Georges-Marie Saulnier

The Catastrophic Flash-Flood Event of 8–9 September 2002 in the Gard Region, France: A First Case Study for the Cévennes–Vivarais Mediterranean Hydrometeorological Observatory

The Cevennes–Vivarais Mediterranean Hydrometeorological Observatory (OHM-CV) is a research initiative aimed at improving the understanding and modeling of the Mediterranean intense rain events that frequently result in devastating flash floods in southern France. A primary objective is to bring together the skills of meteorologists and hydrologists, modelers and instrumentalists, researchers and practitioners, to cope with these rather unpredictable events. In line with previously published flash-flood monographs, the present paper aims at documenting the 8–9 September 2002 catastrophic event, which resulted in 24 casualties and an economic damage evaluated at 1.2 billion euros (i.e., about 1 billion U.S. dollars) in the Gard region, France. A description of the synoptic meteorological situation is first given and shows that no particular precursor indicated the imminence of such an extreme event. Then, radar and rain gauge analyses are used to assess the magnitude of the rain event, which was particularly remarkable for its spatial extent with rain amounts greater than 200 mm in 24 h over 5500 km2. The maximum values of 600–700 mm observed locally are among the highest daily records in the region. The preliminary results of the postevent hydrological investigation show that the hydrologic response of the upstream watersheds of the Gard and Vidourle Rivers is consistent with the marked space–time structure of the rain event. It is noteworthy that peak specific discharges were very high over most of the affected areas (5–10 m3 s−1 km−2) and reached locally extraordinary values of more than 20 m3 s−1 km−2. A preliminary analysis indicates contrasting hydrological behaviors that seem to be related to geomorphological factors, notably the influence of karst in part of the region. An overview of the ongoing meteorological and hydrological research projects devoted to this case study within the OHM-CV is finally presented.

A disaggregation scheme for soil moisture based on topography and soil depth

This paper reports on a new soil moisture disaggregation scheme based on topography and soil depth information. It is designed for low resolution remote sensing data assimilation into hydrological modelling. The scheme makes use of a simple Soil Vegetation Atmosphere Transfer model coupled to the TOPMODEL formalism. Water and energy balance are computed at the catchment scale, taking lateral flows due to topography into account. Lumped values of near-surface and deep soil water content are then disaggregated at local scale using simple relationship between mean quantities, local topography and soil depth information. Results for a small water catchment in South-eastern Australia show satisfactory reproduction of the local soil moisture patterns using a combination of topography and soil depth information. Due to subgrid variability and differences between the simulation and observation scale (the Digital Elevation Model pixel versus the point measurement), the point-to-point comparison between observations and simulations shows a poor correlation. Rescaling shows that a good correlation is obtained when averaging the simulated and observed soil moisture over a length of 100 m.

Including spatially variable effective soil depths in TOPMODEL

Abstract TOPMODEL ( Beven and Kirkby, 1979 ; Beven et al., 1995 ) was one of the first attempts to model distributed hydrological responses based on variable contributing area concepts. It makes use of an index of hydrological similarity based on an analysis of the topographic data. The index approach was later generalised to take account of spatial variability of soil transmissivities, but no similar spatial analysis of the variability in the rate of the decrease of the transmissivity with depth has yet been examined. This paper shows how the TOPMODEL theory can be extended to handle this spatial variability, using a 2D distribution function of a new soil depth-topographic index of hydrological similarity. A first sensitivity analysis of the effect of variable soil depths on the model predictions for the Maurets catchment, France, is presented. Predicted discharges and calibrated parameter values are not sensitive to the patterns of effective soil depth investigated. Distributed predictions may be more sensitive but raise questions of how to obtain the parameter data required.

Analytical compensation between DTM grid resolution and effective values of staurated hydraulic conductivity within the TOPMODEL framework

The recent widespread availability of digital terrain data has resulted in increasing use, in a variety of hydrological models, of different degrees of complexity. Previous experience suggests that neither model results nor effective parameter values are independent of the resolution of the digital terrain data used. Calibration of parameter values can compensate for lack of resolution in the digital terrain data. This paper will show, for one particular model, TOPMODEL, how an analytical link can be established between the grid size of a raster digital terrain model and the effective saturated hydraulic conductivity value used in the model. The work generalizes the results of the recent study by Franchini et al. (1996) and allows the change in effective conductivity to be estimated on the basis of keeping a realistic simulation of saturated contributing areas as the DTM grid size changes.

Digital elevation analysis for distributed hydrological modeling: Reducing scale dependence in effective hydraulic conductivity values

The recent widespread availability of digital terrain data has made automatic procedures for topographic analyses popular. Previous studies have shown that hydrological models and their effective parameter values are dependent on the resolution of the elevation grid. This paper examines the analysis of raster elevation data within the topography-based model, TOPMODEL, framework. It is demonstrated that the algorithm used in processing channel pixels in calculating the topographic index k = ln(a/tanβ) can have a dramatic effect on the sensitivity of effective parameter values to the grid size. Suggestions are made for calculating the topographic index of channel pixels, consistent with the TOPMODEL assumptions, that strongly decrease the sensitivity of the calibrated effective hydraulic conductivity values to grid size.

Coupling the ISBA Land Surface Model and the TOPMODEL Hydrological Model for Mediterranean Flash-Flood Forecasting: Description, Calibration, and Validation

Abstract Innovative coupling between the soil–vegetation–atmosphere transfer (SVAT) model Interactions between Soil, Biosphere, and Atmosphere (ISBA) and the hydrological model TOPMODEL has been specifically designed for flash-flood forecasting in the Mediterranean area. The coupled model described in this study combines the advantages of the two types of model: the accurate representation of water and energy transfer between the soil and the atmosphere within the SVAT column and an explicit representation of the lateral transfer of water over the hydrological catchment unit. Another advantage of this coupling is that the number of parameters to be calibrated is reduced by two, as only two parameters instead of four parameters concern the TOPMODEL formulation used here. The parameters to be calibrated concern only the water transfer. The model was calibrated for the simulation of flash-flood events on the three main watersheds covering the French Cevennes–Vivarais region using a subset of past flash-flood...

Subgrid runoff parameterization

A subgrid parameterization of runoff based on the TOPMODEL hydrological framework is introduced within the ISBA surface scheme. Indeed, by the mean of a topographic index of hydrological similarity, the TOPMODEL framework suggests an explicit scaling equation linking local scale and macro scale water deficit. The idea is then to couple this scaling equation with ISBA to be able to associate the water content of each macro pixel (here 64km2) simulated by ISBA to the corresponding subgrid spatial distribution of the local pixels (here 75×75m2) of the digital elevation model (DEM). The main advantage is that this subgrid runoff parameterization depends only on the topography, without any calibration parameters. The ISBA-TOPMODEL model is tested in the Ardeche basin (France), for the period 1981-1995. Soil, vegetation and atmospheric forcing are taken form the GEWEX-Rhone database. The comparison between the simulation obtained with ISBA-TOPMODEL, the former version of ISBA including the Variable Infiltration Capacity subgrid runoff scheme, and the observations is presented.