Les divers types de modèles déterministes

Ingénieur en Chef à E.D.F. Chef du Département de la Recherche fondamentale du service Hydrologique de l'ORSTOM

Résumé

This report discusses the various conceptions underlying the design of deterministic precipitation/runoff transformation models. After a very brief review of the past development of such models, the author describes what he calls a general physical transformation scheme ; which refers to the various stages of the water circuit in the hydrological cycle of a catchment area. This scheme can be summed up as a number of exchanges taking place between reservoirs containing various storage levels likely to arise in real life, with which losses and flow lag can be associated as appropriate. A precipitation/runoff transformation model is simply the translation by means of methematical expressions of the relationships between some or all of the items forming part of the overall scheme. Irrespective of the form of the model, there is a series of successive operations which can be summed up as follows, the time step being taken as the operating unit : (i) determination of precipitation occurring during the time step ; (ii) reduction of precipitation ; (iii) construction of a unit hydrograph for each surface/element; (iv) "transporting" the hydrographs to the outlet (distributary); (v) summation of the element hydrographs reaching the outlet, giving the overall discharge. Considered from point of view of the vertical water cycle, the models can be subdivided into storage and runoff models. In the former, all the flows are governed by varyingly complex transfers between individual reservoirs. In the latter, runoff is considered to be the most important factor for the flood hydrograph, whilst the base flow is subjected 10 a much more summary analysis ; precipitation can be reduced either by considering a given absorption capacity or by application of a runoff coefficient. For the horizontal distribution of the transformation the catchment area can either be considered to react as a whole (overall model), or alternatively, each surface element can be taken as reacting individually, giving a hypothetical element hydrograph which is then "transported" to the outlet and added to those of the other surface elements to give the hydrograph for the whole catchment area. Outstanding among these models are the matrix models in which the catchment area is divided up on the basis of a characteristic matrix formed by the intersection of Thiessen polygons related to the various rain gauges and the isochronous network for the catchment area.