Forests and water

1
Forests and water

• Dr Ian Prosser
• Water for a Healthy Country Flagship Program
• 21 March 2007

2
National Water Initiative

• aims to increase the productivity and
  efficiency of Australias water use to meet
  increasing water demand through more effective
  water markets.
• New plantations- assessing impacts on water
  users- estimating water access entitlement
  requirements
• Forest management, climate change and bushfires
  in major water supply catchments

3
National Water Initiative key elements

• Water access entitlements that are perpetual
  shares in the resource.
• Reduce over allocation to protect the environment
  and ensure continuing value of the shares.
• Provide greater security of resource into the
  future.
• Free and mature water markets to ensure water
  goes to highest value use.
• Formal risk attribution. Reductions in water
  availability due to change in government policy
  are to be born by the government.

4
Forest interception key elements

• Only covers land use change not current land
  use.
• Is in terms of surface and groundwater resource
  not rainfall.
• Is aimed at providing water resource security to
  holders of water access entitlements.
• In fully allocated water systems
• (i) significant interceptions should be
  recorded (through a licence system).
• (ii) additional interception activities above a
  threshold level will require a water access
  entitlement (taking into account the possible
  benefits of increased water use).
• In less than fully allocated systems, water use
  should be assessed and monitored but does not
  require an entitlement as long as the full
  allocation is not reached.

5
Forests and annual water yield

• Less streamflow from forested catchments
• greater leaf area
• trees can access deeper water stores
• trees intercept more rainfall
• greater atmospheric turbulence
• Within the catchment, water use differences occur
  related to soil, climate, and lateral
  redistribution of water

100 mm runoff 1 Ml/ha
6
Afforestation and flow regime

• Changes in flow regime after afforestation

Flow duration curves
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Water allocations in the River Murray system

• Mean annual water yield differences do not
  solve the problem
• Irrigation water is supplied mainly through
  summer releases from major reservoirs.
• These reservoirs capture winter flows and
  release them in summer
• Critical question for new forest interception
  then is
• - impact on flows into reservoirs?
• - summer flows from unregulated tributaries?
• - operations of reservoirs and rivers?

8
Current land cover
9
Water yield
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30,000 ha of new tree plantations
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Reduction to Tributary Inflows (flow duration
curves) 30,000 ha high water yield reduction
Bungle Creek 5.6
Goobarragandra River 1.4
Gilmore Creek 15.8
Tarcutta Creek 3.7
Hillas Creek 7.5
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Adelong Creek
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Mean annual changes (30,000 ha new plantation)
Reduction in Lowbidgee diversions 0.05
Reduction in total diversions 0.4
Reduction in allocation 0.7
Flow reduction (Balranald) 2.6
Flow reduction (Wagga) 1.2
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Conclusions from Murrumbidgee

• Impacts of 30,000 ha in high impact water yield
  areas
• Overall the reductions are small
• Area planted is 0.4 of basin area and 1 of
  flow contributing area
• Average local impacts 5 of mean annual flow
• 1 mean annual flow reduction at Wagga Wagga
• Impact on total diversions is a 0.4 reduction
• Impact on end of system flows is a 2.6 reduction
• The system buffers impacts on irrigators
• Impacts extend outside of the Murrumbidgee

15
Assessing water access entitlements

• Zhang curves not good enough 30 variation
  about mean
• Other factors
• Forest management
• Access to groundwater
• Soil depth and type

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Groundwater use
Factor affecting groundwater use
Total water in (mm)
Water use (mm)
Site
Species
water table depth
614
612
water table gt 6m
SA
E. globulus
675
1059
water table lt 6m
SA
E. globulus
soil depthstocking
572
608
wide spaced
WA
E. globulus
813
608
1200 sph deep soil
WA
E. globulus
water table depth
667
661
water table gt 6m
SA
Pinus radiata
586
985
water table lt 6m
SA
Pinus radiata
soil type
530
457
heavy sodic soil
NSW
E. grandis
628
988
light soil
NSW
E. grandis
species ?
583
1331
light soil
NSW
C. maculata
species ?
438
580
contour belt
WA
Farm trees
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Stand density blue gums WA
White et al, unpubl.
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Manage for water and productivity Iron bark,
Victoria
Iron bark, Vic


Tivi et al, unpubl
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Water supply in forested catchments

• Most catchments of water supply reservoirs are
  predominantly native forest.
• Wildfires in 1939 have decreased stream flows to
  Melbourne by about 400GL (40) due to increased
  water use in just 65,000 ha of regenerating
  forest.
• Perth catchments have dense regeneration forest.

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Climate change Murray uplands

• The Murray Uplands (an example)
• 3.5 Mha of forests, which have
• 32,000 GL annual water use (evapotranspiration),
  delivering
• 8,000 GL annual stream flow from forests
• A 5 change in water use 20 change in stream
  flow
• 1,600 Gl

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Conclusions

• Plantation interception requires accurate
  assessment in terms of water sources and water
  access entitlements.
• Well targeted plantations in unregulated
  tributaries could have little impact on water
  allocations, and have possible salinity benefits.
• Need full assessments of water systems to avoid
  an unnecessarily restrictive approach to forest
  water interception.
• Need specific plantation scale assessments of
  reduced water yield not Zhang curves.
• Bigger issue is future yield from forested water
  supply catchments.
• Manage these forests primarily for water yield
  and water quality.
• Use forest management to mitigate effects of
  climate change and bushfires.

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Thank You
ian.prosser_at_csiro.au