ASSESSMENT OF TREND AND DETECTION OF MUTATION OF RAINFALL OVER SOKOTO - RIMA RIVER BASIN, NIGERIA

Abstract

Time-based characteristics of hydro meteorological processes are of great significance in the planning, designing and operation of water systems. In view of this, attempt was made to assess general trend characteristics and change point (mutation) of rainfall over the Sokoto -Rima River Basin, in Nigeria. Investigations were carried out by using rainfall data from selected gauging stations across the basin; Three statistical tests: (i) Pettit's Test, (ii) Robust Mann-Kendall Test, and (iii) Sequential Mann Kendall test (SQ-MK test) were employed for the analysis. The results obtained indicated that few stations showed insignificant trends in annual and seasonal series. However, Pettit's' test showed that 100% of the station exhibited significant trend. Mann- Kendall test on annual rainfall series for some stations showed a mixture of varying contrast in negative and positive trend; for instance, Katsina, Gusau and Sokoto showed increasing positive trend (48.86% of the stations) while for Goronyo, there were indications of decreasing trend (14.28% of the stations). On the other hand, Jibiya, Bakalori and Zobe do not indicate discernible trend pattern. Generally, the seasonal Mann-Kendall revealed that the sum percentage of the negative significant trend far outweighed that of the positive trend at the ratio of 4:1 while for SQ- MK, it is of 11:1; i.e., ratio of significant negative trend to positive significant trend. The consequence is that for every 1% increase in rainfall amount, there is a 4% or 11% decrease in trend signature. The results showed that most of the significant mutation points began in 1990s. Considering the results obtained, there is need to examine other hydrometeorology variables in addition to rainfall in order to have a thorough understanding of the time-space dependent behaviour of the hydrometeorological processes and their correlating aggregate effects. It is pertinent therefore that several statistical approaches should be used to capture trend and mutations; as one approach may not truly give a snapshot of hydrological variability in a particular basin; for the purposes of drawing effective conclusions