VARIABILITY AND INTENSITY OF THE “LITTLE DRY SEASON” IN SOUTHWESTERN NIGERIA

AUTHORS J. O. Adejuwon & T.O. Odekunle AF 23 Department of Geography, Obafemi Awolowo University, Ile Ife

Introduction “Little Dry Season” (LDS) is a concept that is used to describe the wet season meso-scale climatological phenomenon which manifests itself as a decline in both the frequency and amount of daily rainfall for a number of weeks half way through the rainy season. The phenomenon is also manifested as double or bimodal maximum rainfall regime, which consequently resulted into the division of the growing season into ‘early’ and ‘late’.

Introduction Cont. The phenomenon appears in the annual rainfall curve in the form of a decline in the July – August rainfall. The reduction in rainfall becomes noticeable in the last week of July, intensifies through, until the third week of August, and dissipates thereafter.

Justification LDS is a phenomenon of great climatological importance with mixed implications for crops and agricultural practices. Thus, knowledge of variability and intensity of this phenomenon would go a long way to alleviate farmers’ problems.

Justification Cont. LDS can adversely affect the yields of early crops because it comes at the time of seed or tuber development. It can also delay the planting of late crops which will then become vulnerable to early termination of the rainy season

Justification cont. On the other hand, LDS is put to good use in the agricultural calendar in three main ways: it is seen as a period of the growing season during which conditions are favourable for weeding; It is also a period when conditions are favourable for the spraying of some crops with insecticides and pesticides,

Justification cont. According to one view, LDS favors good yield of yam. When the LDS occurs with normal intensity it favors good yield of early crops which are ready for harvesting at the times of its occurrence

OBJECTIVES The existing studies on the LDS have only concentrated on the causes and the spatial variation of the phenomenon, relatively neglecting its temporal variability and intensity. Therefore the objectives of this study are to:

Determine the mean period of the occurrence of LDS in Southern Nigeria and; Assess the variability of LDS rainfall in the region;

Methodology The occurrence of LDS can be derived from the slope of the cumulative percentage graph of 5- day mean rainfall. Its occurrence during the course of a year is defined by the first point of maximum negative curvature and the second point of maximum positive curvature on the cumulative percentage graph for each station.

Methodology Cont. The basic steps by which the graphs are constructed can be summarized as follows:- derivation of the percentage mean annual rainfall that occurs at each 5-day interval for every station;

Methodology cont. Cumulation of the computed percentages at each 5-day interval; Plotting of the cumulative percentages at each 5-day interval; and Identification of the graph for every station, the time of onset and retreat of the LDS.

Length of LDS in Lagos

Length of LDS in Benin city

Length of the LDS in Ibadan

Length of LDS in ILORIN

Results The results show that LDS occurrences in Ikeja, Benin, Ibadan and Ilorin are between 15th July and 8th September, 20th July and 3rd September, 20th July and 29th August, and 25th July and 29th August, respectively..

RESULTS CONT. The relative variability indices obtained for the total rainfall amounts of the respective stations are 54%, 38%, 60% and 51%.

Results cont. Their respective relative variability values for the total rain-days are 40%, 22%, 42% and 34%. During the period of observation, on the average, about 70% of the LDS occurrences are of normal climatological intensity. 16% of the LDS are supra normal while 14% are sub normal for all the stations studied

Conclusions Normal intensity LDS may have its own advantages. However, extremely long, wet or dry LDSs are definitely undesirable and can constitute constraints in agricultural practices.