Geographic variation in growing season rainfall during three decades in Nigeria using principal component and cluster analyses

Reports of changes in the seasonal and annual rainfall in Nigeria suggests that a more detailed analyses of the geographic extent of these changes and of their impact on agriculture could be of value. Variation in the growing season (April to September) rainfall from stations across Nigeria was anal ysed over the 30-yr period, 1960-90. Regression analyses were used to exami ne long-term trends. Principal component and cluster analyses were used to group stations with similar trends in standardised seasonal rainfall. Mean accumulated standardised seasonal rainfall were used to examine short- and medium-term trends for each of the groups identified. Significant (P less t han or equal to 0.05) decreases in rainy season rainfall were found at 8 st ations mostly in the Guinea and arid/semi-arid savannas of northern Nigeria , whereas no station showed significant increases. Examination of the month ly (April through September) rainfall showed that only three - Kano, Sokoto and Potiskum in the arid/semi-arid savanna - of the twenty-three stations used in the analysis had declining rainfall trends for each of the months A pril to September and subsequently declining seasonal rainfall trends. Howe ver, 12 to 15 stations had consistently declining rainfall trends in atleas t some but not all the growing season months. However, a similar pattern wa s not the case in terms of increasing rainfall trends, where only one to th ree stations had consistently increasing rainfall trends in some but not al l of the months from April to September. Stations that showed increasing ra infall trends were in the southern parts of Nigeria. Six groups with similar patterns in standardised seasonal rainfall were ide ntified by Principal Component and Cluster analyses. For most of the groups , the period from 1967 to 1973 was that of consistently below average seaso nal rainfall. However, the timing and extent of the decline varied with loc ation. Common to stations in four of the six groups was a negative trend in seasonal rainfall for the period considered. The geographic variation in s easonal rainfall trends has tremendous agricultural significance since ther e are indications that the reliability of the season is decreasing from the humid forest zone with positive seasonal trends to the arid/semi-arid sava nna with significant negative seasonal trends.