The efficiency of the Standardized Evapotranspiration Deficit Index (SEDI) in assessing the impact of drought on vegetation cover

It is important to choose an indicator that can optimally demonstrate the effects of drought intensity on soil moisture access, evapotranspiration and the changes in vegetation cover at a regional scale. Therefore, herein, SEDI was developed by the fit of the experimental distribution of Gringorten on evapotranspiration deficit based on TerraClimate data at the time scales of 1, 3, 6 and 12 months, and its relationship with Standardized Precipitation-Evapotranspiration Index (SPEI), Standardized Precipitation Index (SPI), Standardized Soil Moisture Index (SSMI), Normalized Ecosystem Drought Index (NEDI) and Normalized Difference Vegetation Index (NDVI) were investigated. The results indicated that SEDI has the highest significant correlation (above 95%) with NEDI and SPEI, especially for the 1-month time scale. This index also revealed the lowest correlation (less than 25%) with SPI on short-term time scales. The relationship between SEDI and SSMI indicated the high sensitivity of SEDI to the cumulative reduction of low amounts of soil moisture. According to the findings, the 6-month SEDI with NDVI showed the highest correlation with a 1-month delay (r = 0.64) and the best fit between them occurred in wet months. However, in the dry months, the relationship between SEDI with NDVI was affected via water availability stresses, grazing intensity and pest infestation. Finally, the use of SEDI at a regional scale, especially in arid and semi-arid regions like Lorestan, could be recognized as an important index in depicting the effects of drought on vegetation cover, due to the use of the actual evapotranspiration factor.