A three decades-long study on the variability of drought in relation to the contribution of rainfall was conducted in the Mekerra wadi watershed, located in northwest Algeria, covering the period from 1973 to 2005. The runoff and rainfall data were analysed by the Mann-Kendall test, the double mass curve method and the SPI index. Rupture of the series studied appeared during the 1980s. The rainfall and runoff trends with their contributions were sharply reduced, except in, when an increase was recorded. The region experienced extreme drought from 1981-1989 and from 1993-2001 with rainfall contributions greater than 60%. The area focused on in this study was found to have two distinct seasonal periods with different rainfall-runoff patterns. The first was from November to July and was characterized by precipitation with a small runoff contribution, while the second, from August to October, manifested intense rainfall accompanied by a strong contribution from runoff. The increase, which was recorded in August, September and October for all the parameters studied shows the importance of the superficial runoff component when combined with decreased infiltration. These climatic conditions reduce the natural recharging of groundwater, and cause increased susceptibility to soil erosion, reduced agricultural production and increased risk of floods.