Abstract:
Drought is a slow onset natural hazard which affect sectors like agriculture and water resources. This research investigates the spatial and temporal trends of drought episodes in Nigeria and evaluates trends and variability in drought episodes over Nigeria using Thornthwaite moisture index (MI), Standard precipitation index (SPI), Standard precipitation evapotranspiration index (SPEI) and Self-Calibrating Palmers drought severity index (Sc-PDSI) over 5 agro ecological zones; Mangrove (MG), Freshwater swamp (FWS), Rain forest (RF), Shortgrass Savannah (SGS)
and Tall grass Savannah(TGS). Performance of several drought indices were compared. Statistical approach was also employed to determine the coefficients of kurtosis, skewness and correlations between the meteorological parameters. Rainfall variability index showed that 2010s was the wettest decade (+1.25) while 1980s was the driest (-0.91), with years between 1975 and 1993 being drier than any other comparable period in the last four decades. In all the zones, observed rainfall changes varied between -2.5 and +0.2.2 mm/yr. The warmest years in Nigeria were 1987, 1998 and 2005. The coldest years were 1975, 1978 and 1989. Annual temperature has risen by 0.021oC/year during the last 41 years. Using Feddema climate classification of 1994, SGS and TGS occupy 49% of semi-arid and arid, 24% of the land area is under dry sub-humid climate while
27% of the landscape area is classified as wet sub-humid and humid climate. Significant positive trend was experienced at all zones for all indices except sc- PDSI at the MG zone, which had a negative trend of -0.21 and Sen Slope of -0.003/year. There were similar trends and changes for all zones for all indices used. 1978 – 1988 were classified as the driest period for SPI, SPEI and Sc-PDSI. Positive trends in more than 90% of the landscape from all indices indicate high agreement that there is no drought in the last quadrennial (2012-2015). MG had an annual rainfall increase of 4.24mm/year, FWS; 8.9 mm/yr, RF; 4.9mm/yr, TGS; 5.7mm/yr and SGS;2.3mm/yr.vi Sc-PDSI indicated similar changes with SPI and SPEI in all the zones except MG and FWS where the last quadrennial (2012-2015) indicated dryness. Observed trends indicate positive strong
correlation between SPI and SPEI (r ≥ 0.9). MI indicated average positive correlation between SPI, SPEI and Sc-PDSI (r ≥0.41). Weak negative correlation between indices occurred between SPI and Sc-PDSI at MG. Rainfall generally showed higher and significant correlations with MI and slopes of all moisture indices. Knowledge of historical changes in drought episodes is pertinent to the introduction of new, more productive and more drought resistant varieties of different crops and for introduction of improved farming systems.
