Murray-Darling Basin
30.3 Runoff harvesting into off-channel water store

The volumetric value for the line item for the 2010–11 year was 1,506,485 ML. The line item includes collection of runoff into off-channel water storages including local catchment runoff into farm dams within the Murray–Darling Basin (MDB) region. The following table presents breakdown information for the volumetric value on a surface water resource plan (WRP) area basis.

 

Runoff harvesting into off-channel water storages in the MDB region for the 2010–11 year

Surface water resource plan area		State	Volume (ML) for the 2010–11 year
Code	Name
SW11–12 and SW17–19	Warrego – Paroo – Nebine, Condamine–Balonne, Moonie, NSW Intersecting Streams and Barwon–Darling watercourse	Qld and NSW	325,665
SW15–16	Qld and NSW Border Rivers	Qld and NSW	111,363
SW14	Gwydir	NSW	58,273
SW13	Namoi	NSW	95,134
SW10	Macquarie–Castlereagh	NSW	188,229
Sub-total Northern Basin			778,663
SW9	Lachlan	NSW	188,717
SW1 and 8	Murrumbidgee NSW and ACT	NSW and ACT	196,411
SW2, 4, 5 and 7	NSW Murray and Lower Darling, Vic Murray, SA Murray and Wimmera–Mallee	NSW, Vic and SA	127,832
SW3	Northern Victoria	Vic	191,743
SW6	Eastern Mount Lofty Ranges	SA	23,120
Sub-total Southern Basin			727,822
Total for the region			1,506,485

 

(1) Bureau of Meteorology (the Bureau): National Climate Centre daily climate grids (rainfall, temperature and solar radiation), (2) Commonwealth Scientific and Industrial Research Organisation (CSIRO): Australian water resource assessment – landscape (AWRA-L) model parameters, and monthly climatological average radiation grid data, and (3) Geoscience Australia: MDB human-made waterbody feature class and 9 arc-second digital elevation model (DEM).

The Bureau.

Rainfall runoff harvesting to off-channel water storages was estimated based on the AWRA-L version 1.0.0 streamflow and the tool for estimating dam impacts (STEDI) farm dam water balance model outputs (Sinclair Knight Merz 2011).

Using climate grid data for the MDB region (including precipitation, temperature and solar radiation data), AWRA-L (Van Dijk 2010) was used to estimate the runoff depth at each grid-point within the region.

The MDB was divided into 105 regions for the purpose of modelling the off-channel water store. The off-channel water store consisted of storages filled primarily by local catchment runoff. These were determined from waterbody mapping conducted by Geoscience Australia as those that:

• are not named storages (assuming that any storage with a name is unlikely to be a off-channel water storage)

• are above 600 m in elevation

• are below 600 m in elevation in areas that receive greater than 400 mm per annum in precipitation and are not within 50 m of a major or perennial stream.

The above rules attempt to divide storages into those that are likely to be filled primarily by local catchment runoff and those that are filled by abstraction from surface water, groundwater or floodplain harvesting. The catchment of each individual storage was determined via analysis of the 9 arc-second DEM.

The average runoff depth across the MDB sub-regions was determined as the weighted mean of runoff occurring from the relevant grid points within the region boundary. Points were weighted by the area they represented within the MDB landscape to remove edge effects (where the area represented is not wholly within the MDB region) and the effect of changing area represented with changing latitude. The average runoff depth was converted to a volume by multiplying depth by the total area and was used as an input into the STEDI model.

The gridded climate input data are subject to approximations associated with interpolating observation point data to a national grid detailed in Jones et al. (2007).

The estimated volume of runoff harvested is subject to the assumptions associated with the STEDI model and the parameters used.

 The spatial extent of water bodies subject to the assumptions and methods associated with the data provided by Geoscience Australia.

The use of a 9 arc-second DEM to determine catchment area may result in storages being assigned a catchment much larger or smaller than the true catchment. In some cases a storage may be assigned the catchment of a stream line hundreds of metres away.

The uncertainty estimate was not quantified.

 

In the 2011 Account, the methodology used to quantify the line item was improved and resulted in a material change in volume.

The change in value was primarily due to the use of the AWRA-L v1.0.0 model in the 2011 Account to estimate runoff instead of the average of AWRA-L v0.5 and WaterDyn v26. Some change in the volume could also be attributed to changes the AWAP rainfall datasets as data from manually read gauges entered the dataset. These changes were made to provide more reliable estimates of runoff volumes. The difference of 251,177 ML represents a change of approximately 15.3% of the volume provided for the 2010 Account. The changes and their respective values are detailed in the following table.

 

Restatement of comparative year information made for the line item 30.3 Runoff harvesting into off-channel water store

Segment	Volume for the 2009–10 year reported in the 2010 Account (ML)	Difference due to change in the calculation method (ML)	Volume for the 2009–10 year reported in the 2011 Account (ML)
Northern Basin	825,348	–79,389	745,959
Southern Basin	817,399	–171,789	645,610
Whole region	1,642,747	–251,177	1,391,570