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Siltation of reservoirs is a major concern in Zimbabwe. Therefore, development of prediction tools is of great importance. In the present study a recently developed empirical sediment model (HBV-SED) based on a daily rainfall-runoff model was applied to simulate riverine fine sediment transport in a 2 486 km 2 catchment in eastern Zimbabwe. The model performance was evaluated and changes in the model structure were suggested. The modelling was, however, associated with many uncertainties due to the adopted simplification of transport processes. An analysis of the model structure and a comparison with the rating curve function was done. The required length of data for calibration purposes was evaluated and model validation through split sample and proxy basin comparison was performed. Furthermore, since the empirical model was dependent on monitored runoff and fine sediment concentrations for calibration purposes, a field measurement campaign was conducted to assess the accuracy of observed data at the station studied. The field measurements showed large errors in monitored runoff and fine sediment concentrations for the 1998/ 99 wet season, which illustrated the uncertainty in predictions of fine sediment transport based on observed data. The HBV-SED model, which was applied over a period when data were believed to be fairly accurate, simulated the fine sediment transport volume well for the validation period if it was calibrated for a minimum of four years. A shorter calibration period led to a significant increase in prediction uncertainty. The model failed to simulate individual high fine sediment peaks accurately mainly due to poor performance of the rainfall-runoff model on a daily time-scale even if the seasonal flow dynamics were described properly. In the studied catchment the HBV-SED model application resulted in equally poor R 2 -values as the rating curve technique, while the estimated fine sediment volume was more accurate.

Abstract. International water resources agreements for transboundary rivers in southern Africa are generally founded in system analysis models for water planning and allocation. The Water Resources Yield Model (WRYM) developed in South Africa has so far been the only model applied in official joint water resources studies aimed to form watersharing agreements. The continuous discussion around the model performance and growing distress over it being South African, where it was originally developed, while South Africa is one of the interested parties in the process, results in an increased controversy over the system analysis results that are often only meant to guide in selecting the options for water resources management in a given set of scenarios. The objective of this study was therefore to assess the model performance of two other models; WAFLEX and WEAP21 in the Umbeluzi River Basin system where the WRYM was previously applied as part of a Joint River Basin Study. A set of basin development scenarios was equally tested in the three models and the results compared. The results show that the three models all are possible tools for system analysis of river basins in southern Africa, although the structure and complexity of the models are different. The obtained level of satisfaction for specific water users could, however, vary depending on which model was used, which causes uncertainties. The reason for the diverse results is the structurally different ways of describing allocation and prioritization of water in the three models. However, the large degrees of freedom in all system models cause even larger uncertainty in the results since the model developer can, intentionally or unintentionally, direct the results to favor certain water user. The conclusion of this study is therefore that the choice of model does not per se affect the decision of best water allocation and infrastructure layout of a shared river basin. The chosen Correspondence to: D. Juízo (juizo@hotmail.com) allocation and prioritization principles for the specific river basin and the model developer's experience and integrity are more important factors to find the optimal and equitable allocation.

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Abstract. A semi-distributed conceptual model, HBV-SED, for estimation of total suspended sediment concentration and yield at the outlet of a catchment was developed and tested through a case study. The base of the suspended sediment model is a dynamic hydrological model, which produces daily series of areal runoff and rainfall for each sub-basin as input to the sediment routine. A lumped measure of available sediment is accumulated continuously based on a linear relationship between log-transformed values of rainfall and erosion, while discharge of suspended sediment at the sub-basin outlet is dependent on runoff and amount of stored available sediment. Four model parameter are empirically determined through calibration against observed records of suspended sediment concentration. The model was applied to a 200 km2 catchment with high altitude differences in the tropical parts of Bolivia, where recorded suspended sediment concentrations were available during a two-year period. 10,000 parameter sets were generated through a Monte Carlo procedure to evaluate the parameter sensitivity and interdependence. The predictability of the model was assessed through dividing the data record into a calibration and an independent period for which the model was validated and compared to the sediment rating curve technique. The results showed that the slope coefficients of the log-transformed model equations for accumulation and release were much stronger than the intercept coefficients. Despite and existing interdependence between the model parameters, the HBV-SED model gave clearly better results than the sediment rating curve technique for the validation period, indication that the supply-based approached has a promising future as a tool for basic engineering applications.