Measurement and modelling of irrigation channel seepage in northern Victoria

© 1993 Dr. Anthony Joseph McLeodThe earthen irrigation channel network in northern Victoria is operated and maintained by the Rural Water Corporation of Victoria. Water tables are now within 2 m of the surface throughout a large part of the irrigated area in northern Victoria. Groundwater accessions from seepage losses from earthen irrigation channels contribute to these high water tables and the associated problems of salinisation and water logging. Seepage tests using a pondage method were performed on two irrigation channels near Tatura (36° 26’ S, 145° 16’E) in north-central Victoria. Testing took place over two irrigation seasons (1989-90 and 1990-91) at both channel sites (Tatura East and Dhurringile). Testing also took place during a third irrigation season (1991-92) at the Tatura East site. This study is the most extensive of its kind ever completed within an Australian irrigation system. Seepage rates were calculated for all seepage tests performed (11 at the Tatura East site; 8 at the Dhurringile site). Seepage losses in the upstream pond of the Tatura East channel varied between 20 and 34 mm d-1. In the downstream pond this variation was 14 to 19 mm d-1 under normal pondage conditions. The difference between channel water level and aquifer head was the most significant factor in determining the seepage loss rate from the channel. The related problem of leakage (loss from the channel through holes in the channel bank) was highlighted in this study. Measurements of leakage losses were made in 5 of the 8 tests at the Dhurringile site and estimates of the relative magnitude of seepage and leakage losses were made. The average seepage loss (not including leakage ) during these five tests varied between 5 and 9 mm d-1. The data collected from the Tatura East site included channel water elevation and groundwater elevation in three piezometer transects perpendicular to the channel. Using these data, an existing computer model employing the boundary integral equation numerical method was modified to simulate the seepage processes occurring at this channel site. Physically based algorithms of deep seepage and net evaporation from the water table were added to the model during this study. Through calibration and validation of the model results to the field observations, the aquifer parameters were determined and the processes involved in the transmission of seepage away from the channel were identified