Inverse modeling of the hydrological and the hydrochemical behavior of hydrosystems: Characterization of karst system functioning

Inverse modeling of mass transfer characterizes the dynamic processes affec ting the function of karst systems and can be used to identify karst proper ties. An inverse model is proposed to calculate unit hydrographs as well as impulse response of fluxes from rainfall-runoff or rainfall-flux data, the purpose of which is hydrograph separation. Contrary to what hydrologists h ave been doing for years, hydrograph separation is carried out by using tra nsfer functions in their entirety, which enables accurate separation of flu xes, as was explained in the companion paper [Pinault et al., this issue]. The unit hydrograph as well as impulse response of fluxes is decomposed int o a quick and a slow component, and, consequently, the effective rainfall i s decomposed into two parts, one contributing to the quick flow (or flux) a nd the other contributing to the slow flow generation. This approach is app lied to seven French karstic aquifers located on the Larzac plateau in the Grands Causses area (in the south of France). Both hydrodynamical and hydro geochemical data have been recorded from these springs over several hydrolo gical cycles. For modeling purposes, karst properties can be represented by the impulse responses of flow and flux of dissolved species. The heterogen eity of aquifers is translated to time-modulated flow and transport at the outlet. Monitoring these fluxes enables the evaluation of slow and quick co mponents in the hydrograph. The quick component refers to the "flush flow" effect and results from fast infiltration in the karst conduit network when connection is established between the infiltration and phreatic zones, ind ucing an increase in water head. This component reflects flood events where flow behavior is nonlinear and is described by a very short transfer funct ion, which increases and decreases according to water head. The slow compon ent consists of slow and fast infiltration, underground runoff, storage in annex-to-drain systems, and discharge from the saturated zone. These compon ents can be further subdivided by measuring chemical responses at the karst outlet. Using Such natural tracers enables the slow component of the unit hydrograph to be separated into preevent water, i.e., water of the reservoi r and event water, i.e., water whose origin can be related to a particular rainfall event. These measurements can be used to determine the rate of wat er renewal. Since the preevent water hydrograph is produced by stored water when pushed by a rainfall event and the event water hydrograph reflects ra inwater transfer, separating the two components can yield insights into the characteristics of karst aquifers, the modes of infiltration, and the mech anisms involved in karstification, as well as the degree of organization of the aquifer.