The use of some conceptual models for flood forecasting in South-Western Tanzania

Rainfall-runoff transformation is a complex process and the complexity increases when modelling a tropical regime, and among the many models which have been already used and tested, only a small proportion have been used in the tropics. Two models of the conceptual type, NAM & SMAR were tested for rainfall-runoff transformation, for the Kiwira and Little Ruaha catchments in South-Western Tanzania. The results obtained with the SMAR model has indicated the unsuitability of this model in the area of study, being a simple water balance model it has the difficulty of modelling recessions caused by the base flow contribution, however only simple forms of the model (2 parameters) may give reliable results in arid and semi-arid areas. On the other hand the NAM model provides a simplified description of the entire land phase of the hydrological cycle, and by using all the simulation results, it is possible to obtain rough estimates of the different elements of the water balance and their time distribution. The results obtained are encouraging and further investigations of NAM model in different parts of Tanzania is recommended mainly for water supply aspects where high priority is given to good simulation of low flows. The results of this study has confirmed the general experience that the main problems in hydrological modelling are related to unreliability and insufficiency of input data.