Climate and land use change impacts to flow and water quality in the Botany Bay catchment

The Botany Bay WaterCAST model has been used extensively to estimate flows and pollutant loads to Botany Bay to assess the potential impacts of future land use change and land management practices. Until recently however, the relative impacts of these changes had not be assessed with respect to climate change. This paper describes an extension of this work to investigate the combined hydrological and water quality effects of land use change and climate change for 2030 and 2070 on the Botany Bay region. Climate data used to drive the Botany Bay WaterCAST model was directly scaled on a seasonal basis consistent with available projections for 2030 and 2070 climate change scenarios (CSIRO 2007). Broad scale landuse change data to 2031 was gathered from the NSW Department of Planning (DoP) METRIX Subregional Planning Tool web site. This was then used as the basis to extrapolate landuse change to 2070. Model results show an increase in total flows and pollutant loads for future land use scenarios from baseline conditions without climate change reflecting the relative increases in urban density and resultant directly connected impervious fraction of urban land use. Future land use change results also show increases in medium to high flows in some catchments. This potential impact on low and medium flows may be counteracted in climate change scenarios which show that baseflows are likely to be significantly decreased for nearly all climate change scenarios. No strong climate change signal regarding high flows is apparent. The potential impact of climate change on both the stream hydrology and pollutant exports appears to overshadow the potential impacts of land use change. Climate change scenarios indicate a general drying of the catchment resulting in less runoff and pollutant export.