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A major challenge for water resource management in Western Cape, South Africa, is the reduction of the growing sediment and nutrient loads in coastal areas, which belong to the areas most affected by land use change. We used the WebGIS based software STOFFBILANZ to simulate runoff, soil loss, sediment, phosphorus, and nitrogen input in the surface water and groundwater of study area (ca. 6,450 km²). The simulated runoff shows a large regional variability caused by the heterogeneous distribution of rainfall. For the reference catchment Klein River simulated total daily runoff fit the observed values of the reference year 2012. The calculation of potential input of sediment, phosphorus, and nitrogen into waters is based on aggregated or generalized information on climate data, land use types, crop and fruit types, yields, mineral fertilizers, farm manure, nitrogen fixing by leguminous plants, atmospheric nitrogen deposition, and soil denitrification. Critical source areas for potential sediment input, particulate P input and diffuse N input are mainly agricultural areas. Additionally, point sources of high relevance for N and P are found in urban areas. Based on the potential input of sediment and nutrients the impacts of current land use change on water resources were estimated. We used the web-based information system WebLand for the simulation aiming at the provision of stakeholders with information for decision making in water resource management.

Analysis and interpretation of future climate change impacts on a particular crop, require a number of different models and datasets. Such datasets often operate at vastly disparate spatial scales. Mechanistic crop models, for example, classically operate at a site-specific, point location, for which soil and climate must be described in great detail. Future climate scenarios however, are obtained from various Global Climate Models (GCMs) at a very coarse resolutiontypically gridded to 300 km or more. In order to be useful at a local level they need to be downscaled to a spatial scale useful for local analysis. Weather monitoring station locations in the province are irregularly distributedmuch denser in the fruit and vine areas than in the extensive wheat areas. The Western Cape Province is a highly diverse region with regard to geology, topography, climatic influences and the resulting agricultural systems and practices. Future climate change therefore, is likely to have different impacts in different zones of the province where wheat is produced. To address this heterogeneity, the province was divided into 21 distinct response zones for modelling purposes. Geographic Information Systems (GIS) play a key role in addressing the spatial complexities-facilitating issues such as weighted average zonation, aggregation (or disaggregation) of spatial components, local parameterisation of crop models through interpolation, integration of ancillary data such as satellite imagery within the modelling framework and finally in the spatial analysis and display of modelled scenarios. This paper uses a recent climate impact study in the Western Cape to demonstrate the role of GIS in the assessment of expected climate change impacts on dryland wheat agriculture.

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