Distributed parameter hydrology model (Answers) applied to a range of catchment scales using rainfall simulator data. IV Evaluating pasture catchment hydrology

Measured and simulated runoff data were used to evaluate the effects of surface conditions, particularly vegetative cover, on runoff from a 10 ha pasture catchment in the semi-arid tropics of Queensland. The Answers model, modified to include a multilayered Green and Ampt representation of infiltration, was used to simulate runoff. Development, parametrisation and testing of Answers is described in Parts I and III of this series. Measured annual runoff at the catchment outlet decreased from 30% of rainfall when the catchment was in a bare, grazed condition, to 5% of rainfall when vegetative cover increased to 60% after destocking and revegetation. A thorough comparison of the effect of cover on runoff from the measured record was difficult as rainfall was variable over the experimental period and surface conditions changed with revegetation. Answers was used to simulate runoff for the experimental duration using the measured rainfall record, but with consistent cover and surface conditions. Two cover conditions were simulated, bare (as in the early stages of the experiment) and covered (typical of cover conditions late in the experiment). Simulated high cover conditions increased by 10 mm the amount of rainfall required for runoff to occur, relative to bare conditions. Peak discharge was reduced with increased cover for runoff events with an average recurrence interval of up to 3 years. Several revegetation strategies were simulated. The most effective revegetation strategy, in terms of runoff reduction from all rainfall events, was to increase cover levels modestly across the whole catchment rather than to revegetate small areas intensively.