Role for drains in controlling salt by lindayy

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Role for drains in controlling salt

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                                                                    Role for drains
CSIRO Land and Water




                           Riasat
                             Ali
                                            Mark
                                           Filmer
                           FOR csiRO lAnD AnD WATeR
                                                                    in controlling salt
                       Close-up: Salt at the Yilgarn River–Lake     As part of the Western Australian Government’s Engineering Evaluation
                       Brown outflow in the Avon Basin of Western
                       Australia. Dryland salinity will become      Initiative, CSIRO scientists, together with WA’s Department of Water and
                       an even bigger problem throughout            Department of Agriculture and Food have been involved in a collaborative
                       Australia’s vast wheatbelt unless further
                       steps are taken to manage it.                project to evaluate the effectiveness of deep open drains as an engineering
                                                                    solution to dryland salinity at the farm and catchment scale in the State.
                                                                    Their findings indicate that by intervening and establishing drainage systems,
                                                                    substantial reductions in groundwater levels and salt loads can be achieved
                                                                    under various discharge management scenarios. Importantly, the modelling
                                                                    tool developed can be modified and applied to other dryland catchments to
                                                                    help predict the impacts of different drainage management strategies.
                       At a glance
                       Dryland salinity is a major problem in       Leveed arterial channels discharging into             Currently about five per cent of the Avon
                       many parts of the Western Australian         degraded salt lakes are currently the most          Basin, the largest catchment in the WA
                                                                    practical, environmentally sound and                wheatbelt, is affected by secondary (human-
                       wheatbelt and will become an even
                                                                    economically feasibly drainage discharge            induced) salinity and waterlogging.
                       bigger problem unless further steps
                                                                    system, of the systems tested, for salinity-          If climate trends from the recent past
                       are taken to address it.
                                                                    affected land in Western Australia’s wheatbelt.     continue, 25–30% of the basin will
                       Deep open drains are an additional              This is one of the key findings of a             potentially develop shallow watertables and
                       tool available to farmers to halt the        collaborative research project involving            be at risk of increased salinisation.
                       spread of salinity. Where suitable,          scientists from CSIRO Land and Water to
                       they allow some farmers to                   evaluate the feasibility and impact of              Salinity costs Australian
                       reclaim salt-affected land and make          management strategies for sub-catchment
                       it productive.                               and regional-scale drainage systems to              farmers hundreds of millions
                       Uncontrolled discharge of saline and
                                                                    combat dryland salinity at the farm and             of dollars in lost production
                                                                    catchment scale in WA.
                       acidic drainage water into natural                                                               each year.
                       systems brings a high risk of adverse        The problem and its scale
                       off-site impacts.                              Dryland salinity is widespread in the             Open drains help combat salinity
                       Various drainage discharge                   wheatbelt of Western Australia — about two             Although revegetation is being used
                       management strategies can be                 million hectares are directly affected and          to help manage salinity, current planting
                       implemented to minimise the                  another four million are at risk.                   levels are inadequate to control the problem,
                       detrimental impacts of this discharge          WA has the largest area of dryland salinity       effectively so many landowners are
                       water to natural ecosystems.                 in the nation and the highest risk of               considering engineering options such as
                                                                    increased salinity during the next 50 years.        artificial drainage systems.

                                                                                                                 Farming Ahead April 2008 no. 195 www.farmingahead.com.au   37
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                                                                                                                                   Most of the drains have been constructed
                                                                                                                                 with limited planning or design guidelines
                                                                                                                                 and usually with little analysis or
                                                                                                                                 understanding of downstream effects — the
                                                                                                                                 discharge from most of these drainage
                                                                                                                                 systems, where they flow, is being disposed
                                                                                                                                 into natural creeks, rivers and wetlands,
                                                                                                                                 potentially causing adverse ecological and
                                                                                                                                 hydrological impacts to these waterways.

                                                                                                                                 Researchers analyse drainage systems
                                                                                                                                    CSIRO Land and Water researchers




                                                                                                          CSIRO Land and Water
                                                                                                                                 evaluated the impacts of current and future
                                                                                                                                 salinity on streamflows, salt loads and river
                                                                                                                                 systems if nothing was done to control
                                                                                                                                 salinity. The project was focused at a
                                                                                                                                 regional scale. They also wanted to predict
     Field-day: Landholders gather at Wakeman                                                                                    on- and off-site impacts of various drainage
     Creek near Narembeen, Western Australia,                                                                                    systems, evaluate the effectiveness and
     during a field day focusing on drainage
     discharge management strategies to help                                                                                     economic feasibility of different drainage
     control salinity.                                   increased significantly within the Avon                                 discharge and storage strategies, and
                                                         catchment and indeed the entire wheatbelt.                              examine the effects of revegetation and
                                                                                                                                 climate on streamflows and salinity levels.
       The use of deep open drains to lower the          WA has the largest area of
    watertable is increasingly being seen as a                                                                                   How the modelling was carried out
    viable option to manage salinity and improve
                                                         dryland salinity in the nation                                             A hydrological catchment model,
    the profitability of farming enterprises by          and the highest risk of                                                 LASCAM, was calibrated for the Avon Basin
    allowing farmers to reclaim low-lying, salt-         increased salinity during the                                           using local and historical data (1970–2003)
    affected land and turn it into productive                                                                                    such as rainfall, streamflow, stream salinity,
    cropping country.
                                                         next 50 years.                                                          vegetation and soil property records.
       During the past decade this form of                 The total length of so-called deep drains is                             It also included data relating to lake water
    salinity mitigation has become more popular          estimated to be more than 11,000 kilometres                             and salt balance, natural creek, stream and
    and the construction of farm and sub-                but they are generally scattered and without                            river storage, lake and natural channel
    catchment scale drainage systems has                 extensive regional linkages.                                            evaporation and artificial drainage.




38 Farming Ahead April 2008 no. 195 www.farmingahead.com.au
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   Daily streamflow and salt load                                          fns,dng
measurements from 28 locations within the
basin were used to generate reliable sub-
catchment and regional scale predictions.
   Using the model, researchers generated
hydrological predictions for the baseline
scenario in which nothing is done to treat
dryland salinity (no artificial drainage
is implemented).
   They then estimated the impacts of
current and future salinity on groundwater
levels, streamflows, salt loads and lake
overflow frequencies and volumes under a
variety of drainage, revegetation and climate
conditions for the Avon Basin during the
                                                  CSIRO Land and Water


next 100 years.
Historical patterns
   Groundwater levels, streamflows, lake
storages and salt loads have been increasing
in some parts of the Avon Basin since                                                                                          Channel: A discharge drain at Pithara, Western
European settlement, due to the widespread                                  The streamflow at Yenyening increases by           Australia. Such drains can help lower the
replacement of native vegetation with                                                                                          watertable and allow farmers to return salt-
                                                                         about 45% from 18GL/year to 26GL/year.
shallow-rooted crops and pasture.                                                                                              affected land to production.
                                                                            Very little streamflow is generated in the
   Other studies have shown streamflows                                  50% of the basin that is east of Kellerberrin.
in the basin increased five-fold between                                                                                      this century (such as Kurrenkutten, Jilakin).
European settlement and the end of                                          But even in these areas, due to the
                                                                         development of additional areas of shallow           There also are significant increases in
last century.                                                                                                                 discharge salinity, which reflect the increased
                                                                         watertables under the baseline scenario,
   Researchers took into account the                                     the streamflow generation from most                  salinity of lake inflows.
changing state of the catchment when they                                catchments still increases over time.
assessed the impacts of any future                                                                                            Drainage systems
management or climatic changes — they                                       Increased stream salinities and salt loads           Researchers assessed the effect of building
compared model predictions under various                                 also are predicted. The development of               artificial drains at the sub-catchment and
drainage, revegetation or climate scenarios                              shallow watertables increases the discharge          farm scale. Artificial drains are constructed
against the changing baseline scenario.                                  of usually hypersaline deeper groundwater            to lower the watertable and convey water
                                                                         into shallower aquifers and surface water            from farms to sub-catchment outlets or
Groundwater levels                                                       systems resulting in higher stream salinities        natural channels. They can be either leveed
   The model shows the Avon Basin                                        and salt loads.                                      or open.
currently is not at equilibrium — the shallow                                                                                    Leveed drains handle only groundwater,
groundwater systems are changing with                                    The model predicts that if                           while open drains handle both groundwater
time because the recharge rates from rainfall                            nothing is done to treat                             and surface run-off (both leveed and open
and discharge rates through streamflow and                                                                                    drains were modelled but the results
evaporation are not balanced.                                            dryland salinity, groundwater                        were similar).
   It predicts that if nothing is done to treat                          levels across the wheatbelt                             Researchers examined the direct, on-site
dryland salinity, groundwater levels across
the wheatbelt will continue to rise. But the
                                                                         will continue to rise.                               effects that artificial drains had on the farm
                                                                                                                              and sub-catchment. The impacts examined
rates of rise will slow over time.                                                                                            include groundwater levels, groundwater
   But there is considerable variability in                              Rising salinity                                      salinity and water and salt yields within
groundwater response within, as well as                                     The largest increases in salinity are in the      the catchment.
between, areas within the catchment.                                     eastern wheatbelt. The salt loads increase
                                                                                                                                 Artificial drainage also impacts indirectly
   The model forecasts that by 2100 rates of                             faster than streamflows throughout the
                                                                                                                              off-site, as the drainage discharge can adversely
rise in the Lockhart and Yilgarn sub-                                    catchment. The mean annual salt load at
                                                                                                                              impact the environment and landscape
catchments reduce to 25% of the current                                  the basin outlet almost doubles by the end
                                                                                                                              downstream. Researchers examined the
levels. In western catchments many are                                   of first quarter of this century.
                                                                                                                              off-site impacts and evaluated strategies to
already essentially at equilibrium.                                         For the same period the mean annual salt          manage the drainage discharge.
   Most sub-catchments of the Avon Basin                                 loads from Mortlock North and East also
will reach equilibrium by 2100 and further                               double. At Yenyening, the salt load increases        On-site impacts
substantial rises in groundwater levels would                            to about 2.5 times the current load.                    The construction of farm and sub-
not be expected.                                                            The largest increase in the salt load is from     catchment scale artificial drainage is
                                                                         Lockhart where it increases almost four-fold.        predicted to lower local shallow groundwater
streamflows, salinity and salt loads                                                                                          levels. But it is likely to take 15–40 years
   As a consequence of rising groundwater                                Overflow frequency and discharges                    before regional groundwater levels start
levels, the model suggests annual                                           The model also forecasts increased                declining noticeably.
streamflows also will increase if dryland                                streamflows and increased overflow                      Artificial drainage also slightly lowers
salinity is not addressed.                                               frequencies and volume of lake discharges            groundwater salinity (compared with a slight
   The rate of increase in streamflow varies                             under the baseline scenario.                         increase under the baseline scenario) — this
across the basin and is highest for the                                     Discharge frequencies increase for all but        ‘freshening’ of the groundwater occurs
eastern sub-catchments.                                                  one of the non-terminal lakes, although only         because the increased groundwater discharge
   At the basin outlet the mean annual                                   one of the currently terminal lakes (Gulsen)         due to artificial drainage is more saline than
streamflow increases by about 10% from                                   discharges during 2073–2100.                         the (relatively unchanged) recharge.
298 gigalitres/year to 328GL/year during                                    Several lakes that were near-terminal                Other impacts include increased water
the first quarter of this century.                                       last century will fill quite frequently by late      and salt yields — most of the eastern

                                                                                                                       Farming Ahead April 2008 no. 195 www.farmingahead.com.au   39
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                                                                                                                                feasible system, if the drainage discharge is
                                                                                                                                stored in salt lakes. This requires relatively
                                                                                                                                small channels, so excavation costs are lower.
                                                                                                                                  An open arterial system requires very large
                                                                                                                                channels because both surface run-off and
                                                                                                                                drainage discharge are conveyed. It is more
                                                                                                                                expensive than a leveed arterial system.
                                                                                                                                  Sub-catchment scale evaporation basins
                                                                                                                                are prohibitively expensive, although
                                                                                                                                they can reduce streamflows and salt
                                                                                                                                loads downstream.




                                                                                                         CSIRO Land and Water
                                                                                                                                Benefit-cost analysis
                                                                                                                                   A benefit–cost analysis of full drainage
                                                                                                                                (sub-catchment scale artificial drainage plus
                                                                                                                                drainage discharge management) showed it
      Draining: An artificial drainage discharge                                                                                breaks even in most low cost and medium-
      channel at Narembeen, Western Australia.           loads are greater than the impacts under
      About 11,000 kilometres of surface or              drainage and revegetation scenarios.                                   to high-benefit scenarios. If medium costs
      groundwater drains have been constructed in                                                                               and low benefits are assumed, or if drainage
      the WA wheatbelt.
                                                            The annual streamflows predicted under
                                                                                                                                disposal or treatment costs are included, it
                                                         the drier climate scenario (20% reduction in
                                                         rainfall) are lower than all other scenarios                           become less economically feasible.
    catchments are predicted to yield more               including the baseline scenario.                                         The profitability of drainage investment
    water and salt with drainage than without.                                                                                  is governed by drainage effectiveness,
                                                            The annual streamflow at the basin outlet
    Off-site impacts                                     will reduce from 328GL/year (baseline                                  costs including maintenance, and expected
                                                         scenario) to 127GL/year. Salt exports also                             agricultural returns.
       Once excess groundwater is artificially
    drained and conveyed to sub-catchment                will be reduced significantly under the drier                            At catchment scale, drainage systems are
                                                         climate scenario.                                                      more economically viable in the Lockhart
    outlets or natural channels, significant off-
    site impacts occur.                                     A 10% wetter climate will cause a                                   compared with the Avon and Yilgarn
                                                         substantial increase in the annual                                     catchments. The economic viability of
       To minimise these detrimental effects,
                                                         streamflows and salt loads: the impact is                              drainage improves significantly in all three
    strategies must be implemented to manage
                                                         similar to that from a drainage scenario
    the resulting drainage discharge.                                                                                           catchments when the economic benefits of
                                                         where the drainage discharge from sub-
       In the model, the drainage discharge              catchment scale artificial drainage is                                 remnant vegetation and roads are included.
    was managed using regional scale arterial            managed via arterial channels with no salt
    channels with four storage options or                                                                                       Applying the model to other basins
                                                         lake storage.
    sub-catchment scale evaporation basins.                                                                                       This model is being used in the Blackwood
    Storage options were salt lake storage up            Leveed arterial channels                                               River Basin and can be applied in other
    to existing discharge outlet heights (dead                                                                                  dryland catchments and basins to assess the
    storage capacity); salt lake storage up to           are the most practical                                                 feasibility of water management strategies
    raised discharge outlet heights (increased           and economically feasible                                              and the impacts of climate change. At the
    dead storage capacity); salt lake storage up
    to full capacity (lakes are terminal) and lake       system, if the drainage                                                basin scale, it also can assess localised
                                                                                                                                impacts of drainage management strategies
    bypass (no salt lake storage).                       discharge is stored in                                                 on flows and loads in individual catchments
    Drainage discharge management                        salt lakes.                                                            and the impacts of various mixtures of
                                                                                                                                vegetation and engineering strategies,
       Among drainage scenarios, the biggest                Revegetating half the landscape with
    reduction (from the baseline scenario) in                                                                                   implemented either universally or in
                                                         deep-rooted perennials will reduce annual                              selected sub-catchments.
    annual streamflows and salt loads in the             streamflows — the mean annual flow at the
    major river systems will occur if the drainage       basin outlet will fall 16% from 328GL/year                               The research was funded by Water for a Healthy
    discharge is managed at the sub-catchment            (under the baseline scenario) to about                                 Country Flagship; CSIRO Land and Water;
    scale via evaporation basins.                        281GL/year. Revegetation will have similar                             Engineering Evaluation Initiative; Department of
       The largest increase in annual streamflows        impacts as a drier climate.                                            Water, WA; and supported by the Department of
    will occur if the drainage discharge is                 In both cases the net export of salts at                            Food and Agriculture, WA, Department of
    managed via arterial channels with no salt           the basin outlet will decline — 64% for the                            Conservation and Land Management and WA
    lake storage. In this case, salt loads are           20% drier climate and 54% for the 50%                                  Channel Management Group.
    greater than the baseline measure.                   revegetation strategy.                                                   Acknowledgements: Neil Viney, Geoff
       Only marginal increases in the annual                                                                                    Hodgson, Santosh Aryal, Warrick Dawes and
                                                         economic analysis
    streamflows are expected if the drainage                                                                                    John Byrne, CSIRO; farmers who allowed
    discharge is managed via arterial channels             The most cost-effective drainage discharge
                                                         management strategies (based on current                                access on their properties; Ken McIntosh,
    with salt lake storage at either existing or                                                                                Melinda Burton and David Rowlands,
    increased discharge outlet heights.                  construction costs only) are leveed and
                                                         open drainage with discharge disposed into                             Department of Water, WA; Richard George,
       The adoption of a particular regional             natural channels.                                                      Department of Food and Agriculture, WA.
    drainage discharge management strategy
                                                           But due to adverse environmental
    could have huge implications on annual
                                                         impacts, it is unlikely that the potentially
    streamflows downstream and the capacity              acidic and highly saline drainage discharge
    of the system to accommodate such flows.                                                                                    ContaCt Riasat ali
                                                         will be allowed to be disposed into these
                                                                                                                                   (08) 9333 6329
       effects of climate and revegetation               natural systems in the long-run.                                          (08) 9333 6211
       The impacts of climate change (drier or             With this in mind, leveed arterial channels                              riasat.ali@csiro.au
    wetter climate) on streamflows and salt              are the most practical and economically                                    www.clw.csiro.au

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