Forest activities and Water Quality

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Forest activities and Water Quality

• What can we confidently say after 30 years of
  research?

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DOES LOGGING IMPACT UPON WATER QUALITY?

• 64,000 question that largely remains unanswered
• The question itself is not CONSTRUCTIVE - it
  offers no solution and no practical help to the
  industry

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TRADITIONAL APPROACH

• Paired catchment monitored before and after the
  period of major disturbance.
• Estimates of suspended solids or turbidity are
  used to infer increased sediment delivery to
  streams

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IMPORTANT LIMITATIONS

• Black-Box ie you can NOT relate the increase in
  suspended load at the catchment outlet to
  activities on the slopes
• Tells us nothing about where the sediment comes
  from and hence how to manage it

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NEW APPROACH- OPEN UP THE BLACK BOX

• For water pollution to occur, you need 3 key
  ingredients
• a source of sediment,
• delivered directly to the stream
• complete failure of any sediment control
  strategies or BMPs

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CAN WE BE CONFIDENT?

• BROAD GENERALISATIONS
• WELL ESTABLISHED
• LIMITED EVIDENCE
• SPECULATIVE

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SEDIMENT AND NUTRIENT SOURCES

• Well Established
• Unsealed forest roads are the major sources of
  sediment in managed forests.
• Road usage is a critical factor in explaining
  sediment production rates on roads.

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SEDIMENT SOURCES
Sediment Flux (g/m/s)
Time (secs)
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SEDIMENT AND NUTRIENT SOURCES

• Sediment production rates on roads and tracks
  decline within the time frame of 2- to 5 years
• Sediment yeilds from managed forests are lower
  than those from other landuses primarily
  agriculture

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RECOVERY OVER TIME
Snig
Track
Yield (kg/ m2)
Age (years)
0.04
0.03
Hillslope
Yield (kg/m2)
0.02
0.01
0.00
Age (years)
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SEDIMENT AND NUTRIENT SOURCES

• Nutrient concentrations in streams draining
  forested catchments are considerably lower than
  those reported for other landuses, primarily
  agriculture.
• The dominant cause of increased nutrients in
  streams if observed, is due to the effects of
  prescribed burning and wildfire.

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SEDIMENT AND NUTRIENT SOURCES

• Limited Evidence
• The GHA is not a significant source of sediment
  due to limited sediment availability, high
  retention of vegetation cover and spatially
  variable infiltration rates.

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SEDIMENT DELIVERY PATHWAYS

• The degree to which the source is connected or
  linked to the receiving waters

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Bridge
Partial channel linkage
Direct linkage
Infiltration
Full channel linkage
No channel linkage
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DELIVERY PATHWAYS

• Well Established
• Channelised pathways forming at road drainage
  outlets form the most efficient conduit for
  sediment and nutrient delivery to streams.
• Sediment delivery ratios are closely associated
  with the size composition of the in-situ and
  eroded soil.

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CULVERT
MITRE DRAIN
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Follow the Red Line
Major area of Impact
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The impact can be widespread

• The impact of channelised pathways extends beyond
  a single point or particular road drain and can
  be catchment-wide
• They also become permanent features of the
  landscape

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DELIVERY PATHWAYS

• Well Established
• Channelised pathways forming at road drainage
  outlets form the most efficient conduit for
  sediment and nutrient delivery to streams.
• Sediment delivery ratios are closely associated
  with the size composition of the in-situ and
  eroded soil.

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CULVERT
MITRE DRAIN
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Follow the Red Line
Major area of Impact
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The impact can be widespread

• The impact of channelised pathways extends beyond
  a single point or particular road drain and can
  be catchment-wide
• They also become permanent features of the
  landscape

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Drain discharge hillslope gradient
10
2.5
5.0
7.5
15
20
25
45
-
90
0
110
95
85
80
75
75
90
1
155
110
95
85
80
75
75
90
2
155
110
95
85
80
75
75
90
3
150
110
95
85
80
75
75
Maximum contributing length 95m for road gradient
of 5 degrees and DHG 7.5
90
4
125
110
95
85
80
75
75
90
5
100
100
85
80
75
75
95
90
6
90
90
90
85
80
75
75
Road travelway gradient
80
7
80
80
80
80
80
75
75
70
8
70
70
70
70
70
70
70
65
9
65
65
65
65
65
65
65
60
10
60
60
60
60
60
60
60
55
11
55
55
55
55
55
55
55
50
12
50
50
50
50
50
50
50
45
13
45
45
45
45
45
45
45
40
14
40
40
40
40
40
40
40
40
15
40
40
40
40
40
40
40
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200
100
80
60
50
40
30
20
10 year event for T 30mins
10
8
100 year event for T 30mins
6
5
5
6
8
20
30
40
50
60
80
200
10
100
Inter-bank length (m)
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EFFECTS OF BEST MANAGEMENT PRACTICES

• Well established
• BMPs play a significant role in the reduction of
  adverse effects in forested catchments.
• Forest buffer strips are an effective measure in
  reducing the volume of surface water and
  sediment/nutrients delivered to a stream.

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EFFECTS OF BEST MANAGEMENT PRACTICES

• Limited Evidence
• The best location and design of buffer strips in
  forested catchments of varying topography and
  landuse.
• The specific role and effectiveness of BMPs on
  the hillslope compared with those in the
  near-stream area.

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CONCLUDING REMARKS

• The Most Dramatic Impacts On Water Quality Will
  Come About In Response To The Most Dramatic
  Landuse Impact-
• Broad Scale Slash And Burn- Large Scale
  Conversion To Pasture