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[1] The groundwater-induced flooding that occurred in the Somme Basin during April 2001 damaged numerous dwellings and communication routes, and economic activity of the region was flood-bound for more than 2 months. It was the first time that such a sudden event was recognized as resulting from groundwater discharge, despite the Somme valley not being prone to flooding. Because of a dual porosity of the chalk in the basin, nonlinear processes, involving a hydraulic continuity between the macropores of the unsaturated zone and the chalk groundwater, govern water migration through the unsaturated zone. Such a process is the result of switching behavior of groundwater recharge from matrix flow to macropore flow due to accumulated wetness over several years. There is much evidence to support that the flood probability model is climatedependent for the studied region because nonlinear processes amplify the effects of nonstationarities of climatic inputs. An estimation of the return period of catastrophic flooding depends on the long-term precipitation fluctuations. This has implications for flood risk assessment requiring the need to distinguish between short-and long-term flooding risks. Other basins that may not appear particularly prone to flooding could also be subjected to similar groundwater-induced flooding should the long-term precipitation fluctuations observed in the north of France since the beginning of the 1980s persist. Similar extraordinary situations can occur in Belgium and England, whereby significant flooding results in substantial contribution of groundwater flows.

Inverse modeling of mass transfer characterizes the dynamic processes affecting the function of karst systems and can be used to identify karst properties. 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 have been doing for years, hydrograph separation is carried out by using transfer functions in their entirety, which enables accurate separation of fluxes, as was explained in the companion paper [Pinault et al., this issue]. The unit hydrograph as well as impulse response of fluxes is decomposed into a quick and a slow component, and, consequently, the effective rainfall is decomposed into two parts, one contributing to the quick flow (or flux) and the other contributing to the slow flow generation. This approach is applied to seven French karstic aquifers located on the Larzac plateau in the Grands Causses area (in the south of France). Both hydrodynamical and hydrogeochemical data have been recorded from these springs over several hydrological cycles. For modeling purposes, karst properties can be represented by the impulse responses of flow and flux of dissolved species. The heterogeneity of aquifers is translated to time-modulated flow and transport at the outlet. Monitoring these fluxes enables the evaluation of slow and quick components 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, inducing an increase in water head. This component reflects flood events where flow behavior is nonlinear and is described by a very short transfer function, which increases and decreases according to water head. The slow component consists of slow and fast infiltration, underground runoff, storage in annex-to-drain systems, and discharge from the saturated zone. These components 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 reservoir 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 water renewal. Since the preevent water hydrograph is produced by stored water when pushed by a rainfall event and the event water hydrograph reflects rainwater transfer, separating the two components can yield insights into the characteristics of karst aquifers, the modes of infiltration, and the mechanisms involved in karstification, as well as the degree of organization of the aquifer.

The variability in space and in time of gases (He, 222Rn, CO2) in soils might be used for volcano or seismic fault surveillance or in search of hidden mineral deposits. The gases measured in soils or in the weathered layer of the substratum can, however, be strongly altered by environmental conditions, such as atmospheric pressure, soil temperature, or moisture. An accurate knowledge of the influence of environmental conditions is required to decipher information from deeper phenomena in the earth. Variations in radon concentration, used as a gas flow tracer, are modeled using a new approach based on signal processing techniques in order to express impulse responses from multivariable time series. A general formulation is proposed and the inverse problem is solved by calculating the impulse responses of the parameter to be explained versus the variables responsible for the variability of the parameter. Such problems are generally ill posed and regularization methods must be used to develop the pattern. Due to the block‐Toeplitz structure of the correlation matrix, large orders can be used, enabling signal processing of time series controlled by the superposition of fast and slow phenomena. Two examples are processed to illustrate this approach. In the first example (222Rn in soil gas monitoring over a hidden sulphide deposit), radon concentration is controlled only by moisture and temperature, not by atmospheric pressure. The result seems to reveal a diffusive behavior. The impulse response of 222Rn concentration versus rainfall exhibits a great variability, possibly due to the opening of cracks in the surface, the effect being still noticeable 20 days after the rainfall. In the second example (222Rn in a thermal anomaly at summit of Etna volcano), where radon concentration is controlled by soil temperature and atmospheric pressure, the result indicates a convective/advective flow. Radon flux collapsed for 1 day probably due to a break in the rising gas column. In 1993, dramatic increases of signal were recorded for a few hours; such increases are interpreted as gas pulses not related with local seismicity.

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