Investigating the vegetation's temporal–spatial response to meteorological and hydrogeological drought in drylands

As the most constraining environmental factor of vegetation development in arid areas, soil moisture is mainly supplied by precipitation and groundwater resources. Considering the varying sensitivity of different plant communities to precipitation and groundwater-induced water shortages, the communities' management requires the study of the effects of both meteorological and hydrogeological droughts on vegetation. Therefore, this study sought to model the effects of meteorological and hydrogeological droughts on vegetation indices obtained from MODIS satellite images in the Sirjan plain from 2000 to 2019. To this end, vegetation communities were first identified and separated based on extensive field operations, taking into account the Normalized Difference Vegetation Index (NDVI) and Vegetation Condition Index (VCI) at the plant communities' scale. Then, meteorological drought was calculated using Standardized Precipitation Index (SPI) and the hydrogeological drought was also measured by Groundwater Resource Index (GRI) via the Kriging technique. Finally, the relationship was modelled using Multivariate Linear Regression (MLR). The results revealed that SPI at a 6-month time scale (as an important index) played a significant role in elaborating the changes in VCI in eight communities out of 18 ones at the 6-month scale, acting as a strong and reliable estimator. Moreover, 61.6% of VCI changes in Artemisia sieberi-Seidlitzia rosmarinus community were explained with GRI and SPI at a 6-month time scale, indicating the dependence on groundwater and precipitation. Nonetheless, some communities (i.e. six cases) were unresponsive. These findings draw our attention to the importance of considering the special reaction of each plant community in the management.