Low flow-duration-frequency models. Concept and use for a regional approach to watershed low-flow regimes in the Loire (France) and Crisu-Alb (Romania) regions
The French Department of the Environment entrusted to the CEMAGREF the realization of a study relating to an ecosystemic approach to the management of the Loire watershed. Over the long term, this approach is designed to establish orientations for the protection and management of the natural aquatic environments of the Loire. To this end, a good knowledge of the variability of the hydrological regimes is necessary. The method presented relates primarily to low flow discharges. In spite of the great heterogeneity in space and time of low flows, a regionalization study of watershed low flow regimes of Loire (S=117 000 km2) has been attempted on the basis of the same QdF (discharge-duration-frequency) concept developed for floods (GALEA and PRUDHOMME, 1996). To allow an objective evaluation of developed methodology, we present here the details of the conceptual approach used to model and synthesize the low flow regimes observed, and present the results obtained. The methodology relates primarily to relatively undisturbed basins with perennial flow. The variability of low flow regimes takes into account two hydrologic variables expressing two notions of complementary regimes: the notion of annual minimal average flow (VCNd), and the annual minimal threshold discharge (QCNd) below which the flow does not decrease for a continuous duration (d) which varies from one day to 90 days. The statistical study of annual events demonstrates that the Log-Normal law (two parameters) is generally adequate for a given duration d. From the low flow typology observed on 57 selected sites, the statistical study of annual events revealed four homogeneous groups of hydrological low flow regimes. In each of the 4 groups a reference basin is chosen. Its flow-duration-frequency curves normalized to discharge and duration show a regional representativity and the mathematical formalism yields easy-to-use dimensionless curves. The normative discharge VCN(2,1) is the minimum annual daily flow for a two year return period. The normative duration of flow (De) of a basin results from the recession curves described by MAILLET's equation. In each group, a large diversity of low flow regimes exists, as indicated by the two local descriptors (De and VCN(2,1)), which are the only input parameters for the low flow QdF models. Concerning the Loire watershed, these 4 models and their associated typology characterize the large diversity in space and time of low flows observed for the 57 studied cases. A first validation of the low flow QdF methodology concerned 12 basins of Crisu-Alb watershed, in Romania. It was not necessary to establish new reference models for the regionalization of low flows for these 12 studied cases in the Crisu-Alb watershed. The quality of the transposed models confirms, on another hand, that the two local descriptors of the low flow regime are good integrators of flow processes in relation with low flows. The initial results from the synthetic low flow models are promising. Use of these models on ungauged basins will require additional research to explain the model's choice criterion and the two local descriptors of the low flow regime.