Surplus nutrient exports differ between irrigated and high-rainfall agricultural catchments: a tale of two catchments in South East Australia

Successfully mitigating nutrient pollution requires a clear understanding of sources within catchments. Here we explore differences in nitrogen (N) and phosphorus (P) exports from high-rainfall catchments dominated by dryland agriculture, and irrigated catchments. We use one example of each from Victoria, South East Australia for this investigation. Nutrient surpluses were calculated for different types of agriculture, which were combined with non-agricultural sources to estimate catchment-scale nutrient inputs, and compared with riverine exports. For both catchment types, N and P surpluses were highest for vegetable horticulture, followed in order of decreasing magnitude by dairy farming, beef farming, mixed beef and sheep farming, and sheep farming. Dairying contributed the largest proportion of nutrient inputs to both catchments. Riverine N exports from the Irrigated catchment (2.2 to 6.3 % of inputs) and the High-rainfall catchment (3.5 to 6.6 % of inputs) are low compared with other regions of Australia and the world, indicating landscape features that facilitate N retention or loss to the atmosphere. Low N exports from the Irrigated catchment during dry years may reflect interception by irrigation recycling systems. P losses from the Irrigated catchment (7 to 22 % of inputs), occurring predominantly in dissolved form, were higher than the High-rainfall catchment (2.4 to 6.0 % of inputs). Limited capacity for P retention within the Irrigated catchment suggests that further mitigation measures that remove or recycle dissolved P are required. High nutrient surpluses, and exports driven by rainfall-runoff events indicate future nutrient loss mitigation measures must account for more extreme events anticipated under climate change.