Effect of land clearing and climate variability on streamflow for two large basins in Central Queensland, Australia

Dramatic land clearing occurred in Central Queensland of Australia in the 1960s–1980s, and its impacts on streamflow are of great importance for regional water resources inventory and management. Paired catchment studies at the small scale (around 10–20 ha) have shown conclusively that clearing forests for agriculture has more than doubled the streamflow in the region. Two previous investigations at the large scale (the Comet basin with an area of 16,460 km²) have drawn quite different conclusions about the land clearing effect on streamflow, varying from 40% increases in streamflow due to land clearing to all the changes in streamflow occurring as a result of climate variability. To further examine the effect of land clearing on streamflow at the large scale in Central Queensland, this study attempts to resolve the discrepancy of the two previous studies by applying the same set of methods (double mass curves, flow duration curves, Budyko framework, Tanh function and conceptual hydrologic model SimHyd) for three distinct periods, namely pre-clearing, transitional and post-clearing periods, and for two neighbouring large basins. The main conclusions of this study are: 1) the effect of land clearing on streamflow of large basins in Central Queensland are unmistakably detectable; 2) the impact of land clearing manifested itself mostly during wetter years or for wetter basins; 3) for a given basin, the greater the extent of land clearing the larger the effect on streamflow; and 4) a lack of consistent conclusions about the land use change effect on streamflow at the large scale for this region and for several other large basins around the world are likely a result of the different methodologies used, different contrasting periods, and the differing effect on streamflow in relation to the underlying climate variability.