Approaches for the assessment of
surface water – groundwater
interaction
(after Winter et al., 1998)

FINAL

January 2012
This project is funded by the Australian Government through
the Water for the Future - Water Smart Australia program
Approaches for the assessment of
surface water – groundwater interaction

FINAL

January 2012
Executive Summary
In most instances throughout Australia connected river and aquifer systems are managed as
independent resources and hence, surface water and groundwater are accounted for independently
(i.e. double accounting). This approach often results in the same parcel of water being allocated to
different users (i.e. double allocation). As water resources become over-allocated and streamflow
and groundwater levels decline, the financial (compensatory), legal and environmental implications
of double allocation are beginning to be realised, as is the necessity for integrated management.
To aid in integrated management, the magnitude of double accounting of the water resources must
first be understood. To this end, the Australian Government, through the Water for the Future –
Water Smart Australia program, funded Sinclair Knight Merz (SKM) and the Commonwealth
Scientific and Industrial Research Organisation (CSIRO) to develop a practical and moderately
priced methodology for assessing the range of levels of connection between groundwater and river
systems, both in a spatial and temporal context. The approach was developed on the basis of
existing investigations and refined and tested by conducting field trials in ten representative
catchments. Specific objectives of the project are outlined below:
1)
Develop methods for quantifying the degree of connection between river and groundwater
systems, in both a spatial and temporal context.
2)
Install the necessary monitoring infrastructure and demonstrate the application of these
methods at ten representative catchments in eastern Australia;
3)
Provide estimates of the level of connection between groundwater and surface water in the ten
trial catchments, and the likely level of double-accounting of water resources;
4)
Develop quantitative approaches for assessing surface water – groundwater interaction in
catchments with a range of data availability with consideration of the value of water resources;
and
5)
Communicate the project outcomes to local, State and Australian Government decisionmakers to enable integrated water resource management.
The purpose of the project is to compare methods, and as such, the data and subsequent results for
baseflow estimates in each catchment do not represent agreed absolute estimates of surface water –
groundwater flux volumes. Results are not intended for use as inputs to any rules or regulations
governing water resource extraction in specific catchments. Rather, the results and conclusions of
this study are intended solely as a comparison of scientific methods for measuring surface water –
groundwater exchange.
PAGE 1
Approaches for the Assessment of Surface Water - Groundwater Interaction
In order for a method for the quantitative assessment of surface water – groundwater interaction to
be routinely used by catchment managers over a range of Australian catchments, the method must
be accurate and require data that is readily available. All of the methods discussed below are
considered to be effective in estimating the groundwater discharge component of river flow. The
difference between them is the accuracy of the result and the cost which is required to conduct the
assessment. A method of baseflow separation using river salinity data, referred to as the Tracer
method, is considered the most accurate of low cost methods based on field trials conducted in the
ten catchments and as such, has been used as the means of comparing the accuracy of other
methods.
Before any assessment of surface water – groundwater interaction is made it is considered
important to determine the value of the groundwater and surface water resources in a catchment.
Depending on this value, the appropriate level of assessment can be selected and justified. An
approach for assessing the value of these systems has been outlined based on the commodity value
(i.e. gross value added per megalitre of water used) and the environmental service value to the
community. A catchment can then be categorised as having low, moderate or high value
groundwater and surface water resources.
For low value groundwater and surface water systems, a groundwater balance method may be
applied. This method will first involve developing a conceptual understanding of the
hydrogeological processes in a catchment and then estimate the recharge and discharge components
of the groundwater system to enable the baseflow component to be quantified. The information
required includes all available contextual information, geological, landuse, soil and vegetation
maps where available and rainfall data. The accuracy of the estimate is constrained by the data, the
hydrogeological and conceptual understanding of each catchment and the experience of the
hydrogeologist or water resource practitioner conducting the assessment. Based on the ten
catchments studied in this project, the accuracy of this approach averaged approximately 210%.
For catchments with moderate value water resources, baseflow separations using the Tracer method
and Lyne and Hollick Filter method are recommended. These methods are based on a continuous
historical record of river flow and salinity data at a river gauge. The Tracer method requires the
selection of representative salinity values for two end members, surface runoff and regional
groundwater which are considered the two simplest components of river flow. Using continuous
river flow and salinity data (which is available in many catchments), the proportion of groundwater
in river flow can be calculated at any point in time. The end member selections would be based on
the lowest river salinity (for the runoff end member) and the mean of groundwater bore salinity
where available (for the regional groundwater end member). If groundwater data is unavailable, the
maximum river salinity can be used as an alternative, although this is expected to result in
overestimates of groundwater discharge.
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Approaches for the Assessment of Surface Water - Groundwater Interaction
The Lyne and Hollick Filter method uses continuous river flow data and essentially separates the
rapid river responses (usually associated with surface runoff) from the more delayed responses
(usually associated with groundwater processes). It should be noted that the delayed response is a
combination of many processes, of which groundwater discharge is the most significant. Hence the
baseflow estimates from this method must be considered as a conservative estimate and in practice,
almost always overestimate baseflow compared to the more accurate Tracer method. In addition to
the river gauge data, the assessment will include the development of the hydrogeological
conceptual understanding through a brief literature review. This will ensure that the baseflow
estimates are plausible within the hydrogeological setting of each catchment.
It is important to note that the continuous river salinity data that has recently been collected to
address salinity issues in many catchments across Australia, now represents a highly valuable data
set for estimating groundwater discharge to rivers. This data collection should be continued such
that it can be assessed in greater depth, including changes over time and potential impacts of
climatic changes or groundwater pumping impacts.
Catchments with high value water resources, should implement a number of methods to assess
surface water – groundwater interaction at a range of temporal and spatial scales. The confidence
placed in the accuracy of these results will be significantly higher than more limited investigations
due to the more comprehensive characterisation of the hydrogeological processes occurring. The
cost of the assessment where long historical records and previous studies are available is moderate,
and will enable specific management and/or scientific questions to be answered. Recommended
approaches would include at a minimum, baseflow separations using the Tracer and Lyne and
Hollick Filter methods complimented with run of river sampling methods.
Where the data availability of a catchment limits the methodology that should be implemented,
additional data will be required. For example in a catchment with moderate value water resources
and little data availability, more data would be required to use the Tracer method for baseflow
separation. This may require additional groundwater bores to be sampled for salinity (or monitoring
bores to be drilled) or the installation of a salinity logger at existing river gauges. If a high value
water resource catchment has only moderate data availability, a larger investment in monitoring
infrastructure and instrumentation may be required.
Catchment planners have the challenge of determining how to allocate their resources based on the
current and potential future management issues that may exist. It is expected that this report will
enable decisions regarding surface water – groundwater interaction assessments to be made in a
clear manner, in terms of the expected accuracy of the method and associated cost. In most cases
there will be specific issues that need to be addressed and the type of assessment required should be
considered within this accuracy vs cost context.
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Approaches for the Assessment of Surface Water - Groundwater Interaction
From a water resource management perspective, there are many catchments which require closer
attention in terms of the processes and understanding of surface water – groundwater interaction
and the issues of double accounting and double allocation during periods of water scarcity and in
the face of impacts from climate change. In order to manage the surface water and groundwater
resources in an integrated and informed manner the relationships must be better understood and
communicated to all water users.
It is thought that any of the three levels of assessment presented within this report will enable at the
very least, a basis for the successful understanding and ongoing management of what should be
considered a single resource in most cases. Further work, data collection and analysis should be
justified by specific management issues or objectives which are becoming more and more prevalent
across the country.
PAGE 4
Approaches for the Assessment of Surface Water - Groundwater Interaction
Contents
1.
2.
Introduction
6
1.1
1.2
Purpose of this Report
Larger Project Context and Objectives
6
6
Value of Water Resources and Data Availability
8
2.1
2.2
2.3
3.
4.
5.
6.
Value of Water Resources
Data Availability
Catchment Classification – Value and Data Availability
8
9
10
Groundwater Balance Method
13
3.1
3.2
3.3
3.4
Approach
Results
Recommendations
Cost and Accuracy
13
14
21
21
Baseflow Separation Methods
23
4.1
Approach
23
4.1.1
4.1.2
Tracer method using river salinity data
Lyne and Hollick Filter method using river flow data
23
26
4.2
4.3
4.4
Results
Recommendations
Cost and Accuracy
27
29
29
Baseflow Separation and Run of River Methods
31
5.1
5.2
5.3
5.4
31
32
33
33
Approach
Results
Recommendations
Cost and Accuracy
Discussion
34
6.1
Cost vs Accuracy
34
6.2
Scale of Investigation
36
6.3
Effectiveness of the methods
36
6.4
Approaches for catchments with a range of water resource value and
data availability
38
7.
Conclusions and Recommendations
40
7.1
7.2
40
41
Conclusions
Recommendations
Bibliography
42
PAGE 5
1.
Introduction
1.1
Purpose of this Report
The intention of this report is to build upon the findings of the 10 trial catchment investigations and
the related summary report and put them in context with respect to the following:
1)
2)
3)
4)
5)
The availability of data;
The value of water resources;
Effectiveness of methods;
Scale of investigation; and
Cost versus accuracy.
This discussion is to develop approaches for the quantitative assessment of surface water –
groundwater connectivity with consideration of the value of water resources. These approaches are
to enable water managers to consider which methods of assessment may be possible for catchments
with a range of data availability.
1.2
Larger Project Context and Objectives
In most instances throughout Australia connected river and aquifer systems are managed as
independent resources and hence, surface water and groundwater are accounted for independently
(i.e. double accounting). This approach often results in the same parcel of water being allocated to
different users (i.e. double allocation). As water resources become over-allocated and streamflow
and groundwater levels decline, the financial (compensatory), legal and environmental implications
of double allocation are beginning to be realised, as is the necessity for integrated management.
To aid in integrated management, the magnitude of double accounting of the water resources must
first be understood. To this end, the Australian Government, through the Water for the Future –
Water Smart Australia program, funded Sinclair Knight Merz (SKM) and the Commonwealth
Scientific and Industrial Research Organisation (CSIRO) to develop a practical and moderately
priced methodology for assessing the range of levels of connection between groundwater and river
systems (i.e. double accounting), both in a spatial and temporal context. The approach was
developed on the basis of existing investigations and refined and tested by conducting field trials in
ten representative catchments (Figure 1). The field results, in turn, have been extrapolated to
provide estimates of the range of levels of connection (both in the short, medium and long-term) in
the river systems investigated, and the likely level of double accounting of water resources. The
tools developed, together with an indication of the range of levels of double accounting of water
resources, will be an integral component of future integrated water resource management within
Australia.
PAGE 6
Approaches for the Assessment of Surface Water - Groundwater Interaction
Specifically, the five objectives of this project are to:
1)
2)
3)
4)
5)
Develop methods for quantifying the degree of connection between river and groundwater
systems, in both a spatial and temporal context.
Install the necessary monitoring infrastructure and demonstrate the application of these
methods at ten representative catchments in eastern Australia;
Provide estimates of the level of connection between groundwater and surface water in the ten
trial catchments, and the likely level of double-accounting of water resources;
Develop quantitative approaches for assessing surface water – groundwater interaction in
catchments with a range of data availability with consideration of the value of water resources;
and
Communicate the project outcomes to local, State and Australian Government decisionmakers to enable integrated water resource management.
Since the purpose of the project is to compare methods, the data and baseflow estimates do not
represent agreed absolute estimates of surface water – groundwater flux volumes. As such they are
not intended for use as inputs to any rules or regulations governing water resource extraction in
specific catchments. Rather, the results and conclusions of this report are intended solely as a
comparison of scientific methods for measuring surface water – groundwater exchange.
Figure 1 Locations of ten catchments in which methods for assessing surface water –
groundwater interaction are being trialled.

PAGE 7
Approaches for the Assessment of Surface Water - Groundwater Interaction
2.
Value of Water Resources and Data
Availability
2.1
Value of Water Resources
The value of groundwater and surface water resources varies widely across the country depending
on many factors including the use of the water, the ease by which it is obtained, its quality and
quantity. A dollar value for water is generally difficult to quantify due to the complexity and
variability of the water cycle, water use practices and the qualitative value we as Australians place
on water and the environment. In the context of this study the value of groundwater and surface
water resources is assessed based on two factors: environmental service value and the commodity
value.
The commodity value is a measure of the gross value added as a result of water use ($/ML). This
depends on the crops irrigated and the use of the water resources and varies widely across the
country. For example, irrigating low value grass pasture provides a value of water of somewhere
near $50/ML, lucerne near $100/ML, while higher value crops such as viticulture can be up to
$2,500/ML.
Meanwhile, the cost of supplying water varies from say $5/ML to several hundreds of dollars per
megalitre depending on costs of pumping, transporting, or building infrastructure to facilitate this
(i.e. pipes, channels, drilling groundwater bores). The supply cost for urban use has increased
roughly tenfold over the past decade and much more expensive alternatives are being implemented
in many cases (i.e. desalination), rather than having a sole reliance on more traditional water
resources.
The increasing demand on water resources has resulted in the development of a water trading
market. This is generally considered to be a young market with regulation and control measures
being underdeveloped (with some exceptions). Recent water trades have ranged widely in volume
and cost, with trades occurring from as little as $50/ML to as much as $22,000/ML depending on
the demand. There is also a difference between trading of allocations and trading entitlements. In
periods of low allocation the trade price for allocations is often more variable while the trade price
for entitlements tends to be more consistent. The growers of higher value crops are likely to be able
to pay more for water trades which can cause the price of water to be pushed up, particularly in
times of water scarcity. The impacts of historical and current water use practices, population
growth, climate change, environmental and cultural changes are all thought to contribute to the
increasing value of water resources.
The importance of environmental flows for ecology and the impacts of historical and current water
use practices on the environment are becoming more and more critical. Buyback schemes are being
developed in many regions (namely the Murray-Darling Basin) such that environmental flows can
PAGE 8
Approaches for the Assessment of Surface Water - Groundwater Interaction
be secured in an attempt to preserve the ecological and environmental values of the waterways.
This is a complex issue since the dollar value of a wetland system or stand of river red gums for
example is difficult to define. Because of this, studies have tended to develop methods attempting
to place a value on the environmental characteristics related to the water resource. Morrison and
Bennett (2004) for example, used choice modelling surveys to value the improved river health of
several rivers in NSW. Similar methods have been applied in the Thomson and Macalister
environmental flows study (Branson et al., 2005 and the Stringybark Creek environmental flows
assessment (URS, 2006). However, wider applications are difficult and the inherent value of
existing environmental assets remains difficult to quantify.
This quantitative assessment of the environmental value of water is particularly difficult since it
can change with climatic events/ trends and ecological changes and perhaps more variably, with the
value placed on it by the community. Additionally, understanding (let alone predicting) the
environmental response to a given flow is extremely difficult and is the focus of many research
studies, mostly in relation to floodplains of the Murray-Darling Basin and other significant rivers in
south eastern Australia.
It is somewhat easier to assess the value of water for the environment qualitatively, in terms of
environmental service value. This value is a measure of how important the groundwater and
surface water systems and ecosystems related to those systems are for providing benefits to people.
The environmental service value can be described in terms of three primary classifications:

Provisioning services – products obtained using the ecosystems related to the groundwater and
surface water resources or the resources themselves;

Regulating services – degree to which the ecosystems related to the groundwater and surface
water resources or the resources themselves control the quality, volume and nature of river
flow, local climate, pollution, disease and pollination; and

Cultural services – the value associated with the ecosystems related to the groundwater and
surface water resources or the resources themselves for cultural heritage, religious/spiritual,
recreation, ecotourism, educational or aesthetic values.
2.2
Data Availability
Within Australian catchments there is a large variation in the amount of available data for use in
assessing groundwater-surface water interactions. The amounts of data available are usually closely
related to the level of surface water and groundwater resource development but often the data
availability lags behind the value of the resources. The effective application of different methods
for assessing groundwater – surface water interaction are dependent on the level of data availability
and the accuracy of the results will vary accordingly.
PAGE 9
Approaches for the Assessment of Surface Water - Groundwater Interaction
Within the context of this study, catchments with Little, Moderate or Abundant data are classified
as follows:



Little – catchments with no river gauges or a single river gauge with limited data and little or
no groundwater bore data. The only available resources would include broad scale geological,
landuse, vegetation and soil type maps and rainfall data, with limited literature on the
catchment or region;
Moderate – catchments with at least one river gauge which has continuous river flow and
salinity data and a limited number of groundwater bores will have associated groundwater
salinity data. There would be a small number of studies and literature related to the catchment
but few detailed investigations would have been conducted; and
Abundant – will contain multiple river gauges with good current and historical flow and
salinity data, historical records of groundwater levels and chemistry would be captured from a
moderately extensive groundwater monitoring network. These catchments will be developed
and relatively well studied, mostly in terms of specific study areas, and may have numerical
groundwater models and/or surface water models based on the existing data.
2.3
Catchment Classification – Value and Data Availability
In order to justify surface water – groundwater interaction studies, the cost of such studies should
be put within the context of the value of the water resources being investigated. A simple example
can be used to demonstrate the relative economics of such a comparison. Let us consider a
1,000 ML/year water entitlement to be used for irrigation of a high value crop, say some sort of
horticulture. If the gross value added as a result of growing the crop is $2,000/ML, then this results
in a gross margin of $2,000,000/year. The question then is, how important is it to understand where
this water comes from and the dynamics of how the surface water and groundwater resources
interact? Is a study costing $200,000 justified (i.e. 10% of the gross margin of a single entitlement),
such that greater confidence and accuracy can be placed on a quantitative assessment of these
resources?
The classification of catchments in this report is based on both the value of the water resource
(gross value of water used) and environmental service value into low, medium or high value water
resources. The data availability of each catchment limits the approach that can be taken and it may
be that further data should be collected to undertake a higher level of assessment. The water
resource value and data availability are combined in Figure 2, allowing the recommended approach
to be determined.
To illustrate the logic behind one possible scenario a catchment with moderate value water
resources and little data availability is considered. The recommended level of assessment for a
PAGE 10
Approaches for the Assessment of Surface Water - Groundwater Interaction
resource of moderate value would be to use the Tracer method to quantify the component of
groundwater discharge, but with little data this would not be possible. Hence, additional data would
be needed such that this level of assessment could be completed and result in a baseflow estimation
of higher accuracy. This would most likely require, at some cost, the addition of a salinity logger at
a river gauge and the collection of groundwater salinity data. Other recommended approaches can
be determined using this same logic according to the following steps:
1)
the value of the surface water and groundwater resources determines the approach and hence
accuracy of the result; and
2)
the current data availability determines the additional investment required to collect the
appropriate data.
PAGE 11

Figure 2 Approach Selection Flow Diagram
PAGE 12
3.
Groundwater Balance Method
3.1
Approach
This method should be applied to catchments where the water resources are of low value and can
be applied in catchments of all data availability. In practice these catchments are likely to have
little data (i.e. little or no streamflow or bore data) and will often exist in remote parts of Australia
or areas where there is little development. An important question here is whether it is appropriate to
make an assessment of surface water – groundwater interaction using existing data, or whether the
value of the groundwater resource justifies additional data collection to provide a more accurate
result. If an assessment is to be made using existing data, then the resource managers must be
aware that the accuracy of the resulting estimates will not be high. In the following, we assume that
a decision has been made to carry out an assessment using existing data in a data poor catchment.
The aim of the approach is to estimate groundwater discharge to a river as a component of the
groundwater balance in a catchment. This will be based largely on the conceptual understanding of
the hydrogeological processes occurring in the catchment, with the aid of geological,
hydrogeological, landuse and soils maps where available, satellite images and rainfall data.
In any simple water balance, it is assumed that the system is in steady state. This means that the
inputs to the system are equal to the outputs. In the case of a groundwater balance, the recharge
components are equal to the discharge components. Hence the sum of rainfall recharge is equal to
the groundwater discharge (including throughflow, groundwater discharge to rivers (baseflow),
groundwater evapotranspiration and groundwater pumping). Hence the baseflow can be estimated
based on the following equation:
Baseflow = Rainfall Recharge – Groundwater Pumping (where appropriate) – Groundwater
Evapotranspiration (where appropriate ) – Throughflow (where appropriate)
To simplify this formula as a first pass assessment, we can assume that the catchment is closed,
meaning that groundwater throughflow into and out of the catchment is essentially zero (hence the
throughflow term can be removed). Groundwater evapotranspiration is difficult to quantify and for
the purposes of a first pass assessment, it will initially be removed (see the below discussion for it’s
inclusion). Groundwater pumping can also be set to zero (or applied if this is known, as discussed
below), with the resultant groundwater balance consisting of two terms, baseflow and recharge. A
simple method of estimating recharge, is to multiply the rainfall by the catchment area and then
assume that some fraction of that volume recharges the groundwater system (the recharge fraction).
The recharge occurring over cleared and uncleared land is usually very different and hence the
recharge fraction applied for each type of land can be adjusted accordingly (as identified by using
landuse maps or other literature). Hence the baseflow can be estimated using the following
equation:
PAGE 13
Approaches for the Assessment of Surface Water - Groundwater Interaction
Baseflow = (Rainfall X Catchment Area X Recharge fraction X % Cleared Land) + (Rainfall X
Catchment Area X Recharge fraction X % Uncleared Land)
The following discussion uses the ten field trial catchments as a comparison to the first pass and
subsequent groundwater mass balance approaches. It should be noted that the accuracy of this
method, is dependent on the knowledge of the catchment, relevant literature and the available data.
The approach is completely desktop based and does not require any field investigations or
infrastructure.
3.2
Results
The water balance approach was applied to the ten trial catchment investigations. Initially recharge
fractions of 0.05 (cleared land) and 0.01 (uncleared land) were applied. This is equivalent to 5%
recharge of rainfall on cleared land and 1% on uncleared land. The volume of streamflow depletion
from groundwater pumping has also been included within the groundwater balance since it has
been assessed in the previous work for each catchment. Alternatively, the raw groundwater
pumping estimates could be used, or even a rough estimate based on bore use information (i.e.
stock and domestic, irrigation, urban supply etc).
Table 1 and Figure 3 present a comparison of the annual baseflow volumes calculated from the
water balance, with the annual baseflow volumes determined from the tracer based hydrograph
separation method. The match between the two methods is poor to very poor in some catchments.

Table 1 Baseflow estimates (using consistent recharge fractions)
Water
Water
Tracer
Catchment Streamflow
Balance
Balance
method
Groundwater
Area
Depletion Evapotranspiration Recharge Baseflow Baseflow
%
Catchment
(km2)
(ML)
(ML)
(ML)
(ML)
(ML)
Error
Tarcutta
1660
104
-
44481
44377
8593
416
Logan
1262
553
-
53837
53284
7131
647
Cockburn
1130
1716
-
37358
35642
3814
834
Nambucca
431
2959
-
11172
8213
19693
-58
Cattle
326
-
14083
14083
53906
-74
Elliott
251
3350
-
11196
7846
2681
193
Barron
228
782
-
14025
13243
21753
-39
Ourimbah
83
390
-
1952
1562
5773
-73
PAGE 14
Approaches for the Assessment of Surface Water - Groundwater Interaction
60000
Water Balance Baseflow (ML)
Baseflow (ML/year)
50000
Tracer Method Baseflow (ML)
40000
30000
20000
10000

Ourimbah
Barron
Elliott River
Cattle
Nambucca
Cockburn
Logan
Tarcutta
0
Figure 3 Baseflow estimates (using identical recharge fractions)
Table 1 presents the groundwater balance baseflow estimates with consistent recharge fractions
applied to all catchments. In reality however, the recharge rates are likely to vary based on the
predominant soil type in the catchments. Since soil maps exist for these catchments, the recharge
fractions can be adjusted accordingly. This is a subjective process and dependent on the experience
of the hydrogeologist or water resource professional conducting the water balance. Table 2 lists the
predominant soil within the ten study catchments.
PAGE 15
Approaches for the Assessment of Surface Water - Groundwater Interaction

Table 2 predominant soil type in the study catchments
Catchment
Major Soil
Type
Tarcutta
Loam
Cattle
Loam
Cockburn
Loam
Elliott
Sandy loam
Ourimbah
Loam
Nambucca
Loam
Logan
Clayey loam
Barron
Clayey loam
Loam is a generic soil type and is expected to have recharge consistent with the recharge rate we
have used (5% of rainfall for cleared land). The Elliot catchment has a more sandy soil type and is
expected to have a higher recharge rates and hence higher estimates of baseflow. The Logan and
Barron catchments have more clay rich soils and will be expected to have lower recharge relative to
the other catchments. Based on the soil type the recharge fraction applied to the Elliot catchment is
0.07 for cleared land and 0.02 for uncleared land. This is equivalent to 7% recharge for cleared land
and 2% of recharge for uncleared land. The Barron and Logan catchments have had the recharge
fraction changed to 0.03 for cleared land and 0.005 for uncleared land. The revised groundwater
balance baseflow estimates accounting for soil type, are presented in Table 3and Figure 4.

Table 3 Baseflow estimates (accounting for soil type)
Catchment
Area
(km2)
Streamflow
Depletion
(ML)
Groundwater
Evapotranspiration
(ML)
Water
Balance
Recharge
(ML)
Water
Balance
Baseflow
(ML)
Tracer
method
Baseflow
(ML)
%
Error
Tarcutta
1660
104
-
44481
44377
8593
416
Logan
1262
553
-
31405
30852
7131
333
Cockburn
1130
1716
-
37358
35642
3814
834
Nambucca
431
2959
-
11172
8213
19693
-58
Cattle
Elliott
River
326
-
14083
14083
53906
-74
251
3350
-
16793
13443
2681
401
Barron
228
782
-
8181
7399
21753
-66
Ourimbah
83
390
-
1952
1562
5773
-73
Catchment
PAGE 16
Approaches for the Assessment of Surface Water - Groundwater Interaction
60000
Water Balance Baseflow (ML)
Baseflow (ML/year)
50000
Tracer Method Baseflow (ML)
40000
30000
20000
10000

Ourimbah
Barron
Elliott River
Cattle
Nambucca
Cockburn
Logan
Tarcutta
0
Figure 4 Baseflow Estimates (accounting for soil type)
The results remain quite poor, with large overestimates in some catchments and large
underestimates in others. At this stage groundwater evapotranspiration has not been included. It is
well established however, that where shallow groundwater tables exist the unsaturated zone above
the watertable can be directly evaporated or used by vegetation. Typically, the rate of groundwater
evaporation is difficult to quantify since it is dependent on depth to watertable, soil type, vegetation
type, water availability and water stress and a number of other factors. Without specific knowledge
of groundwater evapotranspiration rates from specific vegetation types, an approximation must be
made. If for example it is assumed that groundwater evapotranspiration occurs in the 50 m area on
either side of a river (i.e. where the watertable is likely to be close to the surface) and a
conservative rate of 1 mm/year applied along the river’s length, this component can be
incorporated easily. This has been done for all catchments as an additional groundwater discharge
mechanism. The influence of estimating groundwater evapotranspiration can be seen in Table 4 and
Figure 5.
It is thought that higher rates of groundwater evapotranspiration occur within the Elliot catchment
compared to the other catchments. The Elliot catchment is very flat and has significant areas where
PAGE 17
Approaches for the Assessment of Surface Water - Groundwater Interaction
the water table is less than 5 m below the surface. There is also a large area of forestry (pine tree
plantations) within the catchment. The pine plantations cover approximately 12 km2 of the Elliot
catchment. If we assume a groundwater evapotranspiration rate of 780 mm/yr from the pine
plantations (SKM, 2010) there is a 9360 ML/yr reduction in baseflow due to groundwater
evapotranspiration. The baseflow estimate after ET has been accounted for is 4083 ML/yr which
compares much better to the Tracer estimate of baseflow of 2681 ML/yr.
Table 4 Baseflow estimates (accounting for soil type and groundwater
evapotranspiration)

Catchment
Area
(km2)
Streamflow
Depletion
(ML)
Groundwater
Evapotranspiration
(ML)
Water
Balance
Recharge
(ML)
Water
Balance
Baseflow
(ML)
Tracer
method
Baseflow
(ML)
%
Error
Tarcutta
1660
104
6059
44481
38318
8593
346
Logan
1262
553
4606
31405
26246
7131
268
Cockburn
1130
1716
4125
37358
31517
3814
726
Nambucca
431
2959
1573
11172
6639
19693
-66
Cattle
Elliott
River
326
1190
14083
12893
53906
-76
251
3350
9360
16793
4083
2681
52
Barron
228
782
832
8181
6567
21753
-70
Ourimbah
83
390
303
1952
1259
5773
-78
Catchment
PAGE 18
Approaches for the Assessment of Surface Water - Groundwater Interaction
60000
Water Balance Baseflow (ML)
Baseflow (ML/year)
50000
Tracer Method Baseflow (ML)
40000
30000
20000
10000
Ourimbah
Barron
Elliott River
Cattle
Nambucca
Cockburn
Logan
Tarcutta
0
Figure 5 Baseflow estimates (accounting for soil type and groundwater
evapotranspiration)

At this stage, there are large errors in the groundwater balance baseflow estimates ranging from -78
to 726% of the Tracer method estimates. The average error is 210% and it appears that the water
balance method underestimates baseflow in smaller catchments (Cattle, Ourimbah, Nambucca and
Barron) with the exception of the Elliot catchment, and overestimates baseflow in the larger
catchments (Tarcutta, Logan and Cockburn). Additionally, the smaller catchments are all coastal
catchments (with the exception of the Logan) while the larger catchments are on the eastern side of
the dividing range. This suggests that either the recharge fractions are too high or too low for large
and small catchments respectively, or that the systems are not closed and throughflow is significant
component of the groundwater balance (or perhaps a combination of the two).
It is possible that there is a difference in the nature of rainfall (i.e. frequency, intensity, duration)
and antecedent soil moisture conditions between inland and coastal catchments. It is expected for
example, that early rain after the summer in the inland catchments would not effectively recharge
the groundwater system due to the dry soil moisture conditions and perhaps higher intensity
PAGE 19
Approaches for the Assessment of Surface Water - Groundwater Interaction
rainfall. There would be some rainfall threshold value which would need to be met before effective
recharge could occur, and hence applying a generic recharge fraction to all rainfall may not be
appropriate. This would account for the over estimates of baseflow in the Tarcutta, Cockburn and
perhaps the Logan due to it’s size. The smaller coastal catchments however, might be expected to
experience more consistent rainfall (and hence wetter soil moisture conditions) which may allow
rainfall to more effectively recharge the groundwater than if the antecedent conditions were dry.
Thus the generic recharge factors used may be too small and hence underestimate baseflow.
The pattern of over estimated baseflow in larger catchments and underestimated baseflow in
smaller catchments may be alternatively explained due to the assumption that the catchments are
‘closed’. The water balance assumes that the surface catchment boundary is the same area as the
groundwater catchment boundary and this may not be the case. In smaller catchments, it is
plausible that groundwater throughflow from outside of the catchment (i.e. regional groundwater
flow) contributes to the baseflow seen in rivers. The water balance based on the surface water
catchment would not include this additional input to the system and hence underestimate baseflow.
In larger catchments it appears that not all of the groundwater that is recharged within the
catchment is entering the stream as baseflow. The catchments are more likely to be open ended,
with groundwater flow bypassing the stream gauge or becoming regional groundwater flow. In the
larger catchments (Logan, Tarcutta and Cockburn) the water balance overestimated baseflow by
between 1.7-3.1% of the total rainfall volume within the catchment (Figure 6). Assuming recharge
fractions are reasonable, this suggests that 2-3% of rainfall recharge is escaping from the catchment
as regional flow (throughflow) rather than discharging to the river.
PAGE 20
Approaches for the Assessment of Surface Water - Groundwater Interaction
4
0
-2
-4
-6
Ourimbah
Barron
Elliott River
Cattle
Nambucca
Cockburn
Logan
-8
Tarcutta
Baseflow Estimate Difference as a % of Rainfall
2
Figure 6 Error in groundwater balance baseflow estimates expressed as a percentage of
catchment rainfall volume.

3.3
Recommendations
For catchments where the value of the water resources is low and there is a limited amount of data,
a groundwater mass balance approach is an appropriate method for estimating the groundwater
contribution to river flow. The accuracy of the method is dependent on the hydrogeological
information available for the catchment and the knowledge and experience of the hydrogeologist or
water scientist conducting the assessment. It is critically important that the results of a groundwater
balance are rationalised, since they may produce values that are not realistic for the climate /
vegetation / nature of the river or catchment. If a catchment is of moderate or high value it is
recommended that more data be collected such that a more accurate method can be applied.
3.4
Cost and Accuracy
Since this approach is purely desktop based and does not involve any complicated data analysis, it
can be completed relatively cheaply and within a short timeframe. The costs will involve the time
required to gather literature and other knowledge, geology, landuse, soil and vegetation maps and
PAGE 21
Approaches for the Assessment of Surface Water - Groundwater Interaction
rainfall or any other hydrogeological data that may be available. This information would then be
assessed and a simple spreadsheet based groundwater mass balance developed. It is estimated that
each groundwater balance could be completed in one week per catchment, including time for the
above and review of the groundwater balance, reporting and additional review. The resultant cost
would amount to approximately less than $10,000. The accuracy of the result is likely to be
somewhere within the variation presented in Table 4 and Figure 5 and is considered to be poor with
an average accuracy error of 210%.
PAGE 22
Approaches for the Assessment of Surface Water - Groundwater Interaction
4.
Baseflow Separation Methods
4.1
Approach
Baseflow separations using the Tracer method and complimented by the Lyne and Hollick Filter
method are recommended for catchments considered to contain water resources of moderate value.
If these catchments have moderate data availability (i.e. at least one stream gauge with continuous
flow and salinity data, and some groundwater bore data) then the assessment can be completed
using existing data. If the catchment is of low data availability then additional data would be
required (i.e. instrumentation of salinity loggers at river gauges and the collection of groundwater
salinity data). It is expected that there are a significant number of Australian catchments that fall
into this category, that is, containing moderate value water resources and limited or moderate data
availability.
It is suggested that the approach taken for catchments with moderate value water resources, will
firstly apply the Tracer method where continuous river salinity data exists in combination with flow
data. This would be supported by the Lyne and Hollick Filter method which can in most cases, be
applied to a longer period of record (since flow data often has a longer historical record than the
river salinity record, which may also be unavailable for many gauges). These approaches, their
assumptions and a comparison between methods are discussed below.
4.1.1
Tracer method using river salinity data
Of the methods tested in this study to assess groundwater - surface water interactions, the tracer
based hydrograph separation method (Tracer method) was considered the most accurate, provided
that the end members used could be adequately defined. The Tracer method produced plausible
results that matched the conceptual understanding of a variety of different catchment conditions
within the study catchments. Other methods trialled, on occasions produced results that appeared to
overestimate or underestimate groundwater inflow conditions, based on the hydrogeological
conceptual understanding of the catchments or limitations of the method.
The Tracer method assumes that the flow in the river is derived from two sources; surface runoff
and regional groundwater. The method determines the relative proportions of surface runoff and
groundwater inflow based on the salinity (i.e. groundwater tracer) of river flow over time. Because
the relative proportions of surface runoff and groundwater inflow will change significantly during
storm events and seasonally, measurements of river chemistry need to be made continuously and at
the same frequency as the flow measurements. At any point in time, the proportion of river flow
that is due to groundwater discharge is calculated using the mass balance equation:
PAGE 23
Approaches for the Assessment of Surface Water - Groundwater Interaction
Qg
Qt

c  cr
c g  cr
where c, cr and cg are the tracer concentrations in the river, in runoff and in groundwater respectively, Qt is
the measured total river flow, and Qg is the volume of groundwater inflow.
The success of the tracer hydrograph separation method ultimately relies on an adequate chemical
differentiation of the source waters, and accurate quantification of the end-member concentrations.
For this approach the groundwater end member has been selected as the mean regional
groundwater salinity and as such the baseflow contribution calculated is a reflection of the regional
groundwater discharge to the river. Within this data there is significant variability and hence this is
the primary source of error in applying the Tracer method. However, this is not unexpected since
the depth beneath the watertable, location relative to the river, geological properties/conditions and
recharge processes influencing each groundwater bore sample are also likely to be variable. Hence
the selection of a representative groundwater salinity value must be within the context of a
hydrogeological conceptual model and an understanding of recharge and discharge processes
within the catchment.
The more data there is to develop this hydrogeological understanding, the greater the confidence
that can be placed in the Tracer method. In the case of the ten trial catchments, the mean
groundwater salinity was selected for the Tracer method since there was a reasonable number of
groundwater samples in the catchments studied and the systems were considered reasonably well
understood. Where there are larger amounts of data, statistical approaches could also be taken to
increase the confidence of the representative groundwater salinity value selected. However, since
groundwater salinity will always be spatially variable, some degree of hydrogeological
interpretation will be required to make appropriate assumptions.
An alternative approach is to use the maximum river salinity as the groundwater end member
concentration. This would result in higher baseflow estimates and be representative of regional
groundwater flow in combination with bank storage and other sources. This alternative method
could be used if the groundwater salinity data was not considered representative of the regional
groundwater end member or was unavailable. A brief comparison of the two possible approaches to
selecting the groundwater end member is shown in Table 5 and Figure 7. It can be seen in Table 5
that the maximum river salinity is, on average, 72% lower than the mean groundwater salinity. Any
relationship between regional groundwater salinity and maximum river salinity would be
dependent on a multitude of factors and it is not considered appropriate to apply a single
relationship broadly, such that the acquisition of groundwater data was not necessary.
Rather than using the maximum river salinity as an alternative groundwater end member value in
catchments where there is very limited or erroneous data, it would be relatively easy to conduct a
PAGE 24
Approaches for the Assessment of Surface Water - Groundwater Interaction
sampling round of existing bores in most catchments. Collecting more groundwater data would
allow greater confidence in the end member selection, increase the accuracy of the result and
strengthen the conceptual model developed for the system. Putting effort into collecting more
groundwater data would be the preferred approach since the alternative introduces more unknowns
and potential errors.

Table 5 Groundwater end member selection
Max
Recorded
River EC
Mean
Groundwater
EC from
Bores*
River EC as
% of Mean
GW EC
GW
EC
min
GW
EC
max
GW EC
median
Number
of Bores
Belubula
800
1200
67
620
2200
1410
9
Tarcutta
751
800
94
140
1690
915
6
Logan
667
1100
61
912
2027
1470
7
Cockburn
540
910
59
295
1950
1123
19
Nambucca
122
133
92
90
446
268
8
Cattle
242
322
75
192
2259
1226
12
Elliot
484
600
81
195
1528
862
9
Barron
105
196
54
103
369
236
6
Ourimbah
220
320
69
88
4889
2489
9
Catchment
*In the Ourimbah, Cattle and Logan catchments the highest outlying salinity value has been excluded from
the mean EC calculations.
PAGE 25
Approaches for the Assessment of Surface Water - Groundwater Interaction
5000
Minimum Groundwater EC
4500
Maximum Groundwater EC
Electrical Conductivity (uS/cm)
4000
Median Groundwater EC
Mean Groundwater EC*
3500
Maximum River EC
3000
2500
2000
1500
1000
500
0
Belubula

Tarcutta
Logan
Cockburn Nambucca
Cattle
Elliot
Barron
Ourimbah
Figure 7 Estimates of catchment groundwater EC
*In the Ourimbah, Cattle and Logan catchments the highest outlying salinity value has been excluded from
the mean EC calculations.
4.1.2
Lyne and Hollick Filter method using river flow data
The Lyne and Hollick Filter method separates river flow data into a rapid component (i.e. surface
runoff) and a delayed component (of which baseflow is usually the major process) as a function of
time. The method relies on the principle that runoff events are of relatively short duration, whereas
groundwater discharge to rivers responds more slowly to rainfall recharge. The results are sensitive
to the operator-controlled parameter alpha (α) that is selected. The value of this parameter can
range from 0.90 to 0.99 but a default of 0.925 is often used as a starting point. This parameter is
subject to hydrogeological interpretation and it is often increased to a value near 0.98.
The dominant delayed component is usually groundwater discharge, while other processes and
pathways such as bank storage, interflow, delayed flow from perched groundwater or wetlands for
example, are also included. Hence the baseflow estimate determined using this method, is the
conservative maximum baseflow component and strictly speaking, should not be directly compared
to regional groundwater discharge (although often this comparison is necessary).
PAGE 26
Approaches for the Assessment of Surface Water - Groundwater Interaction
4.2
Results
Both the Tracer method and Lyne and Hollick Filter method were used to estimate baseflow to
rivers in the ten field trial catchments. Two end members were selected for the Tracer method,
representative of surface runoff (minimum river salinity) and mean groundwater salinity (based on
groundwater bore data), while filter parameters of 0.925 and 0.98 were used in all the catchments
to estimate baseflow using the Lyne and Hollick Filter method. The results from the Tracer method
were considered to be the most accurate since they were the most plausible and fit best within the
conceptual models and hydrogeological understanding developed for each catchment.
Table 6 and Figure 8 present a comparison of the Lyne and Hollick Filter method results using both
Filter parameters with the Tracer method results. It can be seen that the 0.98 Filter parameter
generally produced results closest to the Tracer method while the Filter method was an
overestimate of groundwater discharge in almost all cases. The percent error when compared to the
Tracer method was 59% using a Filter parameter of 0.98 while the percent error when using a Filter
parameter of 0.925 was higher at 92%.
70
Baseflow percentage of streamflow
Lyne and Hollick filter, alpha = 0.925
Lyne and Hollick filter, alpha = 0.98
60
Tracer method, mean groundwater salinity
50
40
30
20
10
Barron
Cockburn
Elliot
Nambucca
Logan
Cattle
Belubula
Ourimbah
Tarcutta
0
Figure 8 Study catchment baseflow estimates using the Tracer method and Lyne and
Hollick Filter method (0.98 and 0.925 Filter parameters)

It should be noted that the results shown above were from the analyses of data over a one year
study period and it is possible that a longer term analysis would yield different results. This is
PAGE 27
Approaches for the Assessment of Surface Water - Groundwater Interaction
evident in Table 6 where short term analysis appears to nearly systematically overestimate
baseflow. The reason for this is likely to be because the Lyne and Hollick Filter method commonly
overestimates baseflow during storm events. This is since it is not a process based method and
relies on the difference in flow values between successive data points (i.e. it is a signal analysis).
For example, the delayed component (i.e. baseflow) of storm events and high flow periods is
systematically overestimated by this method unless they are particularly rapid or flashy events.
Persistent high flows force the baseflow contribution higher with the Filter method, when in reality
it is expected that elevated river flows would force water to flow from the river into the aquifer.
High baseflow estimates during storm events are generally non- intuitive, particularly if large
discrepancies are seen when compared with the Tracer method results. Hence if the flow regime is
dominated by a series of unusual storm events, the average baseflow may be higher than that of a
longer term average.

Table 6 Lyne and Hollick Filter and Tracer method comparison
=0.98
Longer
term
=0.925
Longer
term
=0.98
Short
term
=0.925
Short
term
TracerBased
Separation
=0.98
Difference
(Short Longer)
=0.925
Difference
(Short Longer)
Tarcutta
36
52
32
56
34
-4
4
Ourimbah
30
36
42
46
42
12
10
Belubula
28
41
27
39
-1
-2
Cattle
25
38
49
48
27
24
10
Logan
26
34
44
50
16
18
16
Nambucca
26
31
54
53
39
28
22
Elliot
23
30
57
66
48
34
36
Cockburn
16
24
29
36
27
13
12
Barron
40
62
41
62
17
1
0
Catchment
Without further detailed analysis however, there is no basis to assume that the one year period
assessed in this study is not representative of a longer term flow regime. The long term analysis
presented in Table 6 does fit the Tracer method results better, however these analyses should not be
directly compared since they are representative of different data record periods.
Hence the overestimates made using the Lyne and Hollick Filter method can be considered as a
characteristic trait of the method which should be considered and discussed. Currently, there is no
PAGE 28
Approaches for the Assessment of Surface Water - Groundwater Interaction
way of compensating for this over estimation quantitatively. However, using the hydrogeological
knowledge and conceptual models developed for each catchment, the baseflow estimate could be
qualified by a discussion of potential other sources of delayed flows (i.e. wetlands, significant bank
storage, irrigation spill-over or recharge, controlled releases etc).
It should also be noted that there can be some problems with the application of the Tracer method
when the groundwater salinity is quite low and comparable to surface runoff ( i.e. less than 200-300
EC). It then becomes more difficult to differentiate between end members. Additionally,
catchments with low groundwater salinity usually have rapid recharge mechanisms (i.e.
groundwater is fresh because rainfall has not experienced significant evaporation before recharging
the aquifer). This means that sometimes, rapid groundwater responses may be mixed with the rapid
surface runoff responses and both the Filter and Tracer methods become less reliable. Cases have
even been seen where river salinity remains relatively stable during storm events and this is the
topic of ongoing research.
4.3
Recommendations
For catchments containing water resources of moderate value, baseflow separations using both the
Tracer and Lyne and Hollick Filter methods are recommended. The Tracer method is considered
the most accurate method despite the errors introduced by estimating end member concentrations.
The method requires continuous river flow and salinity data, and groundwater salinity data. The
method can also be applied without groundwater salinity data if the maximum river salinity is used
as the groundwater end member (although this will result in an overestimate of groundwater
discharge since bank storage and other processes will also be captured).
The Lyne and Hollick Filter method requires only river flow data and has the potential to be
applied in the most catchments across Australia. The method is considered to overestimate regional
groundwater discharge to rivers but this can be mitigated to some degree, by using a Filter
parameter of 0.98, which was found to produce more reasonable estimates of baseflow contribution
when compared to the Tracer method.
If a catchment contains water resources of high value it is recommended that more data be
collected such that additional methods can be applied to increase the accuracy of the baseflow
estimation.
4.4
Cost and Accuracy
The Tracer method is considered to have a moderate to high accuracy as long as consideration is
made of the potential error in defining the end member values. The method is relatively
inexpensive to apply if continuous river salinity and flow data exist and groundwater bore salinity
data is available (see Table 7). The Lyne and Hollick Filter method is considered to have a low to
moderate accuracy and is expected to systematically overestimate regional groundwater discharge.
PAGE 29
Approaches for the Assessment of Surface Water - Groundwater Interaction
The method is best applied in combination with the Tracer method and it has resulted in an average
error of 59% using a Filter parameter of 0.98 and 92% using 0.925 compared to the Tracer method.
The accuracy of this method is increased with the development of the hydrogeological
understanding and conceptual models for the catchment. This can be achieved by reviewing the
available literature and creating a groundwater balance and is an important step in ensuring the
results fit within a realistic context, on a catchment by catchment basis.

Table 7 Moderate data availability, indicative costing
Continuous river salinity and
flow data is available in
addition to groundwater
bore salinity
River data is unavailable and
instrumentation is required, no
groundwater salinity data is
available and observation bores
need to be drilled
Desktop Assessment
(Lyne and Hollick Filter
Method)
$5,000
$5,000
Desktop Assessment
(Tracer method)
$5,000
$5,000
Task
Purchase and install river
salinity and level loggers
$10,000
Calibrate loggers and
estimate rating curves
Drill bores (assume 10k
per bore)
TOTAL
$5,000
$50,000
$10,000
$75,000
PAGE 30
Approaches for the Assessment of Surface Water - Groundwater Interaction
5.
Baseflow Separation and Run of River
Methods
5.1
Approach
For catchments containing water resources of high value, the confidence and accuracy of methods
is critical. It is recommended that more detailed assessments of surface water – groundwater
interaction be applied such that the accuracy is very high and the understanding of the systems is
also very high. The methods suggested include baseflow separations at multiple river gauges using
the Tracer and Lyne and Hollick Filter methods with additional run of river sampling. This can be
completed by implementing field investigations in catchments with an abundance of data (i.e.
containing long historical records for multiple river gauges with salinity and flow data, a long
record of groundwater level and chemistry data and established groundwater monitoring networks).
If there is only moderate data availability, further instrumentation and monitoring would be
required in order to apply these methods.
In catchments with high value resources, it is expected that a significant diversity of
hydrogeological and water resource knowledge has been accumulated in the catchment and there
will be a vast amount of literature to draw upon. It is also likely that there have been numerical
models created to solve groundwater related problems or issues and additionally surface water
modelling would have been conducted using a range of methods and datasets. Within Australia
there are many catchments that would fit into this category, mostly located within the historically
productive areas of the country.
To make an assessment of the surface water – groundwater interaction occurring in catchments
such as these, it is firstly important to consider the existing work that has been completed. Often
this work will have been conducted on a site specific scale rather than catchment scale, and as such,
these studies will often set the context for a larger scale assessment. Since the data records are
extensive, greater care should be taken in data processing and analysis to ensure that the periods of
record are comparable. The tracer and Lyne and Hollick Filter methods can be easily applied in
these catchments and comparisons made between river gauges. The greater amount of groundwater
information will allow the end members to be more confidently defined and further, the variability
of groundwater end members can be better considered.
It is likely that in addition to these methods, detailed information would be required to look at the
smaller scale variability of surface water – groundwater interaction along the length of the river and
perhaps major tributaries. This would identify areas of groundwater discharge and river loss along
specific reaches of rivers using multiple methods. These may include some or all of the following
methods:
PAGE 31
Approaches for the Assessment of Surface Water - Groundwater Interaction

Run of river sampling using Radon-222, major ion chemistry and other environmental tracers
(i.e. Strontium or Stable Water Isotopes) in combination with detailed flow gauging;

Flow net and vertical hydraulic gradient analysis using groundwater level data;

Streambed and geomorphological assessments to establish geological controls on surface water
– groundwater interaction;

Ecological surveys to indicate interaction or dependence on groundwater processes; and

Integrated numerical modelling methods (i.e. MIKE-SHE, Hydrogeoshere, IHSim, GSFLOW
etc.)
To increase the confidence in the accuracy of the Tracer method more detailed sampling of the end
members could be conducted. These would include:

Salinity at river gauges to be regularly sampled for calibration with the data logger;

Detailed groundwater sampling of nested near river bores to determine where the boundary
(mixing zone) between the hyporheic zone and regional groundwater is; and

Collection of runoff salinity data at a wide spatial distribution.
To increase the confidence in the accuracy of the Lyne and Hollick Filter method a number of
further steps could be taken:

Develop relationships between gauges with an appreciation for hydrogeological processes
responsible for river responses; and

Regular reviews of the river gauge rating curves (stage to discharge relationship).
5.2
Results
Surface water – groundwater interaction studies such as these are not common due to the cost
associated with instrumentation and ongoing sampling. However there are a number of research
sites across the country where many of these methods (and others) have been successfully applied
to quantify and assess the nature surface water – groundwater interaction.
These study sites have characterised many hydrogeological processes and allowed many
management issues to be better understood. However, they are all still limited by their scale of
investigation as they are site specific. It remains a challenge to upscale the detailed knowledge of
one site or a series of sites to the catchment scale. This is the topic of much ongoing research and is
likely to continue to require assumptions and best estimates to be made.
PAGE 32
Approaches for the Assessment of Surface Water - Groundwater Interaction
5.3
Recommendations
To achieve the highest degree of accuracy, extensive infrastructure and sampling programs would
be required. These can become very costly very quickly when the drilling of new observation
bores, nested sites and bore networks are developed. The instrumentation of river gauges and
ongoing monitoring of surface water and groundwater features would be necessary to collect
valuable data with the aim of allowing a greater understanding of the system such that key
management and/or research questions can be answered. In many cases it is expected that this
detailed, highly accurate knowledge will be justified within the context of the current issues
confronting water managers (i.e. drought, double accounting and double allocation, climate change,
increased demand for water resources). Additional data can be collected at a moderate cost
depending on the existing data availability and scale/methods of data collection.
5.4
Cost and Accuracy
The implementation of the above methods would result in highly accurate estimates of baseflow to
rivers and the detailed interactions between surface water and groundwater resources. This would
be encompassing on both spatial and temporal scales which may be critical in solving historical,
current and future water resource challenges. An indicative cost of such investigations would start
on the order of $150,000 if detailed historical and current data records exist for multiple river
gauges and groundwater monitoring bores. This figure would increase depending on the required
infrastructure (i.e. additional river gauges, groundwater monitoring bores/networks),
instrumentation and sampling regimes to be applied.
PAGE 33
Approaches for the Assessment of Surface Water - Groundwater Interaction
6.
Discussion
6.1
Cost vs Accuracy
It is important to note that the higher the accuracy required for a surface water – groundwater
interaction assessment, the higher the cost will be. This is largely determined by the data
requirements and data availability within a catchment, and the time required to analyse the data. If
a catchment with little or moderate data is to be upgraded to a category with moderate or abundant
data, then a significant investment will be required. This investment cost would be dominated by
the installation of river gauge and groundwater bore infrastructure and instrumentation and
additionally, the cost of surface water and groundwater sampling, analysis and interpretation. The
indicative costs of these assessments are shown in Figure 9 below and Table 8 on the following
page.
$500,000
Indicative Cost of Assessment
$400,000
$300,000
$200,000
$100,000
$Poor to Moderate
(+/- 100%)
Moderate to High
(+/- 30%)
Groundwater
Balance
Moderate to High
(+/- 30%)
Tracer Method, Lynne and Hollick
Method and Groundwater Balance
Accuracy of Result

High to Excellent
(+/- 20 %)
Excellent (+/- 10%)
Tracer Method, Lynne and Hollick Method,
Run of River Sampling, Flownet and
Hydraulic Analysis, Multiple Tracers,
Comparisons with Numerical Models, etc.
Figure 9 Cost vs Accuracy
PAGE 34

Table 8 Cost vs Accuracy
Approximate Cost
per Catchment
Accuracy of
Result
Desktop
Geology maps, rainfall data, catchment
details, landuse and soil maps, other
literature
$10,000
Poor to
Moderate
Desktop
Continuous river gauge flow and salinity
data, a number of groundwater monitoring
bores, contextual information as above,
some water resource reporting
$20,000
Moderate to
High
Desktop and
Instrumentation
Instrumentation and collection of
continuous river gauge flow and salinity
data, a number of groundwater monitoring
bores, contextual information as above,
some water resource reporting
$85,000
Moderate to
High
Tracer method, digital Filter analysis,
gauge comparisons, run of river
sampling, flownet and hydraulic
analysis, multiple tracers for mass
balance
Desktop and
Fieldwork
Continuous river gauge flow and salinity
data at multiple gauges, run of river
sampling, numerous nested bore sites and
regional monitoring network, extensive
hydrogeological literature and detailed
mapping and contextual information
$150,000
High to
Excellent
Tracer method, digital Filter analysis,
gauge comparisons, run of river
sampling, flownet and hydraulic
analysis, multiple tracers for mass
balance
Desktop, Fieldwork
and
Instrumentation
As previous but including additional
drilling, establishment of river gauges,
instrumentation, calibration, sampling
programs, interpretation and assessment
$500,000
Excellent
Approach
Groundwater Balance
Tracer method and digital Filter
analysis, including groundwater
balance
Tracer method and digital Filter
analysis, including groundwater
balance
Assessment Basis
Data Inputs
PAGE 35
6.2
Scale of Investigation
The scale of investigation is another important factor to consider when planning surface water –
groundwater interaction assessments. These can be categorised as follows:

Site Specific and Small Scale– investigation describes processes occurring on the order of tens
to hundreds of meters;

River Reach Scale– investigation describes processes occurring on the order of tens of
kilometres; and
While each scale of investigation is useful for addressing the specific objectives of that particular
study, it is a challenge to be able to apply the findings to a larger or smaller scale. For example, a
detailed study including nested groundwater bores adjacent to a river gauge will enable detailed
knowledge of the spatial and temporal variation of hydraulic gradients to be gained. However, the
nature of the interaction between the groundwater and river at this location is not likely to be the
same 100m upstream or downstream. Similarly investigations conducted at larger scales will are
difficult to apply to a smaller scale due to local variability in hydrogeological conditions and the
nature of surface water – groundwater interaction.
Water resource issues and management objectives also range in their scale of interest and hence,
both of the above scales of investigation are relevant for consideration. The data abundant
catchments allow the widest range of investigation scales, from detailed site specific to river reach
scale. The catchments with moderate data allow investigations to be conducted on a river reach
scale with a higher degree of accuracy while data limited catchments, only allow river reach scale
investigations of low accuracy to be conducted.
6.3
Effectiveness of the methods
Each of the methods discussed in the ten trial catchment investigation reports and related summary
report are effective in estimating the groundwater discharge component of river flow. The
difference between them is the accuracy of the result and the cost which is required to conduct the
assessment. The Tracer method is considered the most accurate of low cost methods based on field
trials conducted in the ten catchments and as such, has been used as the means of comparing the
accuracy of other methods.
An overview of the effectiveness of each method as discussed in the Summary Report is briefly
outlined below:

Hydrographs separation using the Lyne and Hollick Filter – a simple method that can be
applied to readily available data. The method is not physically based and was found to
systematically overestimate groundwater discharge compared to the Tracer method. However,
PAGE 36
Approaches for the Assessment of Surface Water - Groundwater Interaction
an alpha parameter of 0.98 was found to produce the most accurate results (though still an
overestimate) which were plausible within the hydrogeological conceptualisation of the
catchments investigated. This method is considered appropriate for comparative analysis at
catchment scale assessments of surface water – groundwater interaction, and if multiple gauges
are available, reach scale assessments may be possible.




Chemical hydrograph separation using the Tracer method – is a relatively simple method
which uses continuous river flow and salinity data to separate the river into two components
(end members), surface runoff and regional groundwater. This method is considered to be the
most accurate of the methods implemented, although the accuracy is dependent on the
confidence placed on the end member salinity values. If mean groundwater salinity is
determined using available or collected groundwater bore data, the method is considered
representative of the contribution to river flow of regional groundwater discharge. However, if
this data is unavailable or deemed unreliable, the maximum river salinity can be used as a less
preferred alternative. This would then represent the contributions of both regional discharge
and bank discharge and is therefore considered an overestimate of regional groundwater
discharge. The Tracer method is considered appropriate for catchment scale assessments of
surface water – groundwater interaction, and if multiple gauges are available, reach scale
assessments may be possible.
Flow gauging – is a simple method whereby river flow gaugings are made along the length of
river to identify areas of loss or gain. The results of this method are considered to be most
accurate when changes in flow along the river are high (i.e. groundwater inflow rates are high)
such that errors in individual readings are relatively small. The accuracy of results can be
improved when used in combination with run of river sampling.
Longitudinal or run of river sampling – is a method that should be applied during baseflow
conditions (i.e. low flows) such that the areas where significant groundwater discharge occurs
can be most easily identified. There are difficulties using this method when the end members
cannot be clearly differentiated using common tracers (i.e. salinity or chloride) and results
were found to be unreliable in many catchments. The use of Radon-222 was found to give the
most accurate results and additional confidence was gained when applied in combination with
river flow gauging at the sampling locations. This method can be conducted on a river reach or
even catchment scale and may prove successful in identifying significant reaches of
groundwater discharge. An assessment of the rates of river loss along losing reaches is not
possible directly using this method.
Hydraulic analysis – requires a comparison between river levels and nearby groundwater
elevations and result in estimations considered accurate for site specific groundwater discharge
to a river or loss to the groundwater (where rivers experience gaining or losing conditions
respectively). However, the application of this method to other reaches and larger scales is
difficult and hence not useful for larger scale assessments.
PAGE 37
Approaches for the Assessment of Surface Water - Groundwater Interaction
None of the above methods alone measure both the spatial and temporal variation in surface water
– groundwater interaction. This can only be achieved to some degree when different methods are
combined. This requires abundant data to be available or collected within the catchment.
6.4
Approaches for catchments with a range of water resource value and data
availability
Catchment planners have the challenge of determining how to allocate their resources based on the
current and potential future management issues that may exist. It is expected that this report will
enable decisions regarding surface water – groundwater interaction assessments to be made in a
clear manner, in terms of the expected accuracy of the method and associated cost. In most cases
there will be specific issues that need to be addressed and the type of assessment required should be
considered within this accuracy vs cost context. The accuracy vs cost is clearly very closely related
to the data availability and should be based on the value of the water resources of a catchment.
Through the development of methods implemented in the ten trial catchment studies and summary
report, a range of methods for the assessment of surface water – groundwater interaction have been
demonstrated. Recommendations are made for catchments with water resources of different value
which is within the context of the data availability, as outlined below:


Catchments with low value water resources – A review of contextual information and
development of a groundwater balance to estimate groundwater discharge. This would be
conducted at low cost with poor accuracy averaging 210 %. This method is limited by data and
contextual information availability and the experience of the hydrogeologist or water resource
practitioner conducting the assessment.
Catchments with moderate value water resources – A review of contextual information and
then the application of firstly, the Tracer method and then the Lyne and Hollick Filter method
is needed. The Tracer method is considered the most accurate method within the uncertainty of
the selected end member salinity values. In most cases the historical river flow record will be
greater than the salinity record and as such it would be beneficial to also apply the Lyne and
Hollick Filter method to both the period of salinity record and the entire flow record. Lyne and
Hollick Filter method is considered to most closely match the Tracer method when an alpha
parameter of 0.98 is applied although this is expected to act as a conservative maximum
estimate of groundwater discharge. The accuracy error of the Lyne and Hollick Filter method
was found to be on average 59% using a Filter parameter of 0.98 and 92% using a Filter
parameter of 0.925. The total range in error using a Filter parameter of 0.98 was from -6 to
175% over the same short data record (1 year) while the error ranged from -52 to 135 % using
longer term flow data. The accuracy error of the Tracer method itself was approximately +/30% depending on end members used.
PAGE 38
Approaches for the Assessment of Surface Water - Groundwater Interaction
If the assessment is to be made in a catchment with little data, then further data collection
and/or infrastructure would need to be established and instrumented/monitored. This would
come at a cost but would allow a more accurate method to be applied..

Catchments with high value water resources – It is suggested that the hydrographs separation
methods (Tracer and Lyne and Hollick Filter methods) be used in combination with river
sampling and gauging. This will enable the spatial and temporal variability in the nature of
surface water – groundwater interaction to be assessed at a moderate cost and with a high
degree of accuracy (i.e. 10% due to better characterisation of end member concentrations)
provided that the data, literature and previous assessments are available for use and review.
The cost of the assessment where long historical records and previous studies are available is
moderate and will enable specific management and/or scientific questions to be answered with
an accuracy error of near +/- 10%. If the infrastructure and instrumentation is to be established
from a lower level of data availability, a significant financial investment will be required.
The cost of the above methods for making assessments of surface water – groundwater interaction
will be determined by the value of the water resources and hence, the accuracy which is required to
adequately answer specific management questions. The data availability of each catchment limits
the methods (and therefore accuracy) that can be used in the assessment, unless further investment
in infrastructure and instrumentation is made.
It is also important to consider the scale of the investigation required to adequately address specific
water resource management issues. The findings of a surface water – groundwater interaction
assessment conducted at a site specific scale are very difficult to apply to a river reach scale or a
catchments scale. Likewise catchment scale processes are difficult to apply to river reach or local
scales due to the highly variable nature of surface water – groundwater interaction. Perhaps the
most relevant scale to water managers is the river reach scale since it acts as a middle ground
between broad catchments processes and detailed small scale assessments.
PAGE 39
Approaches for the Assessment of Surface Water - Groundwater Interaction
7.
Conclusions and Recommendations
7.1
Conclusions
The field trials developed and implemented in this study successfully applied a number of simple
and low cost methods for estimating the groundwater discharge component of river flow. This
report extended those findings to be applied in catchments where the value of water resources are
low, moderate or high with a range of data availability. The key conclusions of this study are
outlined below:
1)
The Tracer method was found to be the most accurate low cost approach for estimating
groundwater discharge (baseflow) to rivers. This method relies on continuous river flow and
salinity data that is available from many gauges across Australia and the definition of surface
runoff and regional groundwater end member salinities;
2)
To support the Tracer method, the Lyne and Hollick Filter method can also be applied. The
method was found to systematically overestimate regional groundwater discharge, whilst an
alpha parameter of 0.98 most closely matched the Tracer method results. The Lyne and
Hollick Filter method can be applied over the same period of record as the Tracer method and
additionally, can often be applied over a longer historical record. This longer term analysis is
expected to result in more accurate results, based simply on the fact that a longer data record
represents a greater range of the changing nature of surface – water groundwater interaction;
3)
Catchments with low value water resources can be assessed using a groundwater balance
approach within the context of existing hydrogeological knowledge and the use of available
literature and resources. The resulting estimate of groundwater discharge is expected to have a
poor accuracy of perhaps 210% but produce a nonetheless important estimate;
4)
Catchments with moderate value water resources can be assessed using the Tracer method in
combination with the Lyne and Hollick Filter method. This would be supported by a brief
literature review to ensure the results fit within the hydrogeological context of the catchment.
The error of accuracy would be approximately 30% and 59% for the Tracer and Lyne and
Hollick Filter methods respectively. Additional data may be required if the catchment has little
data availability or if greater confidence is required;
5)
Catchments with high value water resources should be assessed at a range of spatial and
temporal scales to address multiple management issues. This would involve the application of
the Tracer and Lyne and Hollick Filter methods at multiple gauges in addition to run of river
sampling. The error of accuracy of the combined methods is expected to be near 10%.
Additional data may be required if the catchment has little or moderate data availability or if
greater confidence is required;
PAGE 40
Approaches for the Assessment of Surface Water - Groundwater Interaction
6)
If estimates of increased accuracy are required, then this will inevitably incur greater cost.
Nevertheless, if the value of the water resource is high, then it is likely that an increased
expenditure can be justified in terms of risk management.
7.2
Recommendations
From a water resource management perspective, there are many catchments across Australia which
require closer attention in terms of the understanding of surface water – groundwater interaction,
the health of riverine environments and the significance of double accounting. The specific
recommendations that can be made as a result of this study are detailed below:
1) Estimates of groundwater discharge to rivers should be made using a range of methods (with
associated accuracy) depending on the value of the groundwater and surface water resources in
each catchment. These assessments will form a fundamental hydrogeological basis for making
current and future integrated water resource management decisions. The level of accuracy and
cost of these estimates will be controlled by the data availability and/or investment made in
infrastructure and instrumentation;
2) If the groundwater salinity data of a catchment is limited, it is likely that the accuracy of the
Tracer method could be improved. Hence, in these catchments, rather than using the maximum
river salinity as the groundwater end member, it is recommended that the definition of the
regional groundwater end member be investigated in more detail. This could include a
sampling round of existing bores (which may not have existing salinity data) or, if very few are
available, the drilling and sampling of monitoring bores. A representative end member value
could then be justified as representative using statistical methods (rather than just the mean);
3) The collection of continuous river salinity data at established river gauge sites, should be
continued at the same frequency as flow recordings. This will allow the most accurate of
baseflow estimate methods (Tracer method) to be applied using a significant historical and
current data record.
PAGE 41
Approaches for the Assessment of Surface Water - Groundwater Interaction
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