How much seepage from irrigation channels is too much? Seepage has an adverse impact at the farm level, and is a major and often the sole contributor to local waterlogging problems.
Irrigators need to know whether on-farm channels and drains are causing significant water losses, and whether it is worthy putting resources into fixing a leaky channel. (As water prices rise, the benefits of remedying a leaky channel become greater.)
A project in the Murrumbidgee Valley is finding out how much water is being lost through the floor and banks of on-farm channels, and how effective various sealing treatments are. The researchers are also testing to see if EM31 surveys can be used to assess seepage, and what monitoring methods are available.
Irrigation water scarcity and rising saline watertables have created a pressing need to study the extent of seepage losses from on-farm channels and drains in the Murrumbidgee Irrigation Area (MIA) and Coleambally Irrigation Area (CIA).
Pilot surveys employed the combined approach of imaging soil conductivity, in order to identify seepage hot spots, and taking spot measurements of seepage rates, using an Idaho seepage meter. Soil conductivity was imaged both with a Geonics EM31 and a towed submerged geo-electric array. Both instruments showed a high level of consistency with direct seepage measurements. The geo-electric array is capable of separating and accentuating conductivity of soils at different depths beneath the channels that respond to or prevent seepage in different ways. Sites of high seepage were targeted for remedial work.
New ways of accurately measuring seepage from irrigation channels are the focus of a research project by the Co-operative Research Centre for Sustainable Rice Production (Rice CRC).
This study describes a field-oriented approach to quantifying channel seepage in the Murrumbidgee Irrigation Area (MIA), and outlines strategies to cut water losses using local waste materials and compaction technology.
The field experiments were carried out in two phases:
A 7-year investigation was carried out at farm level alongside normal farming activities. This part of the project aims to discover how in-situ seepage investigations and remedial techniques could aid in improving the conveyance efficiency of earthen channels. It has determined the magnitude of percolation losses attributable to on-farm channels and drains and suggested possible remedies.
To increase soil density for on-farm channels in identified leaky locations, channel compaction was used. Three compaction patterns were employed in the Coleambally and Murrumbidgee irrigation areas, varying from zero, 4 and 8 passes of a 30-ton heavy compaction machine. The data indicated that, depending on soil conditions and initial moisture content, 4 to 8 machine passes would increase density.
Performance evaluation of the remedial work proved that the recommendations were sound.