The Ecological Reserve (environmental flows) as defined under the South African National Water Act 36 of 1998 was designed to equitably manage water for river sustainability while maximizing economic and social welfare. We investigated the climate change impacts on the Ecological Reserve targets for a seasonal river in an agricultural catchment in the Western Cape region using the Habitat Flow Stressor Response method (which integrates hydrology, hydraulics, water quality, and ecological data) under projected climate scenarios (2041–2070). Current Ecological Reserve model outcomes for 3 sites on the Doring River were compared with the future hydrology using Global Circulation Models associated with four Representative Concentration Pathways (RCP 2.6–8.5). Climate predictions reflected reduced future flows, but the uncertainty band of predicted future flows overlapped with present day flows. Flood flashiness following heavy rains and the increased length of dry periods that are predicted would both contribute to increased erosion and geomorphological degradation, and further compromise biodiversity including the endangered fish populations and threaten both freshwater and estuarine ecosystems downstream. Salinity variation is predicted to increase in future leading to increased seasonal salinity stress and reduced use of abstracted water. We interrogate various options for mitigating the impacts including augmenting dry season flows, developing on-farm, catchment-scale, and strategic water resources management, and removing alien vegetation.

Interactions between surface water 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.

The effects of topsoil addition of rice-husk dust (RHD) and cattle dung (CD), alongside surface mulching with dry grasses/legume, on the infiltration characteristics and intrinsic structural properties of a deep, well-drained soil in southeastern Nigeria are assessed. Treatments are RHD-amended, CD-amended and ‘unamended’, each plot being either surface-mulched ore bare, with the unamended-bare plots as control. Amendments and mulch were applied at 20 t/ha equivalents. Their effects on the soil’s infiltration characteristics 7 months later were not evident; however, there was a tendency for differences: CD-amended ≥ RHD-amended ≥ unamended and surface-mulched ≥ bare-surface. By contrast, saturated hydraulic conductivity (K_s) differed thus: CD-mulched ≥ unamended-mulched > the rest. Values were similar for K_s (50.89 cm/h) and final infiltration rate (50.74 cm/h) only under CD-amended plots, which also showed the highest transmissivity (43.50 cm/h). Soil penetrometer resistance was lowest in CD-amended plots (113.44 kPa) and highest in unamended plots (166.78 kPa). Topsoil addition of cattle dung and surface mulching could increase infiltration, though marginally, and permeability of coarse-textured tropical soils beyond the season of their application when their effects on soil structure have almost waned.

The most appropriate scale to use for hydrological modelling depends on the model structure, the purpose of the results and the resolution of available data used to quantify parameter values and provide the climatic forcing. There is little consensus amongst the community of model users on the appropriate model complexity and number of model parameters that are needed for satisfactory simulations. These issues are not independent of modelling scale, the methods used to quantify parameter values, nor the purpose of use of the simulations. This paper reports on an investigation of spatial scale effects on the application of an approach to quantify the parameter values (with uncertainty) of a rainfall-runoff model with a relatively large number of parameters. The quantification approach uses estimation equations based on physical property data and is applicable to gauged and ungauged basins. Within South Africa the physical property data are available at a finer spatial resolution than is typically used for hydrological modelling. The results suggest that reducing the model spatial scale offers some advantages. Potential disadvantages are related to the need for some subjective interpretation of the available physical property data, as well as inconsistencies in some of the parameter estimation equations.