Catchment modelling scenarios to inform GBR water quality targets

Abstract

The Great Barrier Reef (GBR) Water Quality Plan outlines a clear set of water quality and management practice targets (50% reduction in nutrients and photosystem II (PSII) herbicides by 2013, 20% reduction in TSS by 2020). Five lines of evidence have been used to report on progress towards reef plan targets due to investment into improved agricultural management practices. This paper provides an overview of the results from one line of evidence, catchment modelling. The eWater CRC Source Catchments modelling framework was used to generate sediment, nutrient and herbicide loads entering the GBR lagoon from 35 reef catchments. Six catchment models were built for the GBR (423, 000km 2 ). Data was collected for each industry under an (cutting edge), B (best practice) ,C (common) and D (unacceptable) management practice framework for the baseline year (2008/09) and following implementation of improved management practices. Load reductions were calculated with respect to the baseline load. At the request of the funding agency, Queensland Government Premiers Department, additional scenarios were also run to determine if targets could be met by shifting to an A practice adoption. The modelling results suggest there was a 6.2% reduction in average annual suspended sediment load leaving GBR catchments due to two years of investment with the greatest reduction from grazing areas. In the case of total nitrogen, the average annual load was reduced by 6.7%, with the greatest reduction from the Burdekin and Wet Tropics NRM regions. The average annual PSII herbicide load was reduced by 14.6%, with 70% of the reduction from the Wet Tropics and Mackay-Whitsunday regions. Improved herbicide management practices in the cane industry contributed to the largest reduction. Modelling suggests that TSS targets could be met under a 50/50 A/B management scenario whilst the 50% nitrogen reduction target may not be met under an All management scenario. The Source Catchment modelling framework is now established across the entire GBR region, and enables regional NRM groups to use the model outputs to make more informed decisions as to where investment should occur to achieve the greatest load reductions for the least cost. This paper describes how the Source Catchment modelling framework is firstly used to report on progress in meeting Reef Plan 2009 water quality targets; and secondly to run additional scenarios to assess the potential to achieve the given water quality targets.