Is a rainfall simulator useful for estimating phosphorus runoff from pastures - a question of scale-dependency?

Research on management practices that aim to reduce phosphorus in runoff from agricultural land has been hampered by the need to study large catchments over relatively long time periods to account for both the temporal and spatial effects of scale. The concentrations of pollutants such as phosphorus in runoff, similarly to suspended sediment, may diminish with increasing catchment scale. However, the runoff from well-covered dairy pastures contains predominantly soluble rather than particulate phosphorus. This paper examines the hypothesis that the concentration of soluble phosphorus in runoff from dairy pastures is insensitive to scale, and that small-plot rainfall simulators can be used to estimate concentrations of phosphorus in runoff at the farm or subcatchment scale. Over 2 years and 9 runoff events, the mean concentration of soluble phosphorus in runoff from a 140 ha dairy farm (0.95 mg/L) was not significantly different from a 4 ha representative subcatchment (paddock) within the dairy farm (0.66 mg/L). Relative concentrations from the 2 sources varied between events, depending on the duration of the runoff event. This variation was attributed to changes in the relative importance of different source areas as events progressed; runoff from the more distant parts of the larger catchment area, typically in higher positions in the landscape, apparently increased in importance in longer events. A hand-held rainfall simulator, with plots of only 1 m(2), provided a quick, useful estimate of soluble phosphorus concentration in runoff. The mean concentration of soluble phosphorus in runoff from simulated rainfall at 9 locations in the 4 ha paddock (0.64 mg/L) was very similar to the value obtained from the paddock over a 2-year period (0.66 mg/L). We conclude that the concentration of soluble phosphorus in runoff from dairy pasture does depend on scale, and other variables, but the effect of scale was small for the catchment studied. A rainfall simulator may be used, with adequate replication, to estimate the soluble phosphorus concentrations in runoff that could be expected from dairy pastures over much larger areas.