On-farm storage volumes

Northern basin on-farm storage during the flows

During the event we monitored water captured in eligible farm storages across the Northern Basin. We used remote sensing technology (satellite imagery) to observe the presence of water, and in combination with storage curves, to estimate storage volumes.

There are approximately 1,400 large on-farm storages in the Northern Basin floodplains of the Border Rivers, Gwydir, Namoi, Macquarie and Barwon-Darling valleys. If full, these storages could hold a volume of 1,300 GL. At the start of February these storages held an estimated 23 GL, by mid-February 51 GL and by the end of April 292 GL. We therefore estimate an increase in stored water of approximately 270 GL between February to April 2020. This includes an estimated 30 GL by mid-February, during which there were several days when restrictions on floodplain harvesting were lifted in specified areas.

It is important to note that:

• the stored volumes are derived from remote sensing information, rather than directly measured. They are estimates only. We have used assumptions and there is scientific uncertainty in the results. Future telemetry and measurement of floodplain harvesting will improve how we quantify floodplain water take.
• the analysis captures the change in volume of water held in storages and does not identify the individual sources contributing to this stored water. This stored water might be from direct rainfall, on-farm runoff/tailwater capture, harvesting from floodplains or pumping from rivers and aquifers.

To estimate the stored volumes, we used the method below. This process was repeated three times to represent pre-, mid- and end-of-event stored water volumes.

Volume estimation method:

1. Observe storage inundation by measuring the water surface area by satellite imagery.
2. Validate the observations with a second set of high-resolution satellite imagery from a different satellite source.
3. Estimate the volume of water held in each storage using a storage capacity curve derived from aerial survey (LiDAR) data. The curve matches observed water surface height (AHD) to surface area (m2) and volume (ML). In this way, a measured wet surface area is turned into a stored volume of water for each storage.