The 2000-2017 drought risk assessment of the western and southwestern basins in Iran

The study investigates the relationship between drought and vegetation variations in three main basins of western and southwestern of Iran, using the comprehensive approach of time-series analysis, Standardized Precipitation Index (SPI), and anomaly calculation. A total of 30 ground stations' daily meteorological data and the MODIS 16-day composite NDVI products (2000-2017) were used in this case. The results showed that in the Great Karoun River (GKRW) and Karkheh River (KRW) sub-regions, the NDVI decreased slightly over the study period. Simultaneously, in the West Marginal (WMW) sub-region the NDVI slightly increased somewhat. Overall, the NDVI in the whole study area presented an insignificant decrease. Also the results found that there is a significant relationship (p < 0.02) between the NDVI (0.3-0.7) with spring drought and between NDVI (0.2-0.3) with fall drought (p < 0.01). There is a marginally significant relationship (R = 0.43, p = 0.07) between NDVI (0.3-0.4) with fall drought and a relatively high, but insignificant relationship between NDVI (0.7-0.8) with spring drought (R = 0.41, p = 0.09). Low temperature can play an important role in determining the relationship between SPI and NDVI, which evidenced that it can trade-off the effect of a very wet year. As well as a year with very wet condition can affect the NDVI of the coming years. Since summer is hot season in the study area and most of the vegetation consists of shrubs and grasslands, of which the NDVI ranges between 0.2 and 0.4, the vegetation cover can be significantly affected by a wet summer with abundant precipitation.