Daly
Quantification approaches
Summary of quantification approaches
Table 1 outlines the quantification approaches used to derive the item volumes in the Daly region. For a more detailed description of the quantification approach, click on the relevant item name in the table.
Detail of quantification approaches
Operational storage databases
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 following assumptions were made:
- 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, which in turn affects the accuracy of the storage–volume curves.
Climate grid data
Precipitation and evaporation on/from surface water
Monthly precipitation grids were produced using daily data from approximately 6,500 rain gauge stations and interpolated to a 0.050 (5 km) national grid (Jones et al. 2007).
The precipitation at Copperfield Dam was estimated from the proportionally weighted average of grid cells that intersected the water feature. The volume was then estimated by multiplying by the surface area of the storage. In this case, a static surface area value from the Australian Hydrological Geospatial Fabric was used.
The potential evaporation estimate produced by the Australian Water Resources Assessment system Landscape model (AWRA-L) version 3.0.0 (Van Dijk 2010) was used to calculate evaporation from Copperfield Dam. The AWRA-L model uses a modified version of the Penman–Monteith method to produce the potential evaporation. Daily AWRA-L potential evaporation grids were produced based on daily gridded climate data that were available on a 0.050 (approximately 5 km) national grid.
As a potential evaporation dataset, it is an estimate of the evaporative demand of the atmosphere. The daily gridded climate datasets are generated by the Bureau of Meteorology and include downward solar irradiance, and maximum and minimum air temperatures. The methods used to generate these gridded datasets are outlined in Bureau of Meteorology (2007).
The evaporation at Copperfield Dam was estimated from the proportionally weighted average of grid cells that intersected the water feature. The volume was then estimated using the surface area of the storage. In this case, as daily storage level data were unavailable, a static surface area value from the Australian Hydrological Geospatial Fabric was used.
The limitations associated with this approach are:
- The precipitation and evaportaion and AWRA-L potential evaporation estimates were subject to approximations associated with interpolating the observation point data to a national grid as detailed in Jones et al. (2007).
- The use of the static Australian Hydrological Geospatial Fabric waterbody feature class surface area is an approximation only. It represents the storage at total capacity and therefore is likely to result in an overestimation of precipitation on the storage.
- The method used to provide evaporation from rivers assumes the width of the river and also a standard surface evaporation per day.
Mike 11 hydraulic model
Runoff to surface water
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 Department of Natural Resources, Environment, the Arts and Sport 2010.
The runoff estimates were subject to the assumptions of the MIKE 11 model detailed by Department of Natural Resources, Environment, the Arts and Sport 2010.
Hydstra database
River outflow from the region
The Daly River is the only river that flows out to sea from the Daly region. The river outflow was estimated using instantaneous discharge data (L/s) collected at the most downstream gauging station nearest to the outlet to the sea along the river (Mount Nancar station). These data were converted to daily volume data (ML) to determine the total annual discharge (in ML) at Mount Nancar during the 2013–14 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, that is, there is no adjustment made for the contributing area below the gauging station used to calculate the outflow.
This contributing area below the gauging 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 9,373,000 ML, which is 0.99 times that reported in the Statement of Water Flows ( 9,416,775 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 assigns quality codes to flow data in accordance with Table 2.
Quality code | Description |
41 | Good rating |
91 | Satisfactory rating |
141 | Poor rating |
142 | Telemetry data–no visual verification |
150 | Rating under review–not quality coded |
255 | No data |
The total volume of water that outflows into the sea from the Daly River has a quality code of 142. This quality code indicates that the data provided for the Mount Nancar station during the 2013–14 year has not been verified, and is raw data.
Water resource license database and estimations
Water allocation remaining (or claims on water)
The water supply licences in the Daly region have a water management year that begins on 1 May. Therefore, the allocation remaining at the end of the 2013–14 year for the Daly region is the unused component of the annual allocation for these licences. The allocation remaining at 30 June 2014 is calculated as shown in Table 3.
| Account: Water allocation remaining | |
| Opening balance (at 1 July 2013) | |
| add | Water liability increase (allocation announcement) |
| less | Entitled abstraction of allocated water |
| less | Water liability decrease (forfeiture) |
| equals | Closing balance (at 30 June 2014) |
Adjustment and forfeiture of water allocation/claim on water
The portion of water allocation that has not been abstracted at the end of the licence year is forfeited, that is, there is no carryover of entitlements.
The water supply licences for the Daly region have a water management year that ends on 30 April. Therefore, the forfeiture reported for individual user and urban wtaer system licences is calculated as the total allocation remaining at 1 July 2013 less the total allocation diversion between 1 July 2013–30 April 2014, as shown in Table 4.
| Account |
| Allocation remaining (on 1 July 2013) |
less | Allocated diversion (1 July 2013–30 April 2014) |
equals | Forfeiture on 30 April 2014 |
Entitled abstraction of allocated water to individual users
The entitled abstraction of allocated water by individual users (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 2013–14 year. Metered data are supplied by users to the Department. Where metered data is supplied, expected error for water meters 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 either assumed to be zero, or estimated based on previous year usage and information from property development plans or licences. There is not sufficient information relating to actual abstraction to provide more accurate estimates of abstraction for all licences. Where estimates of abstraction are based on property development plans, it is assumed that the full annual entitlement is abstracted, and that the rate of abstraction is uniform throughout the year. It is unlikely on both counts that this will be the case for these licences.
Entitled abstraction of allocated water to urban water system
The volume presented for this line item is a combination of metered data and estimates.
There are two separate licences for surface water supply to Katherine. The allocated diversion for one licence was based on metered data (approximately 98% of the volume) and the other was based on estimated data. Where metered data are available, the abstraction is calculated as the actual abstraction during the licence year. Metered data are supplied to the Department on a monthly basis. Where metered data is supplied, expected error for water meters 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 diversion is estimated to be the full annual allocation. Less than 2% of the total volume of surface water diverted for Katherine supply was based on estimated data. The estimated abstraction is assumed to equal the full annual allocation, and that the rate of abstraction is uniform throughout the year. It is unlikely on both counts that this will be the case for this licence.
The volume of groundwater extracted for urban water supply is based on measured data collected at each source site within the Tindall aquifer using a cumulative water meter.
Abstraction of water for other statutory rights
The volume presented for diversion of surface water is an estimate. Stock and domestic use was estimated based on property area around each of the major rivers within the Daly catchment and a stock and domestic use factor. The property area was assumed to equal the length of each river by an 8-km wide buffer.
Based on best practice, stocking rates, and daily stock water needs, as recommended by the Department of Primary Industries and Fisheries, total stock use was assumed to equal 0.2 ML/year/km2 for each river. Based on average metered water use by domestic water users in the region as part of a voluntary bore–metering project conducted in 2008, the total domestic use was assumed to equal 5.5 ML/year/km2 for each river.
Total volume of surface water diverted is calculated by multiplying the estimated property area by the stock and domestic use factors. There is not sufficient information relating to actual abstraction to provide more accurate estimates of diversions by individual users under statutory rights.
For groundwater, the volume presented for abstraction under other statutory rights is an estimate. The estimated data are based on water allocation plans for the relevant groundwater resource or spatial assessments that factor in variables such as number of properties, stock water requirements, and 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's website.
Water allocation announcements (or increase of claim on water)
The total surface water and groundwater allocation announcement is equal to 100% of the annual entitlement. See Water rights, entitlements, allocations and restrictions note for more information.
Power and Water operational database
Discharge from urban water system
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. 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 (The Department) 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 1. The volumes reported in this account refer to the natural water movement that occurs within these aquifers within the Daly region boundary.
Figure 1 Map of the 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 in Department of Natural Resources, Environment, the Arts and Sport 2010.
Groundwater flow from/to outside region
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 in Department of Natural Resources, Environment, the Arts and Sport 2010.
Groundwater recharge from landscape
Diffuse recharge from the landscape into aquifers within the region was estimated using the 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 in Department of Natural Resources, Environment, the Arts and Sport 2010.
A limitation of this method is it has been found that this methodology has not quantified the increase in recharge during wetter periods in the rainfall record when compared to groundwater level hydrographs and gauged flows.
Recharge/discharge from/to surface water and groundwater
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 in Department of Natural Resources, Environment, the Arts and Sport 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 flood events.