Radon-in-soil monitoring at the Poas volcano (Costa Rica) has been performed together with water chemistry from the hot crater lake since 1981 and 1983 respectively. The results are discussed as a function of the eruptive evolution of the volcano over a 13 years period (1981-1994). It is shown that no definitely clear precursory radon signals have been recorded. On the contrary, ionic species concentrations are likely to be considered good precursors, together with the temperature variations of the crater lake water.
Understanding the dynamics of flow and recharge processes is crucial information for managing water
resources. However, this is challenging in heterogeneous systems, e.g. karst, due to the strong spatial
variability in flow pathways. In addition, the evolution of mixing at the event-scale is rarely
characterized with sufficient details to constrain or validate hydrodynamic models. The objectives of
this study were to (1) identify event-scale variations in contributions from different flow pathways to a
karst spring, and (2) better constrain the response of spring dynamics to the spatio-temporal variability
of flood events. The study focuses on the main spring (Lez spring) of a Mediterranean karst aquifer
near Montpellier (SE France), which is subject to intense autumn rainfall events. We used innovations
in (1) the continuous and high frequency monitoring of tracers (delta18O, delta2H, and natural
fluorescence+ humic and proteic-like compounds), and (2) the types of tracers monitored (natural and
anthropogenic dissolved gases, radon and radium isotopes, 3D-fluorescence and total organic
carbon). Three rainfall events highlighted distinct spring chemical responses. The first event was
characterized by a contribution of deep groundwater flow (low CFC contents, low (228Ra 226Ra)
ratios and low organic carbon concentrations) resulting from a piston flow effect due to heavy rainfalls
at the basin margins. The second event showed a proteic peak associated with a sharp decline in DO
concentrations and an anomalous peak in delta18O. The third event was characterized by a dilution
effect by subsurface waters (CFC contamination, high turbidity and radon peak) due to high local
rainfall. The simultaneous monitoring of these tracers during multiple rainfall events provides an
innovative approach in spring dynamics analysis, and improves our understanding of a complex
hydrogeological system. The Lez spring supplies drinking water for Montpellier, therefore
understanding the variability of water origins and potential contamination pathways during intense
storm events is essential.
Abstract. This paper presents a coupled observation and modelling strategy aiming at improving the understanding of processes triggering flash floods. This strategy is illustrated for the Mediterranean area using two French catchments (Gard and Ardèche) larger than 2000 km2. The approach is based on the monitoring of nested spatial scales: (1) the hillslope scale, where processes influencing the runoff generation and its concentration can be tackled; (2) the small to medium catchment scale (1–100 km2), where the impact of the network structure and of the spatial variability of rainfall, landscape and initial soil moisture can be quantified; (3) the larger scale (100–1000 km2), where the river routing and flooding processes become important. These observations are part of the HyMeX (HYdrological cycle in the Mediterranean EXperiment) enhanced observation period (EOP), which will last 4 years (2012–2015). In terms of hydrological modelling, the objective is to set up regional-scale models, while addressing small and generally ungauged catchments, which represent the scale of interest for flood risk assessment. Top-down and bottom-up approaches are combined and the models are used as "hypothesis testing" tools by coupling model development with data analyses in order to incrementally evaluate the validity of model hypotheses. The paper first presents the rationale behind the experimental set-up and the instrumentation itself. Second, we discuss the associated modelling strategy. Results illustrate the potential of the approach in advancing our understanding of flash flood processes on various scales.
