National Water Account 2015

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Region overview

Contextual information

Physical information

Administration

Water rights

Climate and water overview

Water Accounting Statements

Water Assets and Water Liabilities

Changes in Water Assets and Water Liabilities

Water Flows

Notes

Accounting policies

Supporting information

Quantification approach

Reconciliations

Water resources & systems

Water access & use

Future outlook

References

Accountability statement

Daly: Quantification approaches

Summary of quantification approaches

Table N11 outlines the quantification approaches used to derive the item volumes in the Daly region. For a more detailed description of a quantification approach, click on the relevant item name in the table.

Table N11  Quantification approach used to derive item volumes

Detail of quantification approaches

Water storage product data

Storages

Storage volume at the start and end of the year was calculated using water level data (metres above Australian Height Datum) collected at Copperfield Dam. Rating tables established for each storage were used to convert the height measurement to a volume. The uncertainty range for the storage volume was +/–5%.

The assumptions made were as follows:

• Storage–volume curves represent specifically surveyed parts of the storage and may not reflect the storage–volume relationship across the entire storage.
• Storages are subject to sedimentation and other physical changes over time that in turn affect the accuracy of the storage–volume curves.

Gridded climate data and AWRA-L model

Precipitation and evaporation

Gridded climate data are only used to estimate precipitation and evaporation at storages (i.e., Copperfield Dam). Precipitation on rivers in the region is estimated using the Mike 11 river hydraulic model and is incorporated within the runoff volume; potential evaporation from rivers is estimated based on the FEFLOW groundwater model and Mike 11 hydraulic model.

Monthly precipitation grids for the region were produced using daily data from approximately 6,500 rain gauge stations across Australia and interpolated to a 0.05 degrees (5 km) national grid (Jones et al. 2009). Potential evapotranspiration across the region was estimated using the Australian Water Resources Assessment system Landscape model (AWRA-L) version 5.0 (Viney et al. 2015). The AWRA-L model uses the Penman method to produce the potential evapotranspiration. Daily AWRA-L potential evapotranspiration grids for the region were produced based on daily gridded climate data (including precipitation, solar radiation and temperature) available on a 0.05 degree (approximately 5 km) national grid (Jones et al. 2009).

The volume of precipitation and evapotranspiration at Copperfield Dam was estimated by multiplying the proportionally weighted average of grid cells that intersected the water feature by the surface area of the waterbody. As daily storage level data were unavailable, a static surface area value from the Australian Hydrological Geospatial Fabric was used.

Mike 11 hydraulic model

Runoff

Runoff to surface water in the Daly region was estimated using a surface water model of the Daly River catchment developed using the MIKE 11 software by DHI. The calibration of the MIKE 11 model involved adjusting the model parameters to ensure model outputs matched observed streamflow data in the region adequately. A detailed description of the model calibration process is provided in the Department of Natural Resources, Environment, the Arts and Sport [NRETAS] (2010).

Stream monitoring data

Outflow

The Daly River is the only river that flows out to sea from the Daly region. The river outflow was estimated using flow data collected at the most downstream gauging station nearest to the outlet to the sea (Station G8140040: see Figure C5 in the 'Contextual information'). These data were used to determine the total annual discharge (in ML) at the station during the year.

It is assumed that the river outflow to the sea is equal to the volume of discharge measured at the most downstream station along a river. There is no adjustment made for the contributing area below that station.

This contributing area below the most downstream stations is 6,913 km² approximately 13% of the total area of the Daly region. Based on a drainage-area ratio equation, estimated outflow is approximately 4,630,000 ML, which is 1.15 times that reported in the Statement of Water Flows (4,032,345 ML). Given, however, that the ungauged component of the Daly region lies mainly on the lowlands, which is an area of relatively high rainfall-recharge, it is unlikely that this area will generate such a large amount of runoff. Instead, it is considered that the reported outflow to sea may be underestimated by up to 5–10%.

The Department of Land Resource Management assigns quality codes to flow data in accordance with Table N12.

The total volume of water that outflows into the sea from the Daly River has a quality code of 91 (satisfactory rating). This quality code indicates the lowest quality of data recorded at Mount Nancar (Station G8140001) during the 2014–15 year.

Water resource licence database and meter readings

Allocation remaining

The water supply licences in the Daly region have a water management year that begins on 1 May. As a result, the water allocation remaining at the end of the reporting year (30 June) is the unused component of the annual allocation for these licences. The allocation remaining at 30 June 2015 is calculated as shown in Table N13.

Adjustment and forfeiture

The portion of water allocation that has not been abstracted at the end of the licence year is forfeited (i.e., there is no carryover of entitlements). Therefore, forfeiture is calculated as the total annual allocation for each licence minus the allocation abstraction during the water management year.

Allocated abstraction

The allocated abstraction (both surface water and groundwater) during the licenced water year is derived from a combination of metered data and estimates.

Where metered data are available, the abstraction is calculated as the actual abstraction during the year. Metered data are supplied by users to the Department of Land Resource Management and the expected error associated with metered data is +/– 2%. The department requires that all water meters, when tested under in situ conditions, must be within 2% accuracy across the full flow rate range.

Where metered data are not available the volume of abstraction is assumed to be zero. There is not sufficient information relating to actual abstraction to provide more accurate estimates of abstraction for all licences.

Abstraction: statutory rights

Surface water diversion for stock and domestic use is estimated based on property area around each of the major rivers within the Daly region and stock and domestic use factors.

The property area was assumed to equal the length of each river by an 8-km wide buffer. Total stock use was assumed to equal 0.2 ML/year/km² for each river; this figure is based on best practice, stocking rates, and daily stock water needs, as recommended by the Department of Primary Industries and Fisheries. Total domestic use was assumed to equal 5.5 ML/year/km² for each river, which is based on average metered water use by domestic water users in the region as part of a voluntary bore–metering project conducted in 2008. Total volume of surface water diverted is calculated by multiplying the estimated property area by the stock and domestic use factors.

Groundwater extraction under other statutory rights is an estimate based on water allocation plans or spatial assessments. The assessments take into account variables such as number of properties, stock water requirements, and the distance stock must travel for water.

More information on water allocation plans in the region (Tindall and Oolloo aquifers) can be found at the Department of Land Resource Management website.

Non-allocated extraction: individual users

Non-allocated groundwater extraction for individual users is an estimate based on water allocation plans or spatial assessments. The assessments take into account variables such as number of properties, stock water requirements, and the distance stock must travel for water.

More information on water allocation plans in the region (Tindall and Oolloo aquifers) can be found at the Department of Land Resource Management website.

Allocations

Water allocations are made after a review by the Department of Land Resource Management of river and aquifer levels in the region. More information on these allocations and the associated water access entitlement is given in the Water rights, entitlements, allocations and restrictions note.

Power and Water operational database

Discharge: wastewater

The volume of treated wastewater discharged from the urban water system to the river is based on measured discharge data collected by flow meters installed at the treatment plants. The uncertainty range for flow meters installed at wastewater treatment plants is estimated to be +/– 10%.

FEFLOW groundwater model and Mike 11 hydraulic model

The Department of Land Resource Management uses the Finite Element Subsurface Flow system (FEFLOW) to estimate the natural water movement to and from the groundwater store within the Daly region.

FEFLOW estimates groundwater movement for the entire extent of the Oolloo aquifer and the Tindall aquifer, which extends beyond the Daly region boundary as shown in Figure N7. The volumes reported in this account refer to the natural water movement that occurs within these aquifers within the Daly region boundary.

Figure N7 Groundwater model area relative to the Daly region boundary

The Oolloo and Tindall aquifers are karstic and were modelled as an equivalent porous media with relatively limited storage. The groundwater model was calibrated using regional aquifer parameters to reproduce the observed groundwater levels and discharge to the rivers, as outlined by NRETAS (2010).

Inter-region flows

The groundwater model derives the lateral flux within the Daly region by defining the appropriate water balance zones and calculating the volume of water flowing across each boundary on an annual basis.

Detailed information on the model calibrations are provided by NRETAS (2010).

Recharge: landscape

Diffuse recharge from the landscape into aquifers within the region was estimated using FEFLOW. Recharge into the aquifers occurs via the following pathways:

• direct recharge of excess soil moisture
• precipitation 'channelled' through the unsaturated zone via macropores
• localised indirect recharge of surface water that is channelled into karstic features such as dolines (sinkholes).

Detailed information on the model calibrations are provided by NRETAS (2010).

A limitation of this method is that it does not quantify the increase in recharge during wetter periods in the rainfall record when compared to observed groundwater level and streamflow data.

Recharge/discharge: surface water

The flow between groundwater aquifers and rivers within the Daly region is estimated using FEFLOW in combination with the one-dimensional river hydraulic model MIKE 11. The flow interactions between surface water and groundwater are estimated where the surface water channels in the MIKE 11 model are coupled to the aquifer boundaries in the FEFLOW groundwater model.

Detailed information on the model calibrations are provided by NRETAS (2010).

There is limited understanding of actual river/aquifer interactions, especially with respect to flows from the river to the groundwater system. It is likely that, when FEFLOW is coupled with MIKE 11, groundwater recharge from rivers is overestimated during large flow events.