In order to evaluate measures to increase water availability in and around reservoirs it is necessary to have reliable reservoir water storage models. For that reason, it is necessary to assess the validity of these models. The aim of this research is to do this assessment of two models in data scarce semi- arid regions. The Nakamb ́e catchment is used as a case study. The study covers important aspects of hydrological modelling, such as model input (data) selection, hydrological model choice, calibration, model performance testing, parameter sensitivity, and reservoir water storage simulation.
For the selection of the optimal model forcing data diverse precipitation products are reviewed: CHIRPS, ERA5, and local measurements. Among the evaluated datasets, CHIRPS emerges as the superior choice validated against the local measurements. With respect to the potential evaporation, the combination of ERA5 and local measurements results in the most suitable potential evaporation data, leveraging the temporal and spatial aspects of ERA5 and the absolute values of the local measurements.
Comparing a lumped hydrological model (HBV) with a distributed model (SBM Wflow) in simulating river discharge reveals that the HBV model outperforms its counterpart in simulating discharge. This contrast in performance is attributed to potential overparameterization in the Wflow model, coupled with the complexities of parameter estimation in data-scarce areas. The HBV model, while bearing simplifications, benefits from a more comprehensive calibration process. The model performance is strongly influenced by the calibration efficiency, where the significantly shorter simulation time of the HBV model facilitates an extensive Monte Carlo sampling-based calibration, in contrast to Wflow’s time consuming manual parameter adjustment.
Additionally, the sensitivity analysis showed that in the HBV model, the parameters affecting actual evaporation are the most sensitive one. This emphasizes the importance of accurately simulating this component for the proper model performance. The Wflow model exhibits strong equifinality due to the many parameters within the model. The complexity of this model made it impossible to test all parameters and therefore only some parameters are tested.
Both reservoir water storage models studied, the HBV Reservoir Water Storage Model (HBV RWSM) and the Wflow reservoir module, can effectively simulate reservoir water storage fluctuations, although they differ in how the components are calculated. Due to data limitations, it is impossible to determine which, if any, of the models is correct. However, based on the downstream discharge the HBV RWSM displays a more promising performance.
In conclusion, the HBV model outperformed the SBM Wflow model in simulating discharge due to its simplicity and ease of calibration. Sensitivity analyses highlighted the significance of accurately representing actual evaporation. Both water balance models, the HBV RWSM and the Wflow reservoir module, performed similarly concerning the NSE values. The fluxes contributing to the water balance in the two reservoir water storage models differ significantly. The lack of data on these fluxes makes it impossible to determine which models performs best. Data limitations remain a significant hurdle in model evaluation, emphasizing the need for additional data collection, particularly upstream and downstream of the reservoir, to enhance reliability and reduce uncertainties.