Ord

The volume recognised in the water accounting statements (7,538,700 ML) represents the volume of run-off from rainfall that entered rivers and major storage in the Ord region during 2009–10.

The breakdown of run-off to rivers and major storages is provided in the table below.

Data source

Bureau of Meteorology: National Climate Centre (NCC), daily climate grids (rainfall, temperature and solar radiation), Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class.

Commonwealth Scientific and Industrial Research Organisation (CSIRO): WaterDyn and AWRA-L model parameters, and monthly climatological average radiation grid data.

Geoscience Australia: Southwest Western Australia man made waterbody feature class.

Western Australian Department of Water: Water Information (WIN) database.

Water Corporation Operational Data Storage System database.

Data provider

Bureau of Meteorology and then Western Australian Department of Water.

Method

Runoff to connected surface water comprises two components:

• runoff into major storages
• runoff into rivers.

Runoff into major storages

Runoff into major reservoirs was calculated based on a water balance approach using measured data collected at each major reservoir. Runoff into a reservoir (inflow) is calculated using the following water balance equation:

Inflow = ΔS + O + E - Pb - T

Where:

ΔS is the change in storage
O is total outflow from the reservoir
E is evaporation from the reservoir
Pb is the volume of water pumped backed into the reservoir
T is the volume of water transferred into the reservoir from the water distribution system.

Total outflow (O) is calculated using the following equation:

O = Sp + Sc + D + R

Where:

Sp is spillage
Sc is scour
D is volume of water abstracted from the reservoir
R is volume of riparian releases from the reservoir. 

The calculation of inflows to each major reservoir in the Ord region is described in the following tables:

Lake Argyle

Lake Kununurra

Lower Ord River

Moochalabra Dam

Runoff into rivers

Rainfall runoff into the river components of connected surface water was estimated based on the average of the WaterDyn discharge and the AWRA-L streamflow model outputs.

Using climate grid data for the Ord region (including precipitation, temperature and solar radiation data), WaterDyn and AWRA-L were used to estimate the runoff depth at each grid-point within the region. Only runoff from the landscape is considered; therefore, the surface areas of the major reservoirs and the local catchment reservoirs were excluded from the analysis.

Runoff from the landscape is divided into two components: runoff into the connected surface water store (rivers and drains); and runoff into local catchment reservoirs. Only runoff into the connected surface water store is considered here.

The average runoff depth from the landscape into the connected surface water store was determined as the unweighted arithmetic mean of the relevant Australian Water Availability Project (AWAP) grid-points within the region boundary. Mean runoff depth was converted to a runoff volume by multiplying runoff depth by the total area of the region (excluding reservoirs).

Uncertainty

Run-off to reservoirs is derived from measured and modelled data. Ungraded.
Run-off to rivers is derived from modelled data. Ungraded.

Assumptions, approximations and caveats/limitations

• The runoff estimates were subject to the assumptions of the WaterDyn model detailed in Raupach et al. (2008) and the AWRA-L model detailed in van Dijk (2010).
• The WaterDyn and AWRA-L models appear to be significantly overestimating dry season runoff in the Ord region when compared to observed streamflow records. This result is consistent with the comparisons of Viney (2010) that show strong positive biases for AWRA-L and WaterDyn in terms of winter runoff in Northern Australia. The current model deficiencies will be addressed over time through further development of the AWRA system including national calibration of AWRA-L and assimilation of satellite derived ET/soil moisture and gauged streamflow.
• The estimated runoff corresponds to the runoff expected from an unimpaired catchment. The impairment on runoff from local catchment reservoirs is estimated using a local catchment reservoir water balance model (STEDI). Where this is applied, the runoff estimates inherit the approximations, assumptions and caveats of the water balance model (STEDI) and parameters used.
• Landscape runoff attributed to grid-cells (5 × 5 km) that intersected the catchment boundary (i.e. that had some part of the grid-cell falling outside the catchment boundary) was included fully in runoff calculations for the catchment. This had a limited influence because the average of runoff from all grid-cells was multiplied by the catchment area.