrated parameter set provided by Flipo et al [2022]. No additional calibration of the hydrological components and groundwater heat transport of CaWaQS were undertaken, but the river heat budget was calibrated by modifying the uniform shading factor ranging from 0% to 50% applied on the net incoming radiation on the river [Beaufort et al, 2016].…”