Evaluation of Meteorological Drought Using SPI and SPEI on Wheat Yield in Southwestern Iran

The objective of this research is to determine the meteorological drought index and the effective rainfall model that exhibit the highest correlation with the yield of rainfed wheat in the Karkheh watershed. Additionally, using spatial statistics analysis, the trend and status of drought in various parts of the watershed will be identified. This will allow for the determination of suitable areas for rainfed wheat cultivation in the near future. In this research, meteorological drought monitoring was conducted using the standardized precipitation index (SPI) and standardized precipitation evapotranspiration index (SPEI) drought indices at 34 synoptic stations of the Karkheh watershed in southwestern Iran from 1981 to 2016. Effective precipitation (EPCP) was calculated using four methods, namely the United States bureau of reclamation method (USBR), the simplified version of soil conservation service of the United States department of agriculture method (USDA-SCS simplified), the food and agriculture organization of the United Nations method (FAO), and the CROPWAT version of the USDA-SCS method (USDA-SCS CROPWAT). The correlation between SPI and SPEI and between wheat yield and these indices was performed using Pearson’s correlation coefficient (R) to select the best index for assessing the effects of meteorological droughts on the agricultural sector. The analysis of the drought features showed that the SPEI reported milder and longer droughts in most synoptic stations. The spatial pattern of drought in the Karkheh watershed was determined using spatial statistics methods, and global Moran’s I statistic showed that the drought in the Karkheh watershed had spatial autocorrelation and a clustered pattern with a 99% confidence level. The results of global and local Moran’s I and the Getis-Ord statistic (Gi*) showed that milder droughts prevailed in the northern clusters and more severe droughts in the southern clusters. Also, the correlation between wheat yield and the SPI and SPEI was positive in all stations except for the stations of Kermanshah province and Aqa Jan Bolaghi, which is due to the existence of numerous dams and springs in these areas. Considering the impact of temperature (Tmean) on meteorological drought, it is suggested that in addition to the SPI, the SPEI should also be used to predict droughts in the Karkheh watershed.