Assessment of Drought Indexes on Different Time Scales: A Case in Semiarid Mediterranean Grasslands

Drought is a significant challenge to semiarid Mediterranean grasslands, Increasing the accuracy of monitoring allows improving the conservation and management of these vital ecosystems. Meteorological drought is commonly monitored by the Standard Precipitation Index (SPI) or the Standard Precipitation Evapotranspiration Index (SPEI). On the other hand, agriculture drought is estimated by the Vegetation Health Index (VHI). This work aims to optimise the correlation between both drought types using the best transformation of VHI and the most appropriate time scale. Two drought-vulnerable Mediterranean grasslands were selected to evaluate the performance of the drought indexes. The SPI and the SPEI were calculated using data obtained from nearby weather stations. MODIS data were used to calculate the VHI. This index was standardised, naming it as SVHI. Our results revealed that SPEI was better correlated with VHI compared to SPI. In addition, SVHI obtained better results in the critical vegetation phases than VHI. Overall, SPEI and SVHI were the best correlated indexes. The quarterly scale showed stronger relationships than the monthly scale and the most correlated time frame were Mediterranean spring and autumn. This fact suggests that SPEI and SVHI could provide a plus point for increasing the precision of vegetation monitoring during these periods.

Automated summary

Drought poses a significant challenge to Mediterranean grasslands, and accurate monitoring is crucial for their conservation and management. The Standard Precipitation Index (SPI) and the Standard Precipitation Evapotranspiration Index (SPEI) are commonly used to monitor meteorological drought, while the Vegetation Health Index (VHI) is used for agricultural drought estimation. This study aimed to optimize the correlation between these two types of drought by determining the best transformation of VHI and the most appropriate time scale. The results showed that SPEI had a stronger correlation with VHI compared to SPI, and the standardized SVHI index performed better than VHI in critical vegetation phases. Overall, SPEI and SVHI were the most correlated indexes, with the quarterly scale showing stronger relationships, particularly in the Mediterranean spring and autumn. These findings suggest that SPEI and SVHI could enhance the precision of vegetation monitoring during these periods.