A new version of the reconnaissance drought index, N-RDI

We present a new, modified version of the reconnaissance drought index (RDI), named the N-RDI. To calculate the N-RDI, precipitation and potential evapotranspiration (PET) were replaced with spatially normalized precipitation and PET. To assess the capability of N-RDI, the RDI and N-RDI were computed at monthly, seasonal, and annual time scales over Iran based on a climatic data series of 12 stations from 1967-2019. Interpolated maps of the RDI, N-RDI, and annual yield of rain-fed winter wheat were then prepared using the inverse distance weighting method. Four regions in Iran (with different climate conditions) were then selected, and 30 random points were chosen in each area. Finally, the capabilities of N-RDI and RDI were compared based on the correlation coefficient (CC) between the difference in the mean rain-fed winter wheat yield (DMRWWY) and the difference in the average RDI (Da-RDI) and the average N-RDI (Da-N-RDI) at selected points, in different regions at different time scales. The CCs were higher between DMRWWY and Da-N-RDI than between DMRWWY and Da-RDI in humid (H) and semi-arid (SA) regions, in H and hyper-arid (HA), in SA and arid (A), in SA and HA, and in A and HA regions in 88.23% of selected periods. Moreover, CCs were greater between DMRWWY and Da-N-RDI than between DMRWWY and Da-RDI in H and A regions in 76.47% of selected periods. The results showed that the capability of the N-RDI for assessing drought in large basins is greater than that of the RDI.