Evaluation of an Evapotranspiration Deficit-Based Drought Index and Its Impacts on Carbon Productivity in the Levant and Iraq

Because the physiological and biochemical activities of vegetation, growth, and productivity of crops, especially ecosystem CO2 fluxes, and other ecological interactions are constrained by (1) varying degrees of drought at various timescales and (2) components of drought indices, it is still difficult to understand how the productivity of terrestrial ecosystems and vegetation types respond to drought. The terrestrial ecosystem in the Middle East has been subjected to severe droughts. It is yet unknown how these conditions have changed the gross primary productivity (GPP) in Middle East terrestrial ecosystems. Herein, we investigate this variability using a coupling analysis between the Standardized Evapotranspiration Deficit Index (SEDI) and the standardized gross primary productivity residuals series (sGPPRS). This was accomplished by using the annual GLASS-GPP data set for the Middle East, covering 36 years (1982-2017). Additionally, the GLEAM-version (v3.3a), which calculates yearly actual and prospective evaporation-one of the components of terrestrial evapotranspiration-was employed. The findings show that in the Levant and Iraq, where 30%-34% of croplands and 25%-35% of grasslands recorded a positive significant correlation against the SEDI, crops and grassland ecosystem productivity are more sensitive to drought. Notably, the GPP showed a sharp decline of more than 60% in the agrarian ecosystems of northern Iraq and northeastern Syria during the 2008 extreme drought, which culminated in a total vegetation deficit with a 100% negative anomaly. The significant drop in yearly SEDI during the extreme drought period in 2008 may have contributed to the decreases in the annual GPP and anomalies from 2009 to 2012. Thereafter, as evidenced by the negative GPP anomaly, the moderate and slight droughts from 2009 to 2012 led to consistently stressed vegetation. This ultimately resulted in a protracted delay in the vegetation cover of the ecosystem being restored.