Recycling of irrigation tail water where possible is attractive in the Lower Burdekin where water is a highly valued commodity . This is even greater for those farms which have blocks which don ’ t have ready access to a water supply channel or where the only water supply is a bore with poor water quality that requires mixing with better quality water ( Rickert and Kenniff , 2018 ). In these instances , recycle pits are doubly useful as a supply channel .
It well recognised by industry bodies and funding agencies that recycle pits must be sized , situated and constructed correctly , however , an additional key aspect integral to maximising the utilisation of recycle pits is ensuring an effective distribution infrastructure exists across the farm for water re-use . This requires appropriately located and sized pipelines and pumps , sufficient to enable the rapid and regular drawn down of the water collected in the pit in readiness for the next irrigation or rainfall event ( Rickert and Kenniff , 2018 ).
Given the high cost to construct recycle pits and the increasing cost to pump water , minimising the cost of both is ideal . The use of furrow irrigation automation has been shown to reduce the total amount of irrigation water applied by 10 per cent on a low infiltration soil ( Gillies et al ., 2017 ). It is reasonable to presume that when automation and sensor detection is used , any ( newly ) constructed recycle pit would need to be considerably smaller than when no automation was used to manage irrigation events . There is additional benefit ; the reduced amount of water needing to be recycled , would have a lower pumping cost .
High concentrations of pesticides have been found in creeks draining the southern Burdekin Haughton Water Supply Scheme area ( Davis et al ., 2013 ), suggesting a substantial release of tail water from farms in that region . This result could be due to either small dam size relative to the volume of run-off water or water being flushed from the dams during large rainfall events , moving nutrients ( and sediments and other chemicals ) off farms , reducing the efficiency with which dams keep nutrients on farms .
Shannon and McShane ( 2013 ) found that the size of on-farm dams ( recycling pits ) was well matched to the volumes of irrigation tail water produced , suggesting flushing of dams during large rainfall events the more likely cause . However , the frequency with which water is flushed out of dams and into local water courses is not known . Capturing and recycling of all tail water and some rainfall run-off is considered best management practice in irrigated sugarcane in the Burdekin region ( NQ Dry Tropics , 2016 ). More information is required on the efficacy of capturing tail water , and the pollutants contained in that water , in irrigated areas in the Lower Burdekin .
Further discussion of the efficiency of recycle pits in the region is provided in Section 7.3 .
5 Modelling pollutant loads from irrigated sugarcane
Paddock scale modelling of pollutant loads through the Paddock to Reef program provides information on water quality in runoff or deep drainage for sugarcane in the Lower Burdekin . It links the management practice adoption data with the catchment model by providing estimates of sediment and nutrient loads and pesticide concentrations for management practice scenarios . A suite of defined farming systems which represent plausible management practice combinations and scenarios are simulated for a large number of combinations of soils and climates .
The catchment scale model , Source Catchments , is used for reporting progress towards the water quality targets however it has limited capability for reflecting changes from improved management at the paddock scale . In contrast , paddock soil water balance models are designed specifically to simulate effects of management practices such as crop rotations , intensity of tillage and the application method , and rate and timing of herbicides and fertilisers . They are also able to supply daily outputs based on the same rainfall inputs used in the catchment models for many combinations of climate , land use , soil type and management practice . The paddock modelling results are a critical input to the catchment modelling . The effectiveness of individual practices and state of key parameters for the paddock modelling is derived from paddock scale monitoring ( historic or within the Paddock to Reef program ). In some cases , sub-components of the paddock scale models are derived directly from the monitoring data such as the DIN runoff model in sugarcane .
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