Abstract
Drought Indices are extensively adopted as a drought detection and monitoring tool in all climatic regions. This study assesses and compares the performance of four rainfall based drought indices, Deciles index (DI), Percent of Normal Index (PNI), Standardized Precipitation Index (SPI) and Z-score Index (ZSI) in detection and classification of drought events in Imo state , a humid environment of Nigeria from (1982 to 2016). The resultshows that droughts do occur in humid climatic region of Nigeria. The various indices differ in their detection and classification of drought events in the study area. However, they classified 2007 and 2008 as the year with the most severe drought event in the state. There is high correlation (r > 0.70) among the four indices, with ZSI and PNI showing the highest relationship to SPI (r= 0.97). The differences observed among the drought indices in detecting drought events in the study area can be reduced by using multiple drought indices in addition to the use of SPI in drought assessments especially in Nigeria since technological advancement in drought monitoring software’s has made it easier. Also it is worthwhile to research on why drought detection varies with indices.
 Keywords: Drought assessment, Rainfall-based drought indices, humid environment, Imo state, Nigeria.
Highlights
Drought Indices are extensively adopted as a drought detection and monitoring tool in all climatic regions
There is high correlation (r > 0.70) among the four indices, with Z-score Index (ZSI) and Percent of Normal Index (PNI) showing the highest relationship to Standardized Precipitation Index (SPI) (r= 0.97)
The differences observed among the drought indices in detecting drought events in the study area can be reduced by using multiple drought indices in addition to the use of SPI in drought assessments especially in Nigeria since technological advancement in drought monitoring software’s has made it easier
Summary
Data: This study utilised daily precipitation amount from January 1982 to December 2016 for Owerri weather station from the archive of the Nigerian Meteorological Agency, Oshodi It is the only weather station in Imo state. The observed precipitation on a given time scale deviates from the long-term average rainfall It is the standard drought index used in monitoring drought (WMO and GWP, 2016). Z-Score index is calculated by subtracting individual (yearly) rainfall values from their climatology, divided by the series standard deviation. It is a relatively unknown as a drought index.
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