Ord

The volume recognised in the water accounting statements (4,352,385 ML) represents the difference between the measured opening and closing balances of surface water assets, after physical water inflows and outflows have been added and subtracted, respectively.

The unaccounted-for difference is calculated according to the table below.

The volume of unaccounted-for difference as a proportion of the total surface water assets is 50%.

Descriptions of the main sources of errors are:

• Estimates of Line item 13.4.1 Runoff to connected surface water comprised two components: runoff into reservoirs (2,742,848 ML) and runoff into rivers (4,795,852 ML). Runoff into reservoirs was calculated using measured data. Runoff to rivers was estimated using models. Due to the previously documented biases in the AWRA-L and WaterDyn modeling methods in Northern Australia, it is likely runoff to rivers has been overestimated, thereby resulting in an overestimation of the volume of water entering the connected surface water store. Also, whilst the models estimate the volume of water entering rivers and streams in the region, not all of that water makes its way to reservoirs.
• Estimates of Line item 14.1 Evaporation from connected surface water were only applied to the surface area of reservoirs. Evaporation was not applied to the rivers component estimates of Line item 13.4.1 Runoff to connected surface water. It is likely that this contributes to an overestimation of the volume of water entering the connected surface water store.
• Some of the gauges used to calculate Line item 14.2 Surface water flow out of region contained incomplete records for long periods of 2009–10, which has been adjusted for but nonetheless is likely to contribute underestimating the outflow from the region. Flow out of the region may also be underestimated because there is contributing area below the gauging stations.