SUMMARY
Northwestern Algeria, like most Mediterranean regions experienced alternating wet and dry periods. In a context where climate change is conjugated news at the impact of human activities on the water overuse, it is essential to analyze the dry spells in various time scales for Prevention of rationalization of water resources. To this end, the rainfall series reconstructed three stations covering the river basin Boumessaoud study were analyzed. The analysis was based on the values of the standardized precipitation index (SPI) calculated for time scales of 10, 35 and 68 years. The results show that medium and long-term periods 1945-1952 and 1978-2010 have experienced significant droughts by intensity, duration and frequency. These dry episodes culminated especially in the early 80s with extremely severe type of drought. The stations Henaya the center of the basin and the Zenata in the far north are the most affected by drought compared to the Tlemcen station located outside near the limit of Southeast Basin.