Hydrology and runoff water quality from three improved pastures compared with virgin brigalow (Acacia harpophylla) woodland over 8 years in semiarid Australia

The Fitzroy Basin in central Queensland has the largest cattle herd of any natural resource management region in Australia, and legumes have been widely used to boost fertility of rundown soil and improve cattle liveweight gains. However, there is a paucity of information on the effect of leguminous pastures on hydrology and water quality. This study investigated runoff water quality over eight hydrological years from virgin brigalow (Acacia harpophylla) woodland and three improved pastures, namely, buffel grass (Pennisetum ciliare), butterfly pea (Clitoria ternatea) and leucaena (Leucaena leucocephala). Runoff event mean concentrations and loads of total and dissolved nitrogen, phosphorus, and carbon in addition to total suspended solids are reported. Brigalow woodland had the greatest loss of sediment and nitrogen attributed to the inherently fertile Vertosols (clay soil), but the low occurrence and amount of runoff meant that it had a low risk to water quality. Despite a similar number of runoff events from the improved pastures, leucaena pasture had less total runoff and a lower maximum peak runoff rate in addition to lower nitrogen and carbon in runoff. Total suspended solids and carbon in runoff were greater from grass pasture than from the leguminous pastures, whereas nitrogen and phosphorus were greatest from the butterfly pea pasture, especially in the first 2 years post-planting. Greater exports of phosphorus from the improved pastures were concerning, given the potential for downstream impacts.