The agricultural landscape
The Mulgrave-Russell catchment is located in the Wet Tropics, one of Australia’s wettest regions. While this region represents only 5% of the Great Barrier Reef catchment area, it contributes over 30% of the total freshwater discharged to the Reef. This uniqueness results from its proximity to high coastal mountains, combined with abundant rainfall, creating a narrow coastal plain that rapidly transports water through the catchment.
Before agriculture expansion, floodwaters in the Mulgrave-Russell once remained on the land for longer periods, naturally filtering water and holding soil and nutrients on the land. But over the past 50 years, government schemes cleared much of this land and installed drainage systems. These modifications quickly remove floodwaters from paddocks to allow crops to grow.
These changes to the landscape have rapidly moved high volumes of water from the land into waterways and the ocean, carrying more nutrients off paddocks. This makes it more challenging for growers to manage water flow and nutrient runoff from paddocks.
Environmental challenges
Research shows that dissolved inorganic nitrogen (a key fertiliser component) discharged from the Russell-Mulgrave catchment into the Great Barrier Reef has increased considerably over the past 50 years, with fertiliser losses from paddocks a dominant contributor.
Paddocks in this region are often close to waterways and the coast, leading to a higher risk of runoff carrying elevated levels of nutrients and pesticides into local creeks, rivers, and the inshore marine environment.
These elevated nutrient levels reach inshore areas, including the Frankland Islands, which can increase macroalgae abundance at inshore reefs at the expense of coral cover. The offshore reefs of the middle and outer shelf are also much closer to the coast in this region and are exposed to land runoff more frequently compared to similar reefs in the southern Great Barrier Reef.
The region also boasts Australia's highest fish diversity, with local estuaries acting as critical fish breeding grounds. Good water quality is needed to maintain fish populations and support habitats for fish, including shelter and food.
Reducing runoff can improve farm productivity
The loss of pesticides and nutrients through runoff from farms can be costly for growers. When more fertilisers and pesticides run off paddocks, input efficiency decreases, and overall productivity suffers. Making good land management decisions benefits both the farm and the environment, including:
Effective use of machinery reduces ground compaction, enhancing water infiltration for better soil health and efficient water use.
When precision technology is used to target applications of fertilisers and pesticides, the use and cost of these products are reduced.
By accounting for organic nutrients added to the paddock, such as fallow crops and mill mud, synthetic fertiliser application rates are lowered. This strategy optimises fertiliser rates, which can increase crop yields and reduce input costs.
Local-scale water quality monitoring validates interventions
While the region's tropical rainfall and drainage systems pose challenges in managing runoff, more growers are trialling and tailoring new farming and management practices to tackle water flow and nutrient runoff from paddocks.
Fine-scale water quality monitoring projects in the Mulgrave-Russell region use advanced sensors to identify local 'hotspot areas' with higher nutrient levels, revealing how and when these elevated losses occur. With accurate and robust data in hand, growers are adopting innovative practices to manage the high and unpredictable rainfall.
These practices include capturing and retaining water, such as diverting it into holding pits or blocking drains, contouring paddocks and creating retention basins to help remove sediment and nutrients from the water. Growers are also making changes to fertiliser placement or product choices in paddocks, or earlier applications, to increase the opportunity for crop uptake before major rainfall events.
The effectiveness of these methods is validated through the local water quality data, building growers’ confidence in adopting improved management practices.
How can growers adjust practices?
There are management options to better control the loss of fertilisers and pesticides while maintaining a profitable farm. Water quality monitoring programs in the Mulgrave-Russell work with growers to monitor peak losses, providing a pathway to improved farming techniques and offering access to better tools, technologies, and support. This includes:
Sensor technology: Scientists use sensor-based technologies throughout the stream networks to identify 'hotspot areas' with higher fertiliser losses. This real-time data is delivered to growers via an app and website.
Understanding peak fertiliser losses: Peak runoff losses commonly occur during initial heavy rainfall events when new crops are fertilised. The real-time monitoring helps growers match nitrate spikes with specific farming practices for timely adjustments.
Monitoring drainage interventions: Drainage interventions to remove nitrogen are being tested by growers. Scientists monitor nitrogen levels in inflow and outflow sections at various sites to measure their effectiveness. Early results show drainage interventions have the potential to mitigate nitrogen losses significantly.
Pesticide and fertiliser management: Growers carefully manage application methods and product choices. For fertilisers, this includes timing, rates, placement, and product selection. For pesticides, it involves timing, product choice, and strategic application. Machinery can allow for precise application.
