International audienceThe Cévennes–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

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Many methods have been proposed and applied individually to rainfall fields in order to estimate point or average values at ungauged sites. This paper reviews some of the usual as well as some of the more recent techniques. They are classified according to their fundamental principles, optimality criteria, and practical aspects. Some theoretical developments, mostly for the statistically based techniques, are offered, and the practical aspects are mainly directed to the particular case of rainfall fields. A case study is presented for a region with particularly high rainfall variability, providing a good sample of events for testing the sensitivity of the estimation to the different methods used. The identification of statistical structure functions, required by certain methods using regionalized variable concepts, is discussed in detail for the case considered. Method evaluation is based on a classical validation technique that divides the available stations into two separate networks, i.e., a base network is used to perform estimations at the stations of a test network for a set of independent storm events. Careful attention is paid to result interpretation and visualisation. Finally, conclusions are drawn as to which method should be implemented, taking into account cost-efficiency considerations.Many techniques have been proposed for mapping rainfall patterns and for evaluating the average rainfall over a watershed. Some of these are reviewed by Hall and Barclay [1975], although others have been proposed since then [cf. Thorpe et al., 1979;Bethlamy, 1976;Pande et al., 1978].Attempts to evaluate and compare the different methods are much more rare. This is understandable for techniques that estimate spatial averages because of the lack of experimentally measured areal values. However, most techniques may also be used for point values, where ground verification is possible. Point values are also of interest for contouring and for filling in gaps in records of existing stations. This paper proposes an objective comparison of methods estimating point values. The evaluation procedure itself relies on a simple validation technique commonly used in statistics. The methods evaluated are those which are or are becoming commonly used in surface hydrology.Although several other classification systems will be used throughout this paper, the various methods may first of all be separated into simple and sophisticated ones. In the first group are the following:1. The nearest neighbor method. The estimated value at any given point is taken as the observed value at the nearest neighboring station. This represents the simplest possible interpolation scheme and is in fact the basic principle of the Thiessen method, as demonstrated in the automated version proposed by Diskin [1970] and widely used in hydrology for average rainfall estimation. The arithmetic mean.This assumes that the rainfall depth is theoretically constant over a given region and can be estimated by the average of the observed values within this region.In the ...

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The issue of the accuracy of areal rainfall estimation is discussed through a case study on a catchment area in the Cévennes region of France. The basic tool for the analysis is a “scaled estimation error variance” which is computed from a scaled climatological variogram model of the rainfall field. We show how this variance provides a theoretical criterion to compare the accuracy that can be expected with three linear estimators (Thiessen, spline, and kriging) for various networks' densities. To support the methodology an experimental validation of the scaled estimation error variance computation is performed, using “reference” areal rainfall values computed with a very high density network.

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