The impact of land use change on catchment hydrology in large catchments: The Comet River, Central Queensland, Australia

The Comet River in Central Queensland, Australia, with a catchment area of 16,440 km2 was largely cleared of a natural forest cover dominated mainly by Brigalow (Acacia harpophylla) during the mid-1960s. This provides an opportunity to examine the scale dependence of the impact on runoff from a large catchment of the conversion of forest to grass and cropland since most studies so far reported have been for very small catchments. The observed data on flow and rainfall indicate that in the post-clearing period there was a 78% increase in runoff compared with the pre-clearing period which can be explained, in part, by an 8.4% increase in rainfall. Two modelling strategies have been used to examine the impact of clearing on runoff. An annual water balance model using a standard hyperbolic function indicated that runoff in the post-clearing period was greater by 58% than if clearing not occurred. A simple conceptual daily rainfall-runoff model, SIMHYD, was calibrated for the pre-clearing period using various calibration strategies and used to estimate runoff in the post-clearing period as if clearing had not occurred. The reverse strategy was also applied and a total of eight runs of the model are reported. From this modelling it is estimated that clearing has generated an increase in runoff of approximately 40%.