Non-linear modelling of rainfall-runoff transformation for some catchments in Tanzania

This work is concerned with the determination of the inadequacies in the Simple Linear Model for rainfall-runoff transformations when applied for four catchments in Tanzania. The residual errors arising from the Simple Linear Model were tested for evidence of seasonality and nonlinearity. It was found that among the four catchments, three catchment's exhibit seasonal behaviour. Also there exists a systematic relationship between the residual errors and the observed flows; which is evidence of non-linearity. Based on these inadequacies modifications to the Simple Linear Model were proposed. In the first case time varying weighting factors were incorporated in the model. In the second case seasonally varying weighting factors were incorporated to account for seasonal behaviour of the system. The results obtained for the second case showed that the inclusion of the seasonal parameter in the Simple Linear Model gives significant improvement in the model performance for catchments with marked seasonal behaviour. On the other hand, the inclusion of the time variant parameters has fair results on catchments without seasonal behaviour and gives poor results in catchments with no seasonal behaviour. From the study it is evident that the Simple Linear Model is inadequate for rainfall-runoff transformation, and therefore inferior to the non-linear modelling approach.