Daly: Methods for final account
Summary of methods
There were five key methods for establishing item volumes in the 2018 Account. Click the down arrow in the table below to view the list of items derived by each method type. For detailed information about each method scroll down this page or click on the links in the table.
Methods approach
AWRA-R model
Stream monitoring data
Water resourcing licence database and meter readings
FEFLOW groundwater model and Mike 11 hydraulic model
Estimated data
Detail of methods
AWRA-R model
AWRA-R is a river network model that represents key hydrological processes and diversions at a daily time step (Dutta et al. 2017; 2015). The model was used in the National Water Account to quantify river fluxes and stores along the river network.
The river system is conceptualised in AWRA-R as a node-link network comprising nodes connected by river reaches. Gauged streamflow data are used where available. For ungauged portions of catchment, the landscape runoff from the AWRA-L model is used (Viney et al. 2015). River processes represented in the AWRA-R model are shown in Figure N1.
Figure N1 Conceptual diagram of AWRA-R reach showing model components (from Dutta et al. 2015)
Rivers
The volume of water in the river channels at 30 June was estimated by using the daily water balance approach within the AWRA-R model. The water balance includes inflow at the upstream nodes and outflow at the downstream nodes; contributing catchment runoff, diversions for consumptive use; overbank flooding and floodplain return; rainfall; evaporation; and losses.
Precipitation and evaporation
Rainfall and evaporation into/from storages and rivers were calculated using climate data from the Bureau of Meteorology interpolated to 0.05 degree (5 km) national grids (Jones et al. 2009). Calculations were done on a daily time step, with the annual totals summed from the daily values.
Climate data for each water body at each time step were estimated from the proportionally weighted average of grid-cells that intersected the water body. Evaporation was estimated using Morton's shallow lake formulation (Morton 1983). Rainfall and evaporation volumes were then estimated by multiplying the surface area of each waterbody by the weighted average rainfall and evaporation respectively. The average daily surface area of rivers was estimated using the AWRA-R model and the average daily surface area of storages was calculated from daily storage levels and capacity tables.
Runoff
Runoff to surface water was estimated using the modelled runoff from the AWRA-R model. Runoff within AWRA-R is in turn derived from landscape runoff modelled in the AWRA-L model, with a scaling factor applied within AWRA-R during the calibration process.
The AWRA-L model is a daily grid-based water balance model that is conceptualised as a small unimpaired catchment (Viney et al. 2015). It simulates the flow of water through the landscape from rainfall entering the grid cell through the vegetation and soil, and then out of the grid cell through evapotranspiration, runoff or deep drainage to the groundwater. Its inputs include gridded climate, soil, vegetation and topographic data. For more information see the Bureau's Australian Landscape Water Balance webpage.
Overbank flow and flood return
The AWRA-R floodplain module was used to model the volume of overbank flow from the river onto the floodplain, and the return flow from the floodplain back into the river. The module applies a simple storage-based floodplain model to each river reach. The floodplain modelling method is detailed in Dutta et al. (2013).
Stream monitoring data
Outflow
The Daly River is the only river that discharges to the 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 R5 in the 'Region description'). 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.
The 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 1.15 times that reported in the Statement of Water Flows (11,362 GL). 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. Therefore, no adjustment is made for the contributing area below that station.
Quality codes are assigned to flow data in accordance with Table N3, as given in the Bureau of Meteorology's Water Data Online.
| Quality code | Description |
| A | The record set is the best available given the technologies, techniques and monitoring objectives at the time of classification |
| B | The record set is compromised in its ability to truly represent the parameter |
| C | The record set is an estimate |
| E | The record set's ability to truly represent the monitored parameter is not known |
| F | The record set is not of release quality or contains missing data |
The total volume of water that discharges into the sea from the Daly region has a quality code of B, which indicates the lowest quality of data recorded at Mount Nancar (Station G8140040) during the 2017–18 year.
Water resource licence database and meter readings
Allocation remaining
The water allocation remaining for a water licence at the end of the reporting year is the unused component of the annual allocation. As there is no carryover provisions for water supply licences in the Daly region, the portion of water allocation that has not been abstracted at the end of the water year is forfeited. Therefore, the allocation remaining at the end of the water year is 0 ML.
Adjustment and forfeiture
The portion of water allocation that has not been abstracted at the end of the licence water 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 licence water 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 Environment and Natural Resources 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 Environment and Natural Resources 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 Environment and Natural Resources website.
Allocations
Water allocations are made after a review by the Department of Environment and Natural Resources of river and aquifer levels in the region. More information on these allocations and the associated water access entitlement is given in the Water access and use note.
FEFLOW groundwater model and Mike 11 hydraulic model
The Department of Environment and Natural Resources 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 N1. The volumes reported in this account refer to the natural water movement that occurs within these aquifers within the Daly region boundary.
Figure N2 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.
Estimated data
Storages
Storage volume data for the Copperfield Dam surface water storage at 30 June was estimated based on measured water level data (metres above Australian Height Datum) collected at Copperfield Dam in previous years. Rating tables established for the storage were used to convert the height measurement to a volume.
Discharge: wastewater
The volume of treated wastewater discharged from the urban water system to the river is estimated based on four years of annual metered data collected between 2012 and 2015. Discharge data are collected by flow meters installed at the Katherine Waste Water Treatment Plant.
During this 4-year period, total annual wastewater discharge from the treatment plant changed little from year to year, so it was assumed that the wastewater discharge during the 2016–17 year was equivalent to the average annual discharge between 2012 and 2015.
The uncertainty range for flow meters installed at wastewater treatment plants is estimated to be +/– 10%.