ABSTRACT:The potential impact of large-scale climate patterns of El Niño Southern Oscillation (ENSO), North Atlantic Oscillation (NAO) and Pacific Decadal Oscillation (PDO) in modulating precipitation regimes across the upper Medjerda River Basin in northern Tunisia is assessed. As the case study is located in the Mediterranean region (North Africa), the regional Mediterranean Oscillation (MOAC) and Western Mediterranean Oscillation (WeMO) are also investigated. Six precipitation time series are also observed. Strong correlations are identified between ENSO and precipitation series at lag −2 years. Extreme ENSO years are reflected in the precipitation as periods of severe water deficit or excess. Wavelet spectra driven to seasonal precipitation reveal that precipitation is organized in preferred bands with distinct activities in each scale band, and that most of the precipitation variance is explained by the 2-8-year scales. Using cross-wavelet analysis, climate patterns that are most associated with precipitation variability are identified. The analysis demonstrates also that precipitations are simultaneously controlled by different climate patterns. Particularly, the influence of ENSO on precipitation is stronger as well as that of PDO and MO. Results indicate that precipitation variability at the upper Medjerda River Basin is associated with global-scale ENSO processes at the annual as well as seasonal time scales and is aligned with changing phase difference between periods. Moreover, separation of annual precipitation into two seasons reveals statistically significant associations between El Niño and La Niña phases of ENSO with dry and wet seasonal precipitation, respectively. Complementing this, three streamflow records with length up to 104 years are used, and relationships with rainfall series are analysed using wavelets. A strong coherence between rainfall and streamflow observations is found and justifies undertaking the study of climate-streamflow relationships where ENSO exhibits potential impacts on annual streamflows.

Résumé Un filtre de Kalman standard a été développé en vue de la mise à jour de certaines variables d'état de la fonction du sous-sol et de la sortie du modèle HBV. La version globale du modèle HBV, fonctionnant au pas de temps horaire, a été calée par essais et erreurs sur les bassins tunisiens des Oueds Barbara et Mellila. Le pas de temps journalier a été considéré pour évaluer la qualité de la reconstitution. Le modèle a été ensuite couplé avec le filtre en reformulant le modèle conceptuel en un système d'équations d'état dynamiques et en implémentant la procédure du filtre de Kalman. L'hydrogramme des débits horaires constitue la mesure introduite pour la correction des états. Malgré un choix suivant la littérature des erreurs sur les mesures et les états du système, ce filtre a permis de réduire nettement l'incertitude sur le débit engendré par le processus pluie-débit et la pluie d'entrée.Mots clefs assimilation des données; filtre de Kalman; modèle HBV; mise à jour; modélisation pluie-débit; temps réel Integration of a Kalman filter in the HBV hydrological model for runoff forecastingAbstract A standard Kalman filter was developed with the purpose of updating certain state variables of the lower zone function and the output of the HBV model. The global version of the HBV model, functioning with an hourly time step, was calibrated by trial and error to the Tunisian basins of the Barbara and Mellila Wadis. The daily time step was considered for evaluating the quality of the data reconstitution. The model was then coupled with the Kalman filter by reformulating the conceptual model as a system of dynamic state equations and implementing the Kalman filter procedure. The hourly discharge hydrograph constitutes the introduced measurement to correct state variables. Despite a literature-driven selection of the errors on measurements and system states, this filter clearly allowed reduction of the uncertainty on discharge produced by the rainfall-runoff process and the precipitation input.