Cevennes-Vivarais Region Research Articles

Temporal Variability of Sea Surface Temperature Patterns in the Gulf of Lions During Heavy Precipitation Events in The Cevennes-Vivarais Region

The Cévennes-Vivarais region in southern France frequently suffers from Heavy Precipitation Events (HPEs), especially during the autumn season. The northwestern Mediterranean Sea is a source of heat and moisture for these HPEs, with strong air sea exchanges, which are mainly controlled by the near-surface wind intensity and the Sea Surface Temperature (SST). The aim of this study is to characterize the SST structures, location and variability related to HPEs. Indeed, the Gulf of Lion is characterized by a cyclonic circulation with three main oceanic features: (1) the Northern Current (NC), (2) the Balearic Front (BF), (3) and the deep Convective Cell (CC). The MEDidterranean ReanalYsiS (MEDRYS1V2), an ocean reanalysis at 1/12˚ resolution was used over the 2000-2011 period to identify the NC, BF and CC oceanic feature's locations and for the calculation of an SST index. Then, an unsupervised cluster analysis method, using Principle Component Analysis for dimension reduction and the Silhouette for clustering, was used 20 in order to determine the most typical periods. The Local Moran's I (LMI) spatial statistical method, was used to highlight the areas of significant temporal variability of SST, considering the periods defined with the clustering method. For each period [early autumn (August-September), October and late autumn (November-December)], the LMI values, only considering the HPE initial stage, are extracted and compared to the averaged LMI values. The results highlight that in August-September, the Rhone river outflow have the most significant effect on SST variability on average, same as during HPEs. In October, for the HPEs initial stage, the largest variability seems to be related to the effects of the Mistral and Tramontane wind. In later autumn, there is a southward displacement of the BF and CC patterns and during the HPEs initial stage, the most significant SST variability is found near the center of the CC.

Investigating the role of geology in the hydrological response of Mediterranean catchments prone to flash-floods: Regional modelling study and process understanding

Summary In this study, a regional distributed hydrological model is used to perform long-term and flash-flood event simulations, over the Cevennes–Vivarais region (south of France). The objective is to improve our understanding on the role played by geology on the hydrological processes of catchments during two past flash-flood events. This modelling work is based on Vannier et al. (“Regional estimation of catchment-scale soil properties by means of streamflow recession analysis for use in distributed hydrological models”, Hydrological Processes , 2014), where streamflow recessions are analysed to estimate the thickness and hydraulic conductivity of weathered rock layers, depending on the geological nature of catchments. Weathered rock layers are thus implemented into the hydrological model CVN-p, and the contribution of these layers is assessed during flash-flood events simulations as well as during inter-event periods. The model is used without any calibration, to test hypotheses on the active hydrological processes. The results point out two different hydrological behaviours, depending on the geology: on crystalline rocks (granite and gneiss), the addition of a weathered rock layer considerably improves the simulated discharges, during flash-flood events as well as during recession periods, and makes the model able to remarkably reproduce the observed streamflow dynamics. For other geologies (schists especially), the benefits are real, but not sufficient to properly simulate the observed streamflow dynamics. These results probably underline the existence of poorly known processes (flow paths, non-linear spilling process) associated with the planar structure of schisty rocks. On a methodological point of view, this study proposes a simple way to account for the additional storage associated with each geological entity, through the addition of a weathered porous rock layer situated below the traditionally-considered upper soil horizons, and shows its applicability and benefits for the simulation of flash flood events at the regional scale.

A high-resolution rainfall re-analysis based on radar–raingauge merging in the Cévennes-Vivarais region, France

Summary This work aims at providing quantitative precipitation estimates (QPEs) for the Cevennes-Vivarais region, France, over temporal (1–6 h) and spatial (1–300 km2) scales relevant for flash-flood prediction in that region. A systematic implementation of three estimation methods (radar QPE, hourly raingauge Ordinary Kriging – OK – and merging of radar and raingauge data through Kriging with External Drift – KED) proves the KED method to systematically outperform the concurrent approaches for the 131 main rain events selected during the period 2007–2014. Error models, assuming the standard deviation of the QPE error to be a bi-linear function of the rain rate and the kriging normalized estimation standard deviation, are parameterized for the KED and OK QPEs for the considered temporal and spatial scales. The error models are shown to depend on the type of rain event (Cevennes rain events, localized convection, widespread rainfall) and physical parameters such as the 0 °C isotherm altitude and the rain intermittency. The added-value of the radar network in terms of QPE with respect to the hourly raingauge network is larger for localized convection rain events as well as for the smallest space–time scales which are those of interest for flash-flood prediction in the region.

Precipitation, soil moisture and runoff variability in a small river catchment (Ardèche, France) during HyMeX Special Observation Period 1

Flash flooding is a potentially destructive natural hazard known to occur in the Cevennes-Vivarais region in southern France. HyMeX (Hydrological Cycle in the Mediterranean Experiment) is an international program focused on understanding the hydrological cycle in the Mediterranean basin. Soil moisture is known to be a useful indicator of catchment response, however, establishing a meaningful estimation of soil moisture at the catchment level can be difficult due to its high variability in space and time. In a small gauged catchment in the Cevennes-Vivarais region in southern France, a series of manual soil moisture measurements was taken from September to December 2012 at both the field and catchment scale during the Special Observation Period 1 (SOP1) as part of the HyMeX program. Six plots were selected along a trajectory of a microwave link installed in the catchment and were chosen to represent different elevations in the catchment. Within each field plot, surface soil moisture was measured along a 50 m transect at 2 m intervals. This allowed the study of changes in within-field variability as well as between-field variability in response to precipitation events and during the drying out phase. Several precipitation events occurred over this autumn 2012 period which caused a significant wetting-up of the catchment, allowing the study of soil moisture processes over a wide range of wetness conditions. The influence of antecedent catchment conditions (soil moisture) on rainfall–runoff dynamics is demonstrated through the comparison of storm hydrographs for the various events. Dry catchment conditions result in minimal response in event flow, whereas large precipitation events occurring during wetter conditions produce much stronger responses in event flow. This further confirms the importance of quantifying catchment initial conditions to enhance the prediction of flash flood occurrences.

Regional estimation of catchment-scale soil properties by means of streamflow recession analysis for use in distributed hydrological models

The estimation of catchment-scale soil properties, such as water storage capacity and hydraulic conductivity, is of primary interest for the implementation of distributed hydrological models at the regional scale. This estimation is generally performed on the basis of information provided by soil databases. However, such databases are often established for agronomic uses and generally do not document deep-weathered rock horizons (i.e. pedologic horizons of type C and deeper), which can play a major role in water transfer and storages. Here, we define the Drainable Storage Capacity Index (DSCI), an indicator that relies on the comparison between cumulated streamflow and precipitation to assess catchment-scale storage capacities. DSCI is found to be reliable to detect underestimation of soil storage capacities in soil databases. We also use the streamflow recession analysis methodology defined by Brutsaert and Nieber in 1977 to estimate water storage capacities and lateral saturated hydraulic conductivities of the nondocumented deep horizons. The analysis is applied to a sample of 23 catchments (0.2–291 km2) located in the Cevennes-Vivarais region (south of France). For regionalization purposes, the obtained results are compared with the dominant catchment geology and present a clear hierarchy between the different geologies of the area. Hard crystalline rocks are found to be associated with the thickest and less conductive deep soil horizons. Schist rocks present intermediate values of thickness and of saturated hydraulic conductivity, whereas sedimentary rocks and alluvium are found to be less thick and most conductive. These results are of primary interest in view of the future set-up of distributed hydrological models over the Cevennes-Vivarais region. Copyright © 2013 John Wiley & Sons, Ltd.

Assessment of High-Resolution Satellite-Based Rainfall Estimates over the Mediterranean during Heavy Precipitation Events

Abstract Heavy precipitation events (HPE) can incur significant economic losses as well as losses of lives through catastrophic floods. Evidence of increasing heavy precipitation at continental and global scales clearly emphasizes the need to accurately quantify these phenomena. The current study focuses on the error analysis of two of the main quasi-global, high-resolution satellite products [Climate Prediction Center (CPC) morphing technique (CMORPH) and Precipitation Estimation from Remotely Sensed Imagery Using Artificial Neural Networks (PERSIANN)], using rainfall data derived from high-quality weather radar rainfall estimates as a reference. This analysis is based on seven major flood-inducing HPEs that developed over complex terrain areas in northern Italy (Fella and Sessia regions) and southern France (Cevennes–Vivarais region). The storm cases were categorized as convective or stratiform based on their characteristics, including rainfall intensity, duration, and area coverage. The results indicate that precipitation type has an effect on the algorithm's ability to capture rainfall effectively. Convective storm cases exhibited greater rain rate retrieval errors, while low rain rates in stratiform-type systems are not well captured by the satellite algorithms investigated in this study, thus leading to greater missed rainfall volumes. Overall, CMORPH exhibited better error statistics than PERSIANN for the HPEs of this study. Similarities are also shown in the two satellite products' error characteristics for the HPEs that occurred in the same geographical area.