Murray–Darling Basin

The volumetric value estimated for the line item is 34,479,014 ML. The line item value comprises mainly evaporation and seepage from river channels within the connected surface water system, and evapotranspiration within the riverine floodplains. The line item also includes unaccounted-for difference volumes in the connected surface water system arising from uncertainties associated with the quantification of surface water line items, such as run-off (Line item 13.4.1). These two components have not been separated out for this line item due to the reasons explained below.

Evaporation and seepage from river channels within the connected surface water store and evapotranspiration within the riverine floodplains

There was no surface water flowing out of the Murray–Darling Basin (MDB) region to sea during 2009–10 (see Line item 14.2). Therefore, the difference between total surface water increases and decreases is a loss to the connected surface water system.

The largest component of surface water increases is landscape run-off to connected surface water (Line item 13.4.1), comprising 44,270,703 ML (or 94%) of surface water increases. This value represents the total overland run-off generated over the whole MDB, less interception by local catchment reservoirs. It was quantified using the WaterDyn and AWRA-L models.

Only losses from the connected surface water system through evaporation from storages (Line item 14.1) were evaluated explicitly. Evaporation and seepage losses from regulated and unregulated river channels were not quantified because of lack of data and a suitable modelling framework. Thus, a portion of the value reported in this line item represents the losses from the connected surface water system that were not explicitly quantified in the National Water Account 2010 (the 2010 Account).

It is also likely that a large proportion of this line item value are losses that have not been explicitly quantified here but occurred through evaporation from the river water surface, evaporation from the floodplain water surface or recharge to groundwater. This is certainly true for terminal and semi-terminal rivers such as:

• the Narran River (an effluent channel from the Balonne River separating downstream from Dirranbandi) to Narran Lakes
• the Murray River to the lower lakes (when the lower lakes are below sea level, such as in 2009–10)
• the Paroo River, which flows to the Darling River
• the Wimmera River, which flows to Lake Hindmarsh and Lake Albacutya
• the Avoca River, which flows to Kerang Lakes
• the Lachlan River, which loses a significant proportion of flow in the Booligal and Great Cumbung swamps before joining the Murrumbidgee.

As an example, the WaterDyn and AWRA-L models estimated 3,356,636 ML of run-off in the Paroo Valley during the 2009–10 (in Line item 13.4.1). The Paroo River joins the Darling River between Tilpa and Wilcannia. Based on actual flow measurement, instead of an increase in flow after passing the Paroo River, the Darling River recorded a loss of 125,112 ML flow from Tilpa to Wilcannia. This indicates that much of the run-off in the Paroo would have flowed over the river channel banks and evaporated from the floodplain. Some water within the channel may have also been lost through evaporation and seepage into the groundwater. These processes could explain the differences in the modelling estimates and actual downstream flow measurements.

Similarly, comparisons between modelled and observed flows show that the Narran River at New Angledool records a loss of 198,703 ML during 2009–10. These examples highlight the extent of evaporation and seepage in the river channels, and evapotranspiration within the riverine floodplains occurring within the MDB region during 2009–10.

The following table provides more details about evaporation and seepage in the river channels within the region.

1. Includes storage volume increases, and evaporation and seepage in the river channels and storages.
2. Included only storage volume increases and evaporation from storages. Evaporation and seepage in regulated, and unregulated river channels are not included.

Comparing precipitation and run-off to connected surface water system in the above table, the 2010 Account accounts only 6,763,000 ML as the volume remaining in the environment for 2009–10. On the other hand, the relevant volume is 19,100,000 ML in an average season. Therefore, the main contributor to the line item is evaporation and seepage in river channels, which is missing in the above table for 2009–10.

Unaccounted-for difference volumes in the connected surface water system arising from uncertainties associated with surface water line items

Uncertainties associated with the measurements and estimations of line item values result in unaccounted-for difference in this account. For the connected surface water system, value for the unaccounted-for difference cannot be calculated separately. This is because a value for evaporation and seepage from river channels within the connected surface water system and evapotranspiration within the riverine floodplains is not separately available (as described earlier).

Data source

Line item values for the connected surface water store for the 2010 Account (MDB region).

Data provider

Bureau of Meteorology.

Method

Line item value is the difference between all increases and decreases associated with the connected surface water system because there was no outflow from the region reaching sea during 2009–10.

Uncertainty

The volume was estimated. Uncertainty is ungraded.

Approximations, assumptions, caveats/limitations

• In addition to actual losses, the line item value includes unaccounted-for difference volumes arising from surface water line items.