New empirical long-range schemes for the prediction of dates of onset and cessation and of the monthly and annual amounts of rainfall are developed for Kano, in the West African Sahel, using only surface synoptic data. They are based on variations in equivalent potential temperature, q e , which occur as a result of the seasonal, monthly and daily variations of moisture in the summer monsoon flow over West Africa. Agricultural activities may begin about 72 days after the day the anomalies of q e (i.e. q%) first become positive for at least 15 days, essentially signifying the beginning of adequate moisture supply associated with a well established monsoon flow.The new schemes ensure that both the cessation date and the annual amount of rainfall can be predicted prior to the onset of the rains, thus providing, in conjunction with the onset date, very important and useful information for reliable and effective planning of agricultural and water resource activities. Performance tests using an 11-year independent data set indicate that the schemes possess reliable skill.Because the weather over Nigeria is very typical of the entire West African region, being affected by the same wind regime and weather phenomena, these prediction schemes will provide tremendous assistance for enhanced and sustainable agriculture, as well as for efficient water resources management, if extended to the whole area. Furthermore, the methods have the important advantage that, bearing in mind the fact that the majority of West African countries have very sparse, if indeed, any upper-air data, the surface synoptic data needed for their use are readily available in all of the countries.
Accurately predicting precipitation trends is vital in the economic development of a country. Ground observed data from the Nigeria Meteorological Agency (NIMET) was analyzed to study the long-term spatio-temporal trends of rainfall on annual and seasonal scales for 23 stations in Nigeria during a 40-year period spanning from 1974 to 2013. After testing the presence of autocorrelation, Mann-Kendall (modified Mann-Kendall) test was applied to non-autocorrelated (autocorrelated) series to detect the trends in rainfall data. Theil and Sen's slope estimator test was used to find the magnitude of change over a time period. Pettitt's test, Standard Normal Homogeneity Test, and Buishand's test were further used to test the homogeneity of the rainfall series. The results show an increasing trend in annual rainfall; however, only nine stations have a significant increase during the period of study. On the seasonal time scale, a significant increasing trend was observed in the pre-and post-monsoon seasons, while only nine stations show a significant increasing trend in monsoon rainfall and a significant decreasing trend in the winter rainfall over the last 40 years. During the study period, 15.4 and 13.90 % increase were estimated for annual and monsoonal rainfall, respectively. Furthermore, seven stations exhibit changes in mean rainfall while majority of the stations considered (Eighteen stations) exhibit homogeneous trends in annual and seasonal rainfall over the country. The performance of the different tests used in this study was consistent at the verified significance level.
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