Modelled estimates of pollutant load reductions play an important role in assessing the impact of the actions undertaken in the LDC Project and provide a more sophisticated representation of the system processes than the relatively simple method described above adopted at a project scale .
The primary platform for catchment scale modelling in the GBR catchments is the Source Catchments model through the Paddock to Reef Program ( P2R ), which at the time of this report , did not produce results for the entire length of the project for the BBB catchments . A number of assumptions underpin this model , including some of particular relevance to this project ( included in Section 4.2.2 ). For example , the model assumes immediate effectiveness of improved grazing management practices in reducing sediment delivery , and therefore does not account for the likely time lags that occur between on-ground implementation of practice and water quality outcomes .
The modelled results for the whole Burdekin basin have been calculated for the period through to the end of June 2020 . Report Card modelling for the full LDC Project period ( including the final year ) is still underway . The cumulative modelled fine sediment reduction for the whole Burdekin basin over LDC Project years 1 to 3 ( 2017 / 2018 , 2019 and 2020 Report Card years ) is 29,300 tonnes . Importantly , this includes all investments reported in the Burdekin basin , not limited to the LDC Project , but also those outside the BBB catchments where other investments may have occurred ( e . g . Upper Burdekin ). The last reported progress towards the regional end of catchment fine sediment target ( not including the final year of the LDC Project ) was 66.8 %.
An assessment of the LDC Project contributions to these Burdekin basin load reductions reported during the project timeframe found that on average the LDC Project contributed 44 % of the Burdekin basin fine sediment load reductions ( 2,410 tonnes per year ), 29 % of the particulate phosphorus load reductions ( 2,220 kg / per year ) and 26 % of the particulate nitrogen load reductions ( 3,230 kg / per year ), from only 8.3 % the total Burdekin basin area . This equates to less than 1 % of the targeted end of catchment load reduction .
Q3 . What are the modelled pollutant reductions in the MIPs sub-catchments as a result of the MIPs investment ?
For this report , it was possible to calculate a relative estimate of the sub-catchment scale reductions using two methods :
1 ) By summing the project-specific estimates ( using the Alluvium tool ) presented above ; and 2 ) Source Catchment model estimates .
Using the Alluvium investment tool , the greatest total reduction of 6,102 tonnes was estimated in the Bowen sub-catchment , which accounts for 58 % of the total fine sediment reduction estimated for the BBB catchment . All other sub-catchments contributed 15 % or less of the total BBB fine sediment reduction . Similarly , when applying the Source Catchments method , the greatest total reduction of 4,935 tonnes was estimated in the Bowen sub-catchment , which accounts for 68 % of the total fine sediment reduction estimated for the BBB catchment . Glenmore Creek contributed 26 % whilst all other sub-catchments contributed 3 % or less . Similar patterns in the relative sub-catchment contributions were found for particulate phosphorus and particulate nitrogen using the Source Catchments model .
The results of the LDC Community Water Quality Monitoring Program can also be used to validate the models , guide future activities and was a useful engagement tool . Nine river and creek sites across the BBB catchment have been sampled by landholders during rainfall-generated high flow events between 2018 and 2021 wet seasons . These sites are additional to the site specific monitoring undertaken to measure the effectiveness of on-ground works . Sub-catchment sediment and nutrient concentration data collected to date is presented and has been used to improve the modelling of the BBB catchments as part of the Paddock to Reef program ( Source Catchments model ), particularly the fine sediment exports for the Broken sub catchment .
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