Satellites Reveal Two Decades of Deepening Drought Across Northwestern Algeria

Northwestern Algeria is drying out, and the evidence now comes from space. A new study published in Theoretical and Applied Climatology has assembled a twenty-one-year picture of drought across the region, combining satellite rainfall estimates with vegetation and temperature measurements to track how one of North Africa’s most water-stressed landscapes has shifted from a climate of alternating wet and dry spells into one of persistent aridity. The analysis, led by Ramzi Benhizia of the University of Debrecen together with colleagues in Algeria, Hungary, and South Africa, covers 2003 to 2023 and identifies the years after 2015 as a decisive turning point.

The research team confronted a familiar problem in North African climatology: ground-based weather stations are unevenly distributed and their records are often incomplete, particularly in the interior and semi-arid zones far from the Mediterranean coast. To overcome this, the researchers turned to two widely used satellite data streams. The CHIRPS dataset, which blends infrared satellite estimates with station observations at a resolution of roughly 5.3 kilometers, provided continuous gridded rainfall records. The MODIS instruments aboard NASA’s Terra and Aqua satellites supplied 16-day vegetation greenness measurements at 250-meter resolution and 8-day land surface temperature data at 1-kilometer resolution. All processing was carried out in Google Earth Engine, the cloud-based platform that has become the workhorse of large-scale environmental analysis.

From these datasets the team computed a suite of complementary drought indices. Meteorological drought was quantified with the Standardized Precipitation Index, or SPI, approximated through standardized precipitation anomalies at three timescales: three, six, and twelve months. The SPI, recommended by the World Meteorological Organization as a standard drought-monitoring tool, expresses how far accumulated rainfall deviates from the long-term norm, with negative values signaling drier-than-normal conditions. On the vegetation side, the researchers derived the Vegetation Condition Index from NDVI, the Temperature Condition Index from land surface temperature, and the Vegetation Health Index, which combines the two with equal weighting to capture both moisture deficit and heat stress on plants.

The temporal patterns that emerged are striking. The early part of the record, from 2003 through 2014, was dominated by wet conditions, with pronounced positive rainfall anomalies in 2007, 2009 through 2011, and 2013. Around 2015, however, the hydroclimate flipped. A persistent precipitation deficit took hold and continued through 2023, intensifying from moderate drought in 2015 and 2017 to severe conditions between 2019 and 2023. The year 2023 recorded the most extreme drought of the entire study period, with large areas experiencing severe to extreme precipitation deficits. Recurrent drought episodes also appeared earlier, notably in 2007 through 2009, 2011 through 2012, and a pronounced dry phase from 2020 to 2023.

Trend analysis confirmed that this is not merely natural variability. Using the non-parametric Mann-Kendall test, which is robust to outliers and makes no assumptions about data distribution, together with Sen’s slope estimator to quantify the rate of change, the researchers found widespread and statistically significant drying across all SPI timescales. At the three-month scale, 56 percent of the study area showed significant negative trends, with a mean Sen’s slope of minus 0.10 per year. The six-month scale showed significant drying across 55 percent of the region, and even the twelve-month scale, which integrates rainfall over a full year, showed significant drying across 39 percent of the area. Notably, no significant wetting trends were detected anywhere in the domain at any timescale.

The vegetation indices told a more nuanced but ultimately consistent story. The Vegetation Health Index revealed a clear imbalance between degradation and recovery: 4.89 percent of the study area showed significant decline in vegetation health, compared with only 0.51 percent showing significant improvement, meaning degraded land outweighed improved land by nearly a factor of ten. The Vegetation Condition Index showed spatially heterogeneous responses, with 3.4 percent of the area experiencing significant vegetation decline and 2.7 percent significant greening, while the Temperature Condition Index showed almost no significant trends at all, with less than 1 percent of pixels changing significantly. The worst drought years for vegetation were 2008, 2012, 2017, and especially 2022, when thermal stress reached its two-decade minimum and affected even typically resilient provinces such as Chlef, Tissemsilt, and Mostaganem.

Geography mattered enormously. The interior, semi-arid steppe provinces of Saida, Tiaret, Sidi Bel Abbes, and Mascara emerged as consistent drought hotspots, while coastal and mountainous areas with Mediterranean forest cover proved markedly more resilient. This gradient mirrors the region’s precipitation pattern, which ranges from more than 700 millimeters per year in the northern Chelif basin to roughly 240 millimeters per year in the interior Tafna basin. The landscape itself shifts from Aleppo pine and oak forests along the coastal fringe to open steppe grasslands of esparto grass and white wormwood in the interior, and these different vegetation types respond to water stress in distinctly different ways.

Perhaps the most technically revealing result came from the correlation analysis linking rainfall to vegetation response. The relationship between SPI and the Vegetation Health Index was moderate but statistically significant at short timescales, with a Pearson correlation coefficient of 0.567 at the three-month scale and 0.509 at the six-month scale. At the twelve-month scale, however, the correlation weakened to 0.364 and lost statistical significance. This pattern indicates that vegetation in the region responds most strongly to short- and medium-term precipitation variability, while longer-term deficits are partially buffered by soil moisture reserves, groundwater, and ecological resilience mechanisms. For drought early-warning systems, the practical implication is that satellite vegetation monitoring is most informative when paired with seasonal rather than annual rainfall indicators.

The study’s authors place these findings in a broader climatic context. The post-2015 intensification aligns with projections for the western Mediterranean basin, widely identified as a climate-change hotspot, and with a documented shift toward persistent dryness in the Maghreb that some researchers trace back to 1976. Rising temperatures compound the problem by increasing atmospheric evaporative demand, producing so-called hot droughts in which even near-normal rainfall fails to prevent soil moisture depletion and vegetation stress. Anthropogenic pressures, including deforestation, overgrazing, and agricultural expansion, further degrade the soil’s capacity to retain water. The team acknowledges limitations, noting that the precipitation-only SPI does not capture temperature-driven evapotranspiration, and recommends future integration of temperature-sensitive indices such as SPEI alongside satellite soil moisture data. What the study delivers now is a scalable, cost-effective monitoring framework built entirely on freely available data, offering Algerian policymakers a spatially explicit foundation for early-warning systems, agricultural planning, and water allocation as the region confronts what the authors describe as a new climatic reality of persistent water scarcity.

Subject of Research: Integrated meteorological and remote sensing assessment of drought trends in Northwestern Algeria from 2003 to 2023

Article Title: Bi-decadal drought assessment in Northwestern Algeria: Integrating meteorological and remote sensing indices

Article References: Bi-decadal drought assessment in Northwestern Algeria: Integrating meteorological and remote sensing indices. (n.d.). https://doi.org/10.1007/s00704-026-06526-y

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06526-y

Keywords: drought, Algeria, remote sensing, Standardized Precipitation Index, MODIS, CHIRPS, vegetation health index, Mann-Kendall trend analysis, aridification, Mediterranean climate, Google Earth Engine, early warning systems