Remote Sensing Standardized Soil Moisture Index for Drought Monitoring: A Case Study in the Ebro Basin

The occurrence and duration of droughts have increased in recent years, reinforcing their role as a major climate risk. This study evaluates a remote sensing soil moisture-based drought index, the Standardized Soil Moisture Index (SSI), as a tool to monitor different types of drought, from meteorological, agricultural to hydrological. The satellite-derived SSI at different integration times (from SSI-1 up to SSI-24) was compared with the Standardized Precipitation Index (SPI), calculated using precipitation data from 239 meteorological stations in the Ebro Basin. A good correlation (R>0.6) was found between the indices at all integration times. Our results suggest that, independently of the time scale, SSI tends to relate better to the SPI with an additional month for its integration time, reflecting soil moisture’s inertia. Comparison with a gridded SPI product further confirmed that SSI captures basin-wide drought variability, also suggesting that it can observe hydrological processes such as snowmelt and irrigation. These findings demonstrate that remote-sensed SSI is a robust and versatile drought index, capable of monitoring multiple drought types without relying on in situ measurements. Provided the existence of quality soil moisture data, satellite-derived SSI stands as a drought indicator with high coverage and enhanced spatial detail. Hence, this methodology paves the way for accurate drought monitoring in data-scarce regions.