Comparative analyses of standardised precipitation index and moisture quality index for wet and dry periods monitoring over the savanna zones of Nigeria

Abstract

This study evaluated the suitability of a monsoon quality index (MQI) within the timescales of 6 months and 12 months, as an alternative to the standardised precipitation index (SPI), in assessing rainfall conditions over the rainfall-sensitive, food-producing savanna zones of Nigeria. The results revealed varied correlation between commensurate rainfall variability classes depicted by the indices at the two timescales (ranging from −0.719 for near normal and fair rainfall pairs to 0.249 for extreme drought and extremely poor rainfall pairs) due to MQI’s emphasis on intraseasonal dry spells. The findings also showed that while the MQI did better at depicting the south to north variability in rainfall, it over-generalised the wet and dry events by implying very sharp drops in rainfall quality between the southerly and northerly locations. It was thus reclassified into a modified monsoon quality index (mMQI) to correct the over-generalisation. The trend analysis of the frequency of wet and dry episodes for the mMQI and SPI demonstrated agreement in a declining tendency in dry periods and a rising tendency in wet periods. While further studies are recommended to assess differences and similarities between the SPI and mMQI for other rainfall regimes, it is clear that the mMQI has shown viability in analysing wet and dry events in Nigeria’s savanna zones and is able to do so even with just 1 year of data. The requirement for long-term records to compute the SPI makes it relatively weak for environments where data records may be sparse and of varying lengths.