A 7-year project investigating seepage losses in southern NSW has some important findings for improving distribution efficiency in on-farm channels.
Water use and groundwater rise are two important issues facing irrigators in the southern irrigated region of New South Wales. Water loss from irrigated farms and major supply channels through percolation has been extensively studied: however this project, conducted from NSW Agriculture's Yanco office, has been the first to study losses through on-farm channels and drains.
Until recently water loss through seepage from on-farm channels or drains was not recognised as a resource loss and a threat to the sustainability of irrigation enterprises. Although the procedures leading to seepage losses are still not well understood, they almost certainly contribute to salinity, waterlogging and water loss.
Distribution efficiency of poorly constructed or poorly located on-farm irrigation channels and drains can be low. This poor efficiency reduces the water available to the farmer and creates problems on-farm with localised salinity, trafficability difficulties, and weed growth. At the larger scale, the losses create problems in the regional groundwater system, including a rise in groundwater levels and soil salinity problems.
The project was a 7-year investigation that took place on-farm alongside normal farming activities. It aimed to discover how better site investigation and construction techniques for earthen channels could aid in improving conveyance efficiency. The researchers determined the magnitude of percolation losses attributable to on-farm channels and drains and considered what remedies were possible. They then evaluated the likely remediation techniques.
When the project started, the first requirement was a reliable methodology for identifying the extent and likely magnitude of potentially high seepage loss sites along channel and drainage lines.
EM-31 surveys were used to characterise soil differences along channel and drainage lines. The apparent electrical conductivity (ECa) values were compared with direct seepage measurements using an Idaho Seepage Meter. Highest seepage rates were found where ECa values were low.
In some areas seepage rates were found to be low despite low ECa values at these locations. These anomalies were attributed to various factors including substrate compaction, clay layers below the channel bed, sodicity, biological activity and sediment deposition.
The project’s combination of seepage monitoring and EM-31 electromagnetic surveys proved to be highly effective in detailing the nature and extent of the problem in the alluvial landscape of the Riverina, identifying sites with significant levels of seepage and other sites with very low seepage rates.
Generally, the soil profiles of on-farm channels have not been adequately compacted and in southern NSW are not consistent in soil texture, with mainly sandy clay soils and gravelly sandy clays in areas where prior streams are present.
Two approaches for channel sealing were investigated:
The research project investigated different compacting actions and found that increasing the required number of passes from 4 to 8 would increase the density of the soils in the bed of the channel up to and below the natural ground level, depending upon the soil condition and initial moisture content. The compaction technique greatly reduced seepage from high seepage sites without disturbing the existing channel system.
Three types of heavy machines were tested for soil compaction in the channels. The BH-1300 impact roller was very effective in reducing seepage rates at moisture levels between –1% and +2% of standard optimum moisture content. It reduced seepage by 60–75% for 8 machine passes, and by 30–40% for fewer passes, compacting by an average of 60 mm for 4 passes and by 90 mm for 8 passes.
The project evaluated several drain lining methods. One section of drain on each farm was selected for bentonite clay lining. Lining was used rather than impact compaction because, in many deep on-farm drains, it is difficult to move the high-energy impact roller in and out the drain.
Water treatment sludge and rice hull ash were used to fill the voids of channel soil. Both were found to increase the impermeability of soils in the existing channels.
Rice hull ash also showed a capacity to conserve moisture. By keeping the channel soil wet between irrigations, long after water had stopped running in the channel, the rice hull ash prevented shrinkage or cracks.
In developing and implementing this project, NSW Agriculture staff and farmers worked with the Cooperative Research Centre for Sustainable Rice Production, the Department of Land and Water Conservation, CSIRO, local contractors, Geotech companies, and suppliers of heavy machinery suppliers (LandPAC PL and Broon Hires PL).
The research project enhanced farmer knowledge of seepage sites and found better ways to save water. The researchers discovered:
The information from this research will enable farmers to avoid seepage losses from on-farm channels and drains. Its methodology can be applied to other irrigation areas as well.