CLIMATIC CHANGE IN NORTH AFRICA DURING THE LAST CENTURY AND ITS IMPACT ON WATER AVAILABILITY

Water scarcity is a major threat to the economy of any country, especially countries that have a semi arid/arid climate. Drought risk is based on a combination of the frequency, severity, and spatial extent of drought and the degree to which a population or activity is vulnerable to the effects of drought. Northern Algeria experienced severe drought that began in the early 1970's. In order to understand the spatial and temporal variations in climate, we developed a new methodology for the establishment of inter annual rainfall maps, for our case study; we worked on the time period between 1922 through 1989. An original dataset of 475 rain gauges distributed along northern Algeria was used. The first step of the study consisted in checking the validity and consistency of the data to detect anomalies/errors within the datasets. A statistical test known as "Ellipse de Bois", which provides a qualitative evaluation of the existence of a discontinuity in the rainfall time series, was used. A discontinuity in the trend would have different significations: accidental, instrumental or natural. Each anomaly was verified by going back to the hard copies on which the values were reported by the original observer. The second step of the study consisted of the development of a new method for data reconstitution in which the missing values are recovered using the regional vectors of the principal component analyses linked to an iterative process. The third step consisted of establishing a multiple regression analysis between the observed annual rainfall at each rain gauge and the digital elevation model, latitude, longitude and distance to the sea. A rainfall map was established using a thorough geostatistical analysis. At each rainfall gauge, a value was reconstructed using the regression equation. The fourth step consisted of subtracting the rain values obtained by multiple regression at each known point and establishing the residual map, knowing that the residuals are second-order stationary. The final map was obtained by summing the map obtained in the third and fourth step. Another conclusion of the study was the identification of a temporal succession of two droughts and two humid periods from 1922 to 1989, and the drought that began in 1975 continues to this day.