The paper focuses on the development of reservoir operating rules for dry and rainfall events, and their implementation in the case of the Ghézala dam located in northern Tunisia (characterized by Mediterranean climate). Rainfall events are defined in terms of depth and duration that are correlated to each other. A depth analysis per event is performed, conditioned on the event duration. The gamma distribution provides a good fit to depth per event, especially for events lasting at least 6 days. The event duration fits a geometric distribution, whereas the dry events during the rainy season fit a negative binomial distribution. The climatic cycle length is fitted to a gamma distribution. On this basis, many 50-year synthetic event series were generated. Every synthetic streamflow sequence obtained from synthetic rainfall sequences as well as the one derived from the historic rainfall events time series were optimized and optimal decisions were formulated. These decisions were assessed by means of multiple regression analysis to estimate the relation between the optimal decision to every stage (dry or rainfall event) and other system variables. Optimal rules, which have a linear form, were derived by predetermined useful storage interval and depend on storage, inflows and downstream demand at dry or rainfall event t. The range of t is 1-13 days (rainfall event) and 1-57 days (dry event).The rules were satisfactory for every predetermined useful storage interval. The simulated dam performance generated by the operation rules was compared with the deterministic optimum operation and the historical operation. Also included is the comparison of the implicit stochastic optimization-based operation policy per event during the water years 1985-2002.
Abstract. This paper analyses a 42 year time series of daily precipitation
in Ichkeul Lake Basin (northern Tunisia) in order to predict extreme
dry-spell risk. Dry events are considered as a sequence of dry days
separated by rainfall events from each other. Thus the rainy season is
defined as a series of rainfall and subsequent dry events. Rainfall events
are defined as the uninterrupted sequence of rainy days, when at last on one
day more than a threshold amount of rainfall has been observed. A comparison
of observed and estimated maximum dry events (42 year return period) showed
that Gumbel distribution fitted to annual maximum series gives better
results than the exponential (E) distribution combined with partial duration
series (PDS). Indeed, the classical Gumbel approach slightly underestimated
the empirical duration of dry events. The AMS–G approach was successfully
applied in the study of extreme hydro-climatic variable values. The results
reported here could be applied in estimating climatic drought risks in other
geographical areas.
Résumé Un modèle hydrologique global a été calé sur des épisodes pluvieux de durée très variable (de un à plus de dix jours). Les épisodes pris en compte sont ceux qui sont encadrés par au moins un jour sec et ne provoquant pas de déversement du barrage. L'entrée de ce modèle comprend la pluie moyenne du bassin versant et l'indice des précipitations antérieures de Kohler & Linsley. Les expressions du modèle hydrologique du début et au cours de la saison humide, selon un seuil du cumul des pluies de la saison depuis le début de l'automne, sont de la forme non linéaire polynomiale du second ordre. Le modèle s'est montré performant dans l'évaluation de la lame ruisselée malgré le faible nombre de postes pluviométriques (un) pour la taille du bassin versant de 48 km 2 . L'absence d'autres postes pluviométriques sur le bassin versant n'a pas permis d'étudier la précision du modèle en considérant une pluie moyenne sur le bassin versant au lieu de la pluie uniquement au site du Barrage Ghézala.Mots clefs barrage; bilan hydrologique; épisode pluvieux; fonction de production; lame ruisselée; Nord de la TunisieModelling of the runoff-rainfall relationship by duration of rainy episode in a northern Tunisian basin Abstract A global hydrological model was calibrated by rainfall events of varying duration (from 1 to 10 days). The events considered were those separated by at least one non-rainy day on each side, and not causing reservoir spillage. The input of this model includes mean rainfall over the catchment, and the Kohler & Linsley antecedent precipitation index. The expressions of the hydrological model for the beginning and the middle of wet season, according to a threshold of the cumulative rainfall since the start of the rainy season, are in a polynomial nonlinear form of the second order. This model proved to be capable of estimating runoff correctly, in spite of the small number of raingauges (one) for the catchment size of 48 km 2 . The absence of other rainfall stations over the catchment area did not allow the accuracy of the model to be studied by assessing mean rainfall over the catchment rather than the rainfall at the single site of Ghézala Dam.
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