Rainfall simulation underestimates runoff phosphorus concentrations from dairy pastures

Abstract

Concentrations of phosphorus (P) in runoff from intensively managed pastures such as those used for dairying are high. Soil P has a large influence on runoff P concentrations. The most common technique used to derive soil P–runoff P relationships is rainfall simulation. A project is underway to test the utility of combining soil P-runoff P relationships with landscape hydrological models to assist in identifying areas for priority remedial action to reduce runoff P losses. However, there have been conflicting reports on the reliability of rainfall simulation to predict runoff P concentrations under natural rainfall at broader scales, e.g. hill-slope or sub-catchment. This paper reports a comparison of two methods of measuring runoff P concentrations, a) large plots (1250 m2) with low intensity simulated rainfall (8 mm/hr) and, b) small plots (1.5 m2) with high intensity simulated rainfall (80 mm/hr). Measurements were made on two occasions and over a range of soil P concentrations. There was a highly significant (P<0.01) effect of the method of measuring runoff P concentration. Runoff P concentrations from the small plots were approximately half of those derived from large plots. We hypothesize that these differences are the result of differences in contact times between the P source and runoff. However, the processes of mobilisation and subsequent forms of P are similar for both methods. Rainfall simulation may be used in the prediction of runoff P concentrations at scales broader than plot, e.g. at hillslope, if the effect of hydrological and chemical interactions are considered.