Flood irrigation assessment on the Tallangatta Focus Farm
The Tallangatta Focus Farm has 85ha of flood irrigation with an extensive 2km channel system. The efficiency of the system had been discussed by the focus farm group at several meetings. So far this season, a section at the end of the channel system comprising 15ha has not been irrigated due to its distance from the dairy and the perceived water losses in the channel that would deliver the water. However, the actual losses are unknown. To fill this knowledge gap measurements where taken to record delivery losses in the channel and losses associated with run-off and leaching.
This was done by measuring the flow rate of the channel near the pump and at the furthest irrigated area from the pump, which was 1.5km further along the channel.
The flow rate in the channel at the pump was 15.6 ML/day, which dropped to 14.9 ML/day 1.5km further down the channel. This equates to a loss of 0.47 ML/day/km of channel, which is not too bad.
Channel Losses - Factoring in the time it takes to irrigate each bay and the distance along the channel it has been calculated that 2.3ML is lost in the channel each irrigation covering the 70ha.
The losses through runoff and leaching - The amount of water applied to each bay can be calculated by factoring in the flow rates onto each bay and the time taken to water each bay. A total of 50.7ML is applied over the 70ha. Comparing this to the 0.5 ML/ha per irrigation likely to be required by the crop/pasture, 35ML is required per irrigation to adequately irrigate the pasture. This means the farm is applying an excess of 15.7ML. This is close to 70 per cent efficiency which is considered reasonable for flood irrigation.
Overall loss and cost - losing 18ML each irrigation. The estimated cost of this water is $20/ ML with an additional $18/ML diesel cost to pump the water. This means each irrigation's water losses is costing a total of $684 or $10/ha. Note some of the excess water runs off and fills the lagoon which is re-used by the centre pivot.