Basic notions and useful concepts for understanding the modeling of flood regimes of basins in QdF models
Using a multi-duration and multi-frequency description of observed flows, we have recently developed the Cemagref QdF model to represent watershed flood regimes. The model is also used for the predetermination of flood characteristics, on both gauged and ungauged watersheds, in the range of frequent to rare floods. By allowing upstream and downstream recalculation of flood characteristics, the model answers questions relevant to integrated river management. In the present article we have emphasized the basic notions and concepts underlying the QdF models; more detailed descriptions can be found in the referenced publications and theses. The studied hydrologic variables, maximal average flow (VCXd) and maximal threshold discharge (QCXd), are linked to continuous duration, d, which can vary from one second to thirty days. For every sample of duration d, composed with a sampling method of independent maximal values above a given threshold, the exponential law is generally adequate for mean return periods, for means in the range from 0.5 to 20 years (O.52T(year)220). A theoretical form, called "esthetic" GRADEX, is used to extrapolate observed floods to rare frequencies floods (20<T(year)21000). From theoretical laws, relative to observations and extrapolations, discharge (Q) - duration (d) - Frequency (F) curves of a basin are then deduced. Using two basin descriptors, one for the transfer function (D parameter) and one for production (QIXA10 parameter), the preceding curves are then converted into an adimensional or normative form. Such dimensionless curves are then used to predetermine floods quantiles for all durations and frequencies, both on gauged and ungauged basins, using known site parameters D and QIXA10. Quantiles are easily computed from a mathematical formulation (QdF model) of normative QdF curves. Validation of QdF curves on about 250 basins revealed three main groups of hydrological flood regimes. Using the same mathematical form, the great diversity in flood characteristics in each group is well represented by one set of model parameters. The great variability of floods in space and time could then be predetermined in all basins, using three sets of fixed model parameters.