An analysis of chemical loads and forms in overland flow from dairy pasture in South Australia

Two adjacent subcatchments on soils with very strong texture contrast betwe en the A soil horizons (sandy loam texture) and B soil horizons (clay textu re) in the Adelaide Hills, South Australia were instrumented to collect ove rland and subsurface flow from dairy pasture. Subcatchments were defined by exclusion drains in the upper reaches and stainless steel barriers at the lower boundary. Water samples were analysed for 20 different chemical fract ions. Chemical loads were examined in order to determine relationships amon g simple empirical event and site characteristics, and between chemical fra ctions. All but five of the chemical fractions could be separated into one of two clear groups using a statistical technique based on the magnitude of simple correlation coefficients. The two groups consisted of either dissol ved or particulate fractions. The clear separation into these groups is con sistent with the majority of chemical fractions moving from pasture by one of only two processes. Simple empirical variables in a multiple regression explained a high proportion of variation in chemical loads in runoff water. These findings may have major implications in the modelling and prediction of chemical loads in runoff from agricultural catchments and the setting o f environmental limits, in simplifying chemical loss predictions by adding a nutrient loss module to runoff volume predictions. Copyright (C) 2001 Joh n Wiley & Sons, Ltd.