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Future land cover changes may result in adjustments to biophysical drivers impacting on net ecosystem carbon exchange (NEE), catchment water use through evapotranspiration (ET), and the surface energy balance through a change in albedo. The Land Change Modeller (Idrisi Terrset 18.08) and land cover for 2000 and 2014 are used to create a future scenario of land cover for two catchment with different land management systems in the Eastern Cape Province for the year 2030. In the S50E catchment, a dualistic farming system, the trend shows that grasslands represented 57% of the total catchment area in 2014 decreasing to 52% by 2030 with losses likely to favour a gain in woody plants and cultivated land. In T35B, a commercial system, persistence of grasslands is modelled with approximately 80% coverage in both years, representing a more stable system. Finally, for S50E, NEE and ET will increase under this land cover change scenario leading to increased carbon sequestration but less water availability and corresponding surface temperature increases. This implies that rehabilitation and land management initiatives should be targeted in catchments under a dualistic farming system, rather than those which are predominantly commercial systems.

Water quality monitoring tools that rely on data from stress-response tests with continuous exposure at constant concentrations are not always appropriately protective when stressor exposure in the field is episodic in nature. The present study identifies various approaches that have attempted to account for episodic stressor exposure, describes the development of a toxicological effects database of episodic stressor exposure collated from published scientific literature, and discusses whether any discernible trends are evident when these data are reviewed. The episodic stressor exposure literature indicated that few generalizations can be made regarding associated biological responses. Instead, when attempting to characterize the hazard of a certain episodic pollution event, the following situation-specific information is required: the specific species affected and its age, the specific stressor and its concentration, the number of exposures to the stressor, the duration of exposure to the stressor, and the recovery time after each exposure. The present study identifies four main challenges to the inclusion of episodic toxicity data in environmental water quality management: varying stressor concentration profiles, defining episodic stressor concentration levels, variation resulting from routes of exposure and modes of action, and species-specific responses to episodic stressor exposure. The database, available at http://iwr.ru.ac.za/iwr/download, could be particularly useful for site-specific risk assessments related to episodic exposures.

Small dams represent an important local-scale resource designed to increase water supply reliability in many parts of the world where hydrological variability is high. There is evidence that the number of farm dams has increased substantially over the last few decades. These developments can have a substantial impact on downstream flow volumes and patterns, water use and ecological functioning. The study reports on the application of a hydrological modelling approach to investigate the uncertainty associated with simulating the impacts of farm dams in several South African catchments. The focus of the study is on sensitivity analysis and the limitations of the data that would be typically available for water resources assessments. The uncertainty mainly arises from the methods and information that are available to estimate the dam properties and the water use from the dams. The impacts are not only related to the number and size of dams, but also the extent to which they are used for water supply as well as the nature of the climate and the natural hydrological regimes. The biggest source of uncertainty in South Africa appears to be associated with a lack of reliable information on volumes and patterns of water abstraction from the dams.Key words farm dams; hydrological impacts; modelling; uncertainty Estimation des incertitudes lors de la simulation des impacts de petites retenues agricoles sur les régimes d'écoulement en Afrique du Sud Résumé Les petits barrages constituent une ressource locale importante, conçue pour augmenter la fiabilité de l'approvisionnement en eau dans de nombreuses régions du monde où la variabilité hydrologique est forte. Il apparaît que le nombre de retenues agricoles a fortement augmenté lors des derniéres décennies. Cette évolution peut avoir un impact substantiel sur les volumes et les chroniques d'écoulement à l'aval, les usages de l'eau et le fonctionnement écologique. L'article traite de la mise en oeuvre d'une modélisation hydrologique pour étudier les incertitudes associées à la simulation des impacts des retenues agricoles dans plusieurs bassins versants SudAfricains. L'étude est centrée sur l'analyse de sensibilité et sur les limitations associées aux données qui sont typiquement disponibles pour l'évaluation des ressources en eau. Les incertitudes proviennent essentiellement des méthodes et des informations disponibles pour estimer les propriétés des retenues ainsi que l'utilisation de l'eau des retenues. Les impacts sont non seulement liés au nombre et à la taille des retenues, mais aussi à l'intensité de leur utilisation pour l'approvisionnement en eau et aux caractéristiques du climat et du régime hydrologique naturel. La principale source d'incertitudes en Afrique du Sud apparaît être associée au manque d'informations fiables sur les volumes et les chroniques des prélévements d'eau dans les retenues.