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The processes that occur in wetlands and natural lakes are often overlooked and not fully incorporated in the conceptual development of many hydrological models of basin runoff. These processes can exert a considerable influence on downstream flow regimes and are critical in understanding the general patterns of runoff generation at the basin scale. This is certainly the case for many river basins of southern Africa which contain large wetlands and natural lakes and for which downstream flow regimes are altered through attenuation, storage and slow release processes that occur within the water bodies. Initial hydrological modelling studies conducted in some of these areas identified the need to explicitly account for wetland storage processes in the conceptual development of models. This study presents an attempt to incorporate wetland processes into an existing hydrological model, with the aim of reducing model structural uncertainties and improving model simulations where the impacts of wetlands or natural lakes on stream flow are evident. The approach is based on relatively flexible functions that account for the input–storage–output relationships between the river channel and the wetland. The simulation results suggest that incorporating lake and wetland storage processes into modelling can provide improved representation (the right results for the right reason) of the hydrological behaviour of some large river basins, as well as reducing some of the uncertainties in the quantification of the original model parameters used for generating the basin runoff. Copyright © 2013 John Wiley & Sons, Ltd.

Interactions between surface and groundwater systems in fractured rock environments, covering large parts of southern Africa, are poorly understood, such that modelling the different water balance components is highly uncertain. Some of these uncertainties are highlighted and attempts are made to resolve them using hypothesis testing with an uncertain ensemble version of the widely used Pitman hydrological model that includes relevant groundwater components. Five study catchments are used to represent many of the major uncertainties in linking surface and groundwater resources, including the relative importance of deep unsaturated zone drainage, the balance between recharge and losses through riparian evapotranspiration and channel transmission losses.The study demonstrates that some uncertainties can be reduced, but this also involves making a number of assumptions about other aspects of a catchment water balance that can be conceptually supported but not completely validated with available data. The remaining uncertainties need to be resolved through improved process quantification and understanding, possibly using environmental tracers.A c c e p t e d M a n u s c r i p t 2 INTRODUCTIONThe interactions between surface water and groundwater (SW/GW) systems remain poorly understood in many catchments throughout the world and yet they are essential to effectively managing the quantity and quality of water resources. It has been well documented that these systems interact in a range of geological, topographical and climatic settings and that many surface water features, such as rivers, lakes, dams and wetlands will have varying degrees of connection with groundwater systems (Ivkovic 2009, Fleckenstein et al. 2010, Haria et al. 2013). There are a number of comprehensive reviews of the current understanding of the physical interactions that occur between groundwater and surface water systems (Winter 1999, Sophocleous 2002, Brunner et al. 2009, Unland et al. 2013, Lamontagne et al. 2014). Within South Africa the focus has tended to be more on management issues surrounding the interactions between surface and groundwater (Xu et al. 2002, Parsons 2004, Le Maitre and Colvin 2008, Hughes 2010, Levy and Xu 2011), while some contributions have focused on the physical processes behind the interactions, albeit through relatively small scale studies (Hughes and Sami 1992, Lorentz et al. 2004, Roets et al. 2008, Wenningger et al. 2008, Kelbe and Germishuyse, 2010). One of the limiting factorsin trying to understand and quantify interaction processes at scales appropriate for regional water resources management is the lack of direct data that is available to quantify almost all of the individual processes, from recharge, through vertical drainage, to the interactions between aquifers and river channels. This is particularly true for fractured rock aquifers that are highly heterogeneous and are the dominant aquifer type in southern Africa. A further limitation is the general lack of availability (for desktop studies at the catchm...

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This study combines the application of a hydrological model with the use of field data derived from short period measurement campaigns at two sites, one a low topography forested area and the other a steep grassland catchment. The main objective was to determine if the structure of the widely used Pitman model could be considered appropriate for simulating the field data. The model is typically applied at coarse spatial and temporal (1 month) scales, while the tests reported here use data from small catchments and are applied in a daily version of the model. The results demonstrate the importance of ensuring that field observations are measuring the same hydrological variables as the model simulations. At one study site, there was a mismatch in the soil moisture data that was corrected by incorporating a two‐layer soil algorithm into the model. The model results from both field sites identified the sensitivity of the model to assumptions about evaporative demands and indicate that the model structure is very sensitive to the potential evaporation inputs. The overall conclusion is that the model structure is generally appropriate for simulating the hydrological responses at the two sites, but that there remain some unresolved uncertainties about specific model components and the use of certain types of input data. The study lends support for the future development of a more complete daily version of this widely used model. Copyright © 2013 John Wiley & Sons, Ltd.