Risks to Murray-Darling Basin Water Resources Program
Project Summaries
Project code:
CD5
Project title:
Risk of Climate Change Impacts on Salinity Dynamics and Mobilisation
Processes in the Murray-Darling Basin
Project timeline:
March – November 2010
Contractors:
Sinclair Knight Merz
Report Authors:
Greg Holland, Keith Collett and Nicole Caruso
Project objectives:
The aim of this project was to gain an improved understanding of the risk that climate change poses to
salinity dynamics and salt mobilization processes across the Murray-Darling Basin. It was expected to
provide a sound understanding of how the fundamental processes that drive salinity may be affected by
climate change, and to provide input into the development (or future review) of the Water Quality and
Salinity Management Plan. The project was one of several examining the impacts of climate change on
vegetation and land use and on conservation of biodiversity and other environmental assets, and was
linked with the development and implementation of the Basin Salinity Management Strategy (BSMS).
The project was to use the agreed Risk Assessment Program portfolio climate change scenarios:
1. A ‘most favourable 2030 scenario’ that is based on a continuation of the long-term (1895 to
2006) averages for rainfall and runoff across the MDB,
2. A ‘medium 2030 scenario’ that is based upon the medium global warming scenario and
associated rainfall and runoff described in the CSIRO report Water Availability in the Murray–
Darling Basin of October 2008, and
3. A ‘least favourable 2030 scenario’ that is based upon the actual climate of the MDB in the
period 1997–2006 (this includes 15% less rainfall and 50% less runoff in the southern MDB
when compared with the long-term average).
Methods:
The method used in this study in assessing the salinity risk arising from climate change impacts on
salinity processes and salt mobilisation was to complete an assessment for each of the broad landscape
types found within the Basin, along with an assessment of the cumulative impacts of all landscapes on the
lower Murray below Swan Hill. The landscapes are broadly summarised as follows:
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Northern Plains (this includes the lower parts of northern valleys and saline inflows upstream of
Broken Hill off-take)
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Northern Uplands
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Southern Uplands
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Riverine Plains
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CD5 Project Summary
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Mallee (NSW, Vic, SA)
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Mallee floodplain
For each landscape type, key situational factors (such as salt storage, surface topography and aquifer
characteristics), and land and water management drivers (such as water allocation policies, land clearing,
irrigation efficiencies, construction of salt interception schemes), were identified and the potential
impacts on them of climate change were assessed. The method used to quantify potential changes in
salinity in the Murray main stem was as follows:
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The key outcomes from the assessment for each landscape type were used as a basis for
identifying those regions for which climate change is likely to generate significant changes in salt
exports relative to the historical salt load record as represented by the 1975 – 2009 simulated data
set (from MSM-BIGMOD).
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For those regions likely to generate significant changes in salt loads, revised salt loads were
estimated using available data and previous investigations.
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Expected climate adjusted changes in salt loads were combined with 1975-2009 climate adjusted
flows already in MSM-BIGMOD to provide river salinity outcomes.
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The predicted outcomes for 1975-2009 were used to provide an indication of the likelihood of
salinity behaviour for the flow scenarios as represented by the 1895 - 2009 period. This was done
by looking at events in the long term (114 year) monthly flow simulations that resemble events in
the 1975-2009 period that were found using MSM-BIGMOD to be significant for salinity.
Determining climate change implications for historic stream salinities within the tributaries was difficult
due to a lack of data, this meant that the analysis was constrained to general commentary around what
salinity processes in the northern and southern uplands may mean for the Basin’s streams. For some salt
mobilisation processes within the landscape, there are significant delays between climatic driven recharge
regimes, and associated salt load impacts within the river. Future impacts based upon historic climatic
regimes, (termed the Legacy of History) are a feature of the BSMS Salinity Registers. To take into
account these future impacts of past climate events, climatic scenario simulations were compared against
a baseline that assumes 2050 stream salinities. In other words, the baseline simulation assumed the salt
load inputs predicted for 2050. The Year 2050 was considered distant enough for the effects of climate
change to manifest themselves, yet not so distant as to be outside of reasonable planning horizons. Only
changes in salt load greater than plus or minus 150 t/d were included in the river modelling.
Summary of key results:
Climate change may lead to wetter or drier conditions with salinity impacts occurring:
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directly in terms of a changed groundwater recharge and surface water flow regime and hence
differential salt mobilisation across the landscape; and,
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indirectly through land and water management decisions and policies, which through a feedback
process will over time, be adjusted to prevailing climate conditions at any particular time.
The least favourable climate scenario will have a more profound impact on the flow regime. Given the
combined contribution of salt load and flow regime determining the salinity outcome, this flow regime
could be expected to significantly impact on salinity outcomes. The wetter and medium climate scenarios
are not significantly different from historic, so salinities are not anticipated to change dramatically from
the past. There does however, remain some uncertainty in the legacy of history impacts in the Mallee.
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CD5 Project Summary
Landscape impacts on lower Murray salinities
Climate interacts with situational aspects of the landscape such as salt storage, terrain and hydrogeology
to generate flow regimes and salt mobilisation. Together, these two elements determine in-stream salinity
outcomes. Within the Northern Basin, low gradients, overbank losses and significant distances between
the source of salt and the downstream Murray River, means that climate change is unlikely to overtly
deliver increased salt loads from the Darling to the Murray River. However, some increase in flooding
sourced from the Darling may arise if there is increased incidence of tropical climatic events within
northern Australia.
Within the Riverine Plains, substantial change in land and water use brought about by the water trading
regime, particularly “buyback”, and by improved system and irrigation efficiencies, is expected to lead to
reductions in salt mobilisation via groundwater and the surface drainage system as has occurred in recent
years.
Within the southern uplands, some uncertainty remains as to the extent to which a wetter climate will lead
to salt mobilisation beyond the historic past. However, it is not expected to have a great impact on
downstream users, and under a drier climate could be expected to be somewhat less than the past. There
may be some deviations but these are expected to be insignificant compared to salt mobilisation in the
Mallee region where the groundwater salinities are an order of magnitude greater.
Under current understandings of groundwater drivers from the dryland and irrigated Mallee, base salt
loads are expected to remain similar under climate change as recharge rates under rain-fed systems are
relatively small. It is anticipated that the Mallee irrigation footprint will be maintained, even under the
least favourable scenario, as a result of the tendency for water diversions to move to the highest value
uses. These base salt loads are anticipated to pose the greatest threat under climate change because of the
substantial reduction in the flow regime that would emerge under the least favourable scenario. Under
this scenario, salinities downstream of Euston will diverge upwards from historic salinities, with the
impacts most pronounced downstream of Morgan where most flows will have been diverted. Existing
threats within the Mallee region from post flood accessions under historic climatic patterns may be
slightly accentuated under the most favourable scenario, but would be expected to diminish in frequency
and magnitude under the least favourable.
Other dimensions to these threats may emerge with more frequent floods emanating from the Darling and
possibly leading to greater salinity outcomes if drought conditions compromise dilution flows from
Murray storages when Darling floodwaters subside. There is also some uncertainty as to whether reduced
frequency of floods will exacerbate salt accumulation such that salt load fluxes in the longer term will be
substantially higher. Even under the dry scenario, large floods would still be expected to occur from time
to time.
Salinity impacts within tributary catchments
Understanding salinity impacts within tributaries under both the most and least favourable scenarios is
somewhat problematic due to the role that rainfall plays both in mobilising salt, and determining the flow
regime available to dilute salt. As tributary salinities range substantially in both the northern and southern
uplands, existing salinities may already exceed the threshold values that impinge upon environmental
uses in many river reaches. In other reaches, the existing salinities may be approaching the threshold or
be well below. However, assuming a resource condition limit (RCL) value of around 800 EC, climate
change could either lead to local improvements or decline in environmental values, depending upon the
net effect upon salt load and the dilution regime at the local scale. Significantly more work is required at
the local scale to properly understand both the salinity thresholds for environmental values, and the likely
local impact of either a drier or wetter climate scenario.
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CD5 Project Summary
Comparisons with other projects:
Much of the work and results of this project are specific to the topics of salt mobilisation and river salt
loads. However, the demonstrated differences in response of the northern and southern regions of the
Basin to climate change match those identified by other projects. The importance of considering the
combined effects of climate change with changes in land use has also been identified by several other
projects.
Knowledge gaps identified:
In addition to the gaps in understanding of the salinity risks within tributary catchments discussed in the
Final Report, the salinity risks in the Lower Murray under climate change expose a range of uncertainties
that need to be focused upon as new data and understandings become available. In particular:
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An improved understanding of the validity of assumptions underpinning the continuity of historic
salt loads to the river under a least favourable scenario. For example, the results of current work
underway assessing the role that vegetation may play in reducing near river groundwater levels
and hence reducing river salt loads under dry conditions should be taken into account, noting that
if vegetation is playing this role, then over the longer term, such a role may lessen as salinisation
of the floodplain reduces its transpiration capacity.
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The MDBA should maintain a watching brief on the likelihood of contraction of irrigated
agriculture under a drier climate scenario; an outcome that is likely to be dependent upon the
longer term value of water, and upon prevailing financial returns on horticulture.
Implications for policy:
Mitigation of risks associated with climate change can be considered within the policy/management
context and on-ground works and measures. The existing BSMS can be considered as a risk management
framework such that management of the risks arising from climate change could be improved if this
framework is able to be adjusted. As the principal threat in relation to salt mobilisation and loads appears
likely to be the impact of a lower dilution regime, changes to the benchmark period upon which salinity
registers and BSMS targets are assessed, may be an appropriate response. Such a step should be
considered after the role of the registers are confirmed in terms of guiding future salinity mitigation
investment. River operation rules, management of SDLs, and use of environmental water under the Basin
Plan and by the Commonwealth Environment Water Holder, may also need to take account of benefits to
be gained from periodic dilution flows under future climate change.
In terms of on-ground works and measures, there is a range of options to mitigate water salinity impacts.
However, those appropriate to the moderate to high salinity risks identified by this project are largely
those that would enable the impact of low flow periods to be addressed through the delivery of dilution
flows, or works that intercept salt loads or reduce accessions.
Recommended Communication Approach:
The project Final Report should be made widely available in electronic format as it contains a detailed
description of the landscape types used in the analysis as well as the results of the river salinity
modelling, changes in salinity risk and exceedance durations for several locations. Much of the
project’s work will be of particular relevance to the BSMS, but the project team should also be
encouraged to present their findings at workshops, conferences and through journal publications.
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CD5 Project Summary