Determining Rainfall Onset and Retreat Dates in Nigeria

Abstract

This study assessed the relative efficiency of the use of rainfall amount and rainy days in the determination of rainfall onset and retreat dates in Nigeria between 1961 and 2000. Daily rainfall data were sourced from the archives of the Nigerian Meteorological Services, Oshodi Lagos. The specific locations where data were collected include: Ibadan, Ilorin, Kaduna and Kano. The method of percentage cumulative mean rainfall values was employed in the determination of the rainfall onset and retreat dates.The results obtained clearly showed that both the use of rainfall amount and rainy days are equally effective in the determination of the mean rainfall onset and retreat dates in Nigeria. With regards to the rainfall onset and retreat dates of the individual years however, the method based on the rainy days is more efficient than that based on the rainfall amount, as the formal yielded a much more realistic dates than the latter. It is thus recommended that studies investigating rainfall onset and retreat dates series of the individual years should be based on rainy days rather than rainfall amount.