Bureau Home » Water Information » National Water Account » 2017 Account » Daly » Reference information » Methods

National Water Account 2017

Daly: Methods

Katherine Gorge, Northern Territory (iStock © witte-art_de)

Summary of methods

Table N3 outlines the methods used to derive the item volumes in the Daly region. For a more detailed description of a method, click on the relevant item name in the table.

 

Table N3  Method used to derive item volumes

Assets
Approach or data usedItemSource
Not quantified
  • Storages
  • Rivers
  • Lakes and wetlands
  • Water table aquifer
  • Underlying aquifers
 

 

Liabilities
Approach or data usedItemSource
Water resourcing licence database and meter readingsDepartment of Environment and Natural Resources

 

Inflows and outflows
Approach or data usedItemSource
Stream monitoring dataOutflowBureau of Meteorology
Mike 11 river hydraulic modelRunoffDepartment of Environment and Natural Resources
Gridded climate dataBureau of Meteorology
AWRA modelBureau of Meteorology
FEFLOW groundwater model and Mike 11 hydraulic modelDepartment of Environment and Natural Resources
Estimated dataDischarge: wastewaterPower and Water Corporation
Not quantified
  • Inter-region coastal flows
  • Discharge: landscape
  • leakage: urban system
 

 

Abstractions
Approach or data usedItemSource
Water resourcing licence database and meter readingsDepartment of Environment and Natural Resources

 

 

Detail of methods

Gridded climate data

Precipitation and evaporation

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). The precipitation at each waterbody (e.g., storages and rivers) was estimated from the proportionally weighted average of grid cells that intersected each water feature. The volume was then estimated by multiplying the surface area of each waterbody by the weighted average precipitation.

For rivers, the daily dynamic surface area was calculated by multiplying the length of the river section between two gauging stations by the average river width. The average river width was based on the measured flow at the two stations and the stations’ flow-width relationship, which was developed for each gauging station using cross-sectional data (Dutta et al. 2015). Where the cross-section data were not available, an approximate estimate of river width was made using nearby stations. For Copperfield Dam, the only storage in the region, no daily storage level data were available so a static surface area value of the dam from the Australian Hydrological Geospatial Fabric was used.

Evaporation from water bodies (e.g. storages and rivers) was calculated on a daily basis using the Morton's shallow lake formulation (Morton 1983a, 1983b, 1986). For annual evaporation estimate, there is no difference between shallow and deep lake evaporation (Sacks et al. 1994). The climate data required for the Morton's method are maximum temperature, minimum temperature, vapour pressure and solar radiation. The climate data for each waterbody were estimated from the proportionally weighted average of grid cells that intersected each water feature and input to the Morton's program to obtain the evaporation values. The volume was then estimated by multiplying the surface area of each waterbody by the evaporation values.

The limitations associated with this approach are:

  • The dynamic storage surface areas calculated from the levels and capacity tables represent a monthly average and therefore will not capture changes that occur on a shorter temporal scale.
  • River widths are estimated at the upstream and downstream gauging stations and the average width is assumed to be representative for the entire reach.

 

AWRA model

The AWRA-R is a node-link river network model developed with explicit representation of key hydrological processes and anthropogenic water uses. It is used to quantify various river fluxes and stores at a daily temporal resolution along the river network. The river network is conceptualised as nodes at stream gauging stations connected by river reaches. Model parameters are estimated for each river reach, optimised to an objective function combining bias with the daily Nash-Sutcliffe coefficient of efficiency using a shuffled complex evolution algorithm (Duan et al. 1992).

 

Flood return and overbank flow

The volume of overbank flow from the river onto the floodplain, and the return flow from the floodplain back into the river are estimated from AWRA-R model outputs.

The 'overbank threshold' and the 'return flow rate' are calibrated parameters in the AWRA-R model. The annual volume of overbank flow in the region is estimated as the:

  • excess flow above the 'overbank threshold' if the excess flow is less than 1 m3/s, or
  • square root of the excess flow above the 'overbank threshold' if the excess flow is greater than 1 m3/s.

The annual volume of flood return in the region is estimated by multiplying the 'return flow rate' by the volume of water in the floodplain.

Detailed information on the model calibrations are provided by Dutta et al. (2015). The estimates of both flood return and overbank flow have high uncertainty as they were not validated due to a lack of available data.

 

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 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.

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 12,495,000 ML, which is 1.15 times that reported in the Statement of Water Flows (10,886,001 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. Therefore, no adjustment is made for the contributing area below that station. Instead, it is considered that the reported outflow to sea may be underestimated by up to 5–10%.

Quality codes are assigned to flow data in accordance with Table N4, as given in the Bureau of Meteorology's Water Data Online.

 

Table N4 Quality codes
Quality codeDescription
AThe record set is the best available given the technologies, techniques and monitoring objectives at the time of classification
BThe record set is compromised in its ability to truly represent the parameter
CThe record set is an estimate
EThe record set's ability to truly represent the monitored parameter is not known
FThe 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 E, which indicates the lowest quality of data recorded at Mount Nancar (Station G8140040) during the 2016–17 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 rights, entitlements, allocations and restrictions 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 N1 Map of groundwater model area relative to the Daly region boundary
Figure N1 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

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%.