Conceptual frameworks for estimating the water quality benefits of improved agricultural management practices in large catchments

► Conceptual frameworks link agricultural practices to Great Barrier Reef pollution. ► Dissolved inorganic N exports from catchments were well related to N Surpluses in cropped lands. ► Catchment fine sediment exports were well related to field-scale erosion in grazing lands. ► Government targets for reducing pollutant exports to the Reef will not be met with current BMPs. ► The frameworks are applicable to other pollutants with particulate and/or soluble phases. Chemical and sediment losses from agricultural lands are threatening coastal marine and aquatic ecosystems in many parts of the world. This is an acute problem in Australia, where the condition of Great Barrier Reef (GBR) ecosystems is threatened by increased pollutant loads from agricultural lands, and Governments have enacted policies to reduce pollutant exports. These policies raise the question of how to identify changes in land management that will effectively reduce exports. The scale of the GBR catchments (> 400,000km2) precludes detailed modelling investigations, especially within the time scale of policy implementation. Therefore, we developed conceptual frameworks linking agricultural land management to river pollutant exports for two contrasting agricultural pollutants posing threats to the health of GBR ecosystems; dissolved inorganic nitrogen (DIN) and fine sediment (silt and clay), based on a synthesis of past studies. We argue that nitrogen (N) Surpluses (N inputs relative to crop N off-take), are the primary driver of DIN losses from agricultural land to rivers. Similarly, previous studies in GBR grazing lands and elsewhere have quantitatively defined how sediment losses from hill slopes, gullies and stream banks are related to grazing land condition, ground cover and riparian management, which are products of recent climate and grazing practices. From these frameworks we derive relationships between firstly, estimated N Surplus and DIN exports, and secondly ground cover and river fine sediment exports. Using these relationships we examine how DIN and fine sediment exports to the GBR may respond to a range of management scenarios for reducing N inputs, and increasing ground cover and improving riparian management. We predict that widespread adoption of the most extreme scenarios would approximately meet water quality improvement targets set/implied by governments for these two pollutants. However, it is unlikely that these extreme scenarios will be adopted to the extent needed and