Drought assessment and rainfall trend analysis in a southern Mediterranean watershed during the last century (1914-2011)

The current study was carried out in the Cheliff watershed in Algeria, to assess the drought severity and analyze the significance of the spatiotemporal trend of annual rainfall during the period from 1914 to 2011. In this study, the rainfall series of eight stations were subject to standardized precipitation index (SPI) analysis, homogeneity tests, trend analysis and exponential smoothing in order to make a short time rainfall prediction. The results showed that the annual rainfall amount was highly dependent on latitude and elevation. Through the SPI analysis, a severe dry period closely related to the North Atlantic Oscillation (NAO) was observed during the last three decades in the Cheliff watershed, the most significant negative trend in the annual rainfall was registered above a latitude of 35°; indeed, above this latitude, there was a highly significant decrease in the probability of exceedance of a specific precipitation amount during the last 30 years. The major rainfall shift occurred around 1980, and according to simple exponential smoothing, the area will witness relative rainfall stability below a latitude of 35° and a small increase above this latitude over the next 10 years.