progress towards the higher-level targets and outcomes of the Reef 2050 Water Quality Improvement Plan ( Reef 2050 WQIP ) through the Great Barrier Reef Report Card .
Monitoring of pollutant loads at various scales show significant year to year load variation , mainly due to differences in annual rainfall and run-off . Therefore , catchment modelling , supported by monitoring data for validation and verification where possible , is used to estimate long-term annual pollutant load reductions due to the adoption of improved land management practices . This removes the impact of factors such as climate variability by assessing the results over long term climate periods . In addition , the modelling also allows for the assessment of changing pollutant loads under predicted future climate variability .
Time lags to observe water quality improvement from land management demonstrated in monitoring data could be decades due to the high degree of variability between years , and for the outcome of on-ground actions to take effect ; it takes time for vegetation to provide effective stabilisation of riverbanks and affect a reduction in streambank erosion . Long-term water quality monitoring data is also needed to validate and improve the models , continuously improving confidence in the estimates of water quality over time and to fill emerging knowledge gaps ( Lewis et al ., 2018 ).
Due to these lag times and ‘ noisy ’ water quality signals associated with inter-annual flow variability , it can take many years to capture the flow and pollutant concentration variability at a site ( e . g ., Melland et al ., 2018 ). It is estimated that at least a 25-year flow period is suitable for measuring changes in run-off ( Chiew and McMahon , 1993 ), and up to 50 years is needed to monitor a sediment load reduction of 20 % ( Darnell et al ., 2012 ). However , these estimates depend on an array of factors such as the size of the intervention ( area covered ) and the impact of management change ( i . e ., the building of a large trapping feature such as a dam versus farm-scale land management practice changes ). These response timeframes are well outside the current timeframes for this project , and even the target timeframes for the Reef 2050 WQIP . A model framework supported by monitored data that links management action in catchments to water quality is , therefore , required to report progress in the timeframes of the Reef 2050 WQIP targets and is a fundamental underpinning to the Paddock to Reef program .
Modelling can also be useful where there are limited resources for extensive monitoring programs . While it is intended that the evaluation design for the LDC Project addresses the full geographic scope of the LDC Project , there are limitations in the ability to monitor and report across the whole geographic extent due to accessibility and resourcing . These limitations are overcome , to some extent , by the integrated monitoring and modelling approach adopted in LDC WQ MMR Strategy , where modelling can be used to infill spatial gaps in the monitoring . Ongoing and future monitoring building on recommendations from this and similar concurrent reports can be used to further develop the models .
There are limitations in the level of accuracy that can be achieved through both monitoring and modelling leading to some uncertainty in estimated reductions . These challenges are explored further in the LDC WQ MMR Implementation Strategy . The modelled estimates at the catchment scale are generally accepted as the best estimates based on the best available knowledge at this time .
It is also important to acknowledge that improvements in land condition can have a large influence on runoff and associated sediment losses which need to be benchmarked against natural variability in land condition ; in particular , groundcover can change greatly seasonally and year to year . This is driven by management but is also heavily driven by climatic conditions in terms of coverage ( e . g ., high rainfall typically leads to higher coverefforts ) and composition , which are both important for sediment losses and local hydrology . Improvements in land condition are expected to have additional ecological , economic , and social benefits within the local area . Therefore , the success of the program hinges on increased stewardship by local communities and reporting of the local social , economic , and cultural benefits is critical to achieve water quality outcomes .
Accordingly , the following additional Key Evaluation Questions ( KEQs ) are also relevant to long-term water quality outcomes :
• To what extent has LDC contributed to an improved culture of stewardship ?
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