Drought investigation and trend assessment in Macta watershed (Algeria) by SPI and ITA methodology

In this study, severe and extreme episode (dry and wet) periods were analyzed by the Standardized Precipitation Index (SPI), using 42 weather stations in the Macta watershed (Northwest Algeria) over a period of 42 years (1970–2011). First, short- and medium-term (1, 3, 6, and 9 months) and long-term (12 months) SPIs were calculated, and then their trends were detected using the innovative trend analysis (ITA) method, which identifies trends in low, medium, and high values within series. Results show that in the short and medium term, the 42 studied stations, in the Macta watershed, are gradually subject to meteorological and then agricultural drought, marking a water deficit since 1970, affecting vegetation. On the other hand, and in the long term, this deficit is compensated by severe and extreme wet conditions (a water balance will be established between dry and wet episodes). In anticipation, the ITA shows that, on average, 82% of stations, with severe and extreme dry conditions, underwent a gradual reduction, accompanied by 59% of stations showing a reduction in severe and extreme wet conditions. In the long term, convergence between severe and extreme dry conditions and wet ones has been detected, making the situation towards normal conditions. In addition, the majority of the stations suffer from a severe meteorological drought in the center-East of the basin (drought frequency above 100), whereas on the hydrological scale (SPI-12), the situation is balanced (drought frequency below 50).