Two Modelling Approaches for Predicting Water and Salt Generation to Upland Streams: BC2C & 2CSalt Gilfedder, M. 1 , M. Littleboy 2 and M. Stenson 1 1 CSIRO Land and Water, 120 Meiers Rd, Indooroopilly, Queensland, Australia 2 NSW Department of Environment and Climate Change, PO Box 189, Queanbeyan NSW 2620 Email: mat.gilfedder@csiro.au Keywords: Catchment water balance, spatial modelling, hydrology, salt export EXTENDED ABSTRACT Predicting the impact of land-use change on water and salt generation from upland areas at a catchment scale is a difficult task. Across large catchments there are often limited field measurements, and so predictions are forced to rely on modelling. However, at this scale input data for models is usually restricted to information surfaces such as rainfall, geology, and surface topography, and hence the complexity of modelling approaches need to match the complexity of the available data. This paper presents a comparison of two recently developed catchment-scale models which predict the impact of land-use change on water and salt generation from upland areas to streams. Both models consider a surface water balance, incorporate groundwater processes, and generate water and salt export to streams. BC2C is an annual time-step model which uses a simple water balance approach, and groundwater response time theory to estimate the impact of changes in forest cover on stream volume and salt load. 2CSalt differs from BC2C in that it uses the results of daily time-step 1D modelling to provide the water balance, and includes both a hill-slope and an alluvial groundwater store. It produces monthly stream flow and salt load estimates. We compare results from three sub-catchments in the mid-Murrumbidgee, NSW, Australia, using both the BC2C and 2CSalt models. This highlights differences between the two approaches, and allows inspection of their relative merits for catchment management issues. The two models (BC2C and 2CSalt) provide complementary approaches to aspects of investigating the impact of land-use change on generation of water and salt. While there is an overlap in the scale of applicability, the two models are quite different and provide answers to different questions. BC2C is intended for regional prioritisation across large catchments, and for examining the variation in possible impacts of afforestation scenarios between catchments. 2CSalt operates at a scale finer than BC2C, and is computationally more intensive. Its strengths are in providing access to a broader range on land-use options, providing output to examine seasonal impacts, the ability to calibrate to measured gauged data, and output which can feed into river routing models. 1478