Changing characteristics of meteorological droughts in Nigeria during 1901-2010

摘要

The objective of this study was to assess the changes in the severity and return periods of meteorological droughts during major cropping seasons of Nigeria for the period 1901-2010 in order to understand the impacts of climate variation on seasonal droughts. Gauge-based gridded global precipitation climatology center (GPCC) rainfall and climate research unit (CRU) potential evapotranspiration (PET) data having a spatial resolution of 0.5 degrees were used for the reconstruction of droughts using standardized precipitation evapotranspiration index (SPEI). SPEI values were fitted with the best distribution function to estimate seasonal droughts return periods and the modified Mann-Kendall test was used to assess the secular changes in climate variables and drought index. The temporal variations in droughts and their interrelations with rainfall and temperature trends were assessed using a 50-year moving window with a 10-year time step. The results showed that meteorological drought severity has increased for all the cropping seasons of Nigeria. Temperature was found to be the dominating factor for defining droughts in semi-arid regions in the north while rainfall in the monsoon and tropical savanna climatic zones dominates in the south. Rises in temperature in the range of 0.14-0.42 degrees C/decade and almost no change in rainfall have caused decreases in SPEI up to -0.25/decade in some regions. The number of grid points with decreasing SPEI was found to vary between 50 out of 323 for millet and 152 for yam growing season. The moderate droughts were found to become more frequent compared to other classes of droughts. Amongst the cropping seasons, the highest increases in drought frequency were observed during corn growing season. The highest decrease in the median value of the return period of moderate, severe and extreme droughts during the two corn growing seasons were observed to be 6.25, 12.86 to 13.20, 23.15 to 24.31 years in 1901-1950 to 6.25, 12.92 to 12.94, 23.51 to 23.64 in 1961-2010. Compared to other drought severities, the return periods of the moderate droughts were found to decrease more. A general decrease in the return periods of droughts indicates more frequent droughts during all cropping seasons of Nigeria. Therefore, it can be concluded that the rising temperature due to global warming would increase drought severity and frequency in all the cropping seasons of Nigeria.