Lecture14

Learning objectives

1.  List three forestry activities that can impact a watershed and explain how they influence the local hydrology.
2.  Explain how soil disturbances from forestry activities can alter runoff mechanisms in a watershed.
3.  Describe the effect of forest service roads on the drainage network.
4.  Provide an example of how logging practices can negatively impact water quality.
5.  List at least three solutions to reduce the impact of forest harvesting on water resources.
6.  Define with your own words, and put into context, all the terms in the glossary at the end.

 

Forestry Activities

• Road construction
  • For moving around equipment (harvest, planting, everything else)
• Felling, skidding, yarding
  • Cutting down tree
  • Remove tree
  • Sorting out logs; lots of compaction due to lots of moving equipment
• Vegetation removal
• Planting
• Chemical use

Physical effects

• Soil exposure and loss
  • Transport of soil particles from the surface by water and wind caused by a combination of:
    • Mineral soil exposed
      • Sheet erosion
      • Rill erosion
      • Splash erosion
        • From rain splashing
      • Gully erosion
      • Stream bank erosion
    • Soil compaction (lower infiltration rates)
      • Reduction of pore space and soil permeability
      • Caused by
        • Road construction
        • Machinery transit
        • Animals
      • Hydrologic effects:
        • Increased runoff (Hortonian overland flow)
        • Higher peak flows
        • Long term effects on site productivity
    • Soil instability
      • Landslides: processes that involve a transfer of slope materials from higher to lower ground by gravity, without the primary assistance of a fluid transportation agent.
        • Caused by:
          • Weak material
          • Saturation with water
          • Layered or fissured material
          • Vegetation changes
          • Road construction malpractice
        • Triggered by:
          • Rainfall, snowmelt
          • Thawing
          • Earthquakes
          • Erosion / excavation
          • Vegetation removal
        • Hydrologic effects:
          • Erosion (watershed stability)
          • Sediments & water quality
          • Stream blocking & change
          • Damming
        • Vegetation is important
          • After harvest, roots will slowly disintegrate.
          • Period of time between harvest and when new plants re-stabilize the terrain.
    • Precipitation
    • Runoff
    • Wind
  • Hydrologic effects:
    • Water quality (sediment loads)
    • Long term effects on site productivity
• Soil compaction
• Soil instability
• Drainage network changes
• Potential contamination

 

Roads

• Road cuts are very important
  • There is a natural slope for a material. When a cut is made too steep, the terrain will try to become the original slope again.
• Drainage network
  • Artificial increase of drainage network due to road construction and skidding (smaller roads for removing trees)
  • Certain road networks may reduce streamflow by diverting it to other watersheds.
• Roads are operationally very critical for two reasons:
  • Road effects are more permanent because they do not recover unless decommissioned (rarely done in practice)
  • Road effects become more critical for larger watersheds as the chances of road ditches interconnecting with stream network increases.

Potential contamination

Caused by:

• Pest control
• Fertilization
  • Nitrogen
• Industrial spills
  • Fuel, oil

Hydrologic effects:

• Water quality
• Groundwater

Hydrologic effects

• Water quantity
  • Nearly all harvesting activities result in increased streamflow:
    • Soil compaction, increased stream density, vegetation removal (less ET → more Q)
  • Magnitude, frequency, and duration of low, mean, and high flows must be evaluated
  • Must characterize watershed "behavior" in response to forest operations
    • Paved watersheds make it flashier
    • Forested watersheds make it calmer
• Water Quality
• Extreme events

Deterministic determinations do not invoke probability.

Guidelines of best practices

• Apply silvicultural methods that resemble the natural disturbance & regeneration strategy for particular forest types.
• Protect streams with no-harvest buffers (e.g., minimum 20 m to each side of the stream).
• Avoid land use practices that reduce infiltration capacity and soil permeability (DO NOT COMPACT SOIL)
• Minimize exposure of mineral soil / protect the soil
• Avoid logging on steep slopes
  • At the least, use proper equipment such as helicopters to prevent undue damage to the slopes
• Channelize runoff towards stable areas (avoiding steep & susceptible)
• Applying erosion control techniques and promote infiltration
• Minimize road density
  • Few roads as possible
• Use guidelines for proper road & drainage construction
• Carefully analyze soil susceptibility to erosion and landslides with maps of terrain stability.

Off topic ramblings

Cause ≠ effect; If we look at a single event, we can't say anything one way or another. We must look at multiple events to make any inference about anything.  Professor Lauri Daniels knows stuff about wildfire but she has no idea about watershed hydrology. Therefore she needs to stop talking about watershed stuff.

 

Supposedly Yunes is ignorant outside of hydrology. Turns out British Law used in the commonwealth, cause is determined via probabilistic methods. Convincing a judge about the watershed hydrology is much easier than convincing ignorant foresters. 

 

The deterministic determinations do not invoke probability. It is old reasoning. It leads to learned helplessness. Probabilistic thinking challenges old held ideas, therefore foresters are against it.