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The objective of this study is to estimate the extreme floods corresponding to different recurrence intervals in Trois Rivieres watershed, a large sub-basin of the Macta basin (NW-Algeria). Two regional models are used flow-duration-frequency (Qdf) modelling and gradex method. A methodology for modelling QdF curves based on the so-called continuous converging model has been applied. The second model is gradex method which is a rainfall-runoff probability approach to computing extreme flood discharges in a river. The QdF model gives a complete description of the flood dynamics of Trois Rivieres basin. Nevertheless, it is recommended to use this method with vigilance for the predetermination of severe floods for important return periods. However, the gradex method overestimates the extreme floods for different recurrence intervals. This study highlights that diverse methods should be employed for the regional analysis of extreme hydrological events.

In Algeria, the climate change in recent decades has a negative impact on water resources. The goal of this study is to determine the influence of these hazards on surface water resources in the basin of Macta (Northwest of Algeria). The study of climatic pulsations is implemented from the climate coefficient changes and its moving averages over 3 years. Variations of this coefficient based on years, for the period 1949–1973, show an alternation of wet and dry years. Starting from 1973, a very dry period of more than 20 years was installed in the basin, with deficits ranging from 22% to 72%. The consequences of the surface water deficit can damage the environmental balance and consequently affect the various human activities that are directly or indirectly related to the use of these resources.

En Algérie, les conditions climatiques qui prévalent depuis trois décennies ont une influence négative sur la ressource en eau. Ce travail a pour finalité de déterminer leur impact sur les cours d'eau du bassin versant de la Macta (Nord-Ouest de l'Algérie). L'étude des précipitations annuelles (de septembre à août) sur la période 1930-2002 montre une diminution très nette des pluies à partir de la première moitié des années 1970. Les débits des cours d'eau aux stations de Trois Rivières (sur l'oued El Hammam), de Sidi Ali Ben Youf et de Sidi Bel Abbès (sur l'oued Mekerra) en ont été affectés. Sur la période 1976-2002, les lames d'eau écoulées annuelles moyennes sont de 28 à 36 % plus faibles que sur la période 1949-1976. En valeurs moyennes, les précipitations se révèlent plus faibles pour toutes les saisons, mais les écoulements sont peu ou pas diminués en automne.In Algeria, the weather of the last three decades has a negative impact on water resources. The aim of this paper is to determine their impact on the rivers of the Macta basin (North-West of Algeria). The study of annual rainfall (September to August) over the period 1930-2002 shows a sharp decrease of rainfall from the first half of 1970. The flow of Trois Rivières station (wadi El Hammam), Sidi Ali Ben Youb station and Sidi Bel Abbes station (wadi Mekerra) were affected. Over the period 1976-2002, the annual average depth of runoff of runoff is 28 to 36% lower than for the period 1949-1976. On average, rainfall may be lower for all seasons, but the flows are not reduced in the autumn.

At the end of the 1970s, south-western Algeria was marked by a severe, wide-ranging and persistent drought due to its extraordinary rainfall deficit. Rainfall in this region is characterized by taking into consideration the annual data of a certain number of rainfall stations with more than 60 years of observations (1930– 1999). For each station we analyzed a series of annual rainfall (P) and considered their coefficient of variation (CV). This coefficient expresses the rate of rainfall in time and can be used for comparative studies. We noted that in this region, rainfall was very low with the greatest variability. In order to study the evolution of the total rainfall for winter and spring, we employed two methods: the moving average and the average of accumulated variations. We noted that the winter and spring rains evolved differently; however, from the 1980s to the present, these seasonal totals are the lowest and show the greatest deficit. For each of the stations selected, a rainfall index was calculated defined as a reduced central variable. We established a cartography of the average for each decade of these indexes with a view to revealing the contrast between the different periods studied. Generally speaking, it seems that there is an upward trend in the 1930s and mid-1940s. From the 1950s, we note an increase in rainfall deficit at the level of the zone studied. The fall in total annual precipitation is greatest during the two decades that follow. However, the most sudden and significant fluctuation (in the statistical sense) was observed around the 1980s. This period of deficiency has since been characterized by its intensity and duration.