INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 18: 1261–1271 (1998) CHANGES IN RAINFALL CHARACTERISTICS IN NORTHERN NIGERIA AONDOVER TARHULE 1 and MING-KO WOO* School of Geography and Geology, McMaster Uni6ersity, Hamilton, Ontario, L8S 4K1, Canada Recei6ed 4 No6ember 1996 Re6ised 27 February 1998 Accepted 2 March 1998 ABSTRACT This study examines recent changes in several rainfall characteristics in northern Nigeria. The records at 25 locations were analyzed for the occurrence of abrupt changes and trends using the Pettitt and the Mann-Kendall tests. Variables analyzed included annual total rainfall and number of rain days, the dates of onset, termination and duration of the rainy season as well as monthly rainfall, monthly number of rain days and various categories of rainfall above certain intensities. An abrupt change occurred in the time series of annual rainfall, number of rain days and affected areas north of latitude 11° N. However, the sub-periods prior to and after the change points may be considered to be homogenous. The series of variables related to the duration of the rainy season exhibit no significant trends or jumps. It is concluded that recent changes in rainfall over the Sahel were driven by a reduction in the frequency of rain days of high rainfall intensities during the months of August and September. The fact that the high intensity rainfall does not contribute significantly to crop growth may explain the continuation of agricultural activities in the Sahel despite massive reductions in annual rainfall. © 1998 Royal Meteorological Society. KEY WORDS: drought; trend; change point; rainfall, annual; rainfall, variables; Nigeria; Pettitt test; rainy season; Sahel 1. INTRODUCTION Because rain-fed agriculture is the most important mode of employment and food production in West Africa, records of several rainfall variables are used extensively in planning for agricultural and water resources projects. To obtain reliable estimates of the characteristics of rainfall as well as to determine the magnitude of climatic fluctuations, it is necessary that segments of the time series analyzed should be relatively homogeneous. Previous research applied in northern Nigeria was often based on the so called standard climatic normals, or 30-year, non-overlapping periods. Other work analyzed arbitrary periods, sometimes determined solely by the availability of data (Hulme, 1992). These approaches do not consider the possibility of inhomogeneities introduced by significant changes in the rainfall time series such as the positive trends during the 1950s and the onset of a 25-year drought in the late 1960s. Indices presented by Lamb (1985), Nicholson (1993), and Jones and Hulme (1996) indicate that the onset of the drought was abrupt such that the statistical characteristics of the rainfall series during the drought are significantly different from the pre-drought period. Similarly, Hubert and Carbonnel (1987) and Demaree (1990) demonstrated the existence of possible multiple-stable climatic regimes in some Sahelian rainfall records. Inhomogeneities in climatic series may be introduced by an abrupt change (or ‘jump’), by a gradual trend or by a jump superimposed on a trend (Easterling and Peterson, 1995). It is * Correspondence to: School of Geography and Geology, McMaster University, Hamilton, Ontario, L8S 4K1, Canada; E-mail: woo@mcmaster.ca 1 Current address: Department of Geography and Planning, University of Jos, Jos, Nigeria. Contract grant sponsor: Rockefeller Foundation (African Dissertation Internships Award), Natural Sciences and Engineering Council of Canada CCC 0899–8418/98/111261 – 11$17.50 © 1998 Royal Meteorological Society