Two problems are addressed which arise when using monthly water balance models as an aid to making decisions in water resources engineering: what is the influence of data errors on model performance, and what is the data length required in order to obtain reliable models? Two previously defined types of models are used: in PE type models the input series are precipitation and potential évapotranspiration; in P type models the only input is precipitation. The main conclusions are:(1) random errors in precipitation data, when great enough, affect model performance significantly; (2) systematic errors in precipitation data are less important for the estimation of river flow; and (3) a data length of 10 years is necessary and sufficient for a reliable calibration of monthly water balance models of humid basins.Sensibilité de modèles pluie-débit à l'échelle mensuelle aux erreurs de mesure concernant les précipitations et à la durée des chroniques de mesures utilisées Résumé Deux problèmes se posent lors de l'utilisation de modèles de bilan d'eau en tant qu'outils d'aide à la décision en matière de planification des ressources en eau: quelle est l'influence des erreurs de mesure sur les performances du modèle et quelle est la durée des chroniques nécessaires à l'obtention d'un modèle fiable? Deux types de modèles, définis auparavant, ont été étudiés. Les données d'entrée des modèles PE sont les précipitations et l'évapotranspiration potentielle. Les précipitations constituent la seule entrée des modèles P. Les principales conclusions sont les suivantes: (1) des erreurs aléatoires supérieures • à 10 % sur les précipitations affectent de façon significative la qualité des résultats du modèle; (2) des erreurs systématiques sur les précipitations revêtent moins d'importance, en particulier pour les modèles de type P; et (3) une chronique de mesures d'une durée de dix ans est nécessaire et suffisante pour un calage fiable.
A point rainfall generator is a probabilistic model for the time series of rainfall as observed in one geographical point. The main purpose of such a model is to generate long synthetic sequences of rainfall for simulation studies. The present generator is a continuous time model based on 13.5 years of 10-min point rainfalls observed in Belgium and digitized with a resolution of 0.1 mm. The present generator attempts to model all features of the rainfall time series which are important for flood studies as accurately as possible. The original aspects of the model are on the one hand the way in which storms are defined and on the other hand the theoretical model for the internal storm characteristics. The storm definition has the advantage that the important characteristics of successive storms are fully independent and very precisely modelled, even on time bases as small as 10 min. The model of the internal storm characteristics has a strong theoretical structure. This fact justifies better the extrapolation of this model to severe storms for which the data are very sparse. This can be important when using the model to simulate severe flood events.
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