Development of Rainfall Empirical Models for Osun Watershed, Nigeria

The aim of this study was to conduct a sequential detection for the possible trends in seasonal rainfall data series using various statistical packages. Farmers in Osun basin lack adequate planning with respect to agricultural activities for its maximum productivity as rainfall trends are often cited as one of the causes of socio-economic problems such as food insecurity. The available rainfall records provide information on monthly basis as obtainable from other meteorological stations nationwide. Rainfall records from 1980 to 2009 which were most consistent required for the development of rainfall models were used. A time series analysis was used to develop various models. The volume of rainfall within the study location was observed to fluctuate with December having the lowest amount of rainfall of 1.10 mm. The average rainfall was observed to be on the increase from the month of February through to the month of November with the highest amount of rainfall recorded in the month of September with an average rainfall of 556.60 mm. The polynomial equation was used to develop a best line of fit of y = 1.4227x3 – 29.502x2 + 154.72x + 9.2348 with a R Square value of 0.7541. A double maxima of June and September was observed while April which is used to be the beginning of the raining season tends. October which used to be the beginning of dry season was also observed to tend towards raining month. The linear, exponential growth and s-curve equation models of the forms Yt = 16141 + 84.5t; yt = 15557.6 * (1.00676t); and yt = 105/(5.77969+3.26815 (0.6591t)) were developed respectively. The data set were further decomposed and it was observed that there was a seasonal effect. The seasonal indices show the average downward movement within the first 3 months and the last 2 months of the season and average upward movements from the 4th to the 10th month. In conclusion the Linear model from this study proved to be the best for predicting rainfall events for the Osun watershed. It is therefore recommended that the Linear model be used to predict rainfall for the Osun watershed.