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Basic Hydrology

Basic Hydrology. Streamflow: Hydrographs; Case studies of logging effects on streamflow; Peak flow. Streamflow Hydrographs. A hydrograph is a graph of stream discharge over time A hydrograph can cover any time period from an individual storm to a year or more Hydrographs can be used to show:

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Basic Hydrology

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  1. Basic Hydrology Streamflow: Hydrographs; Case studies of logging effects on streamflow; Peak flow VFR Research - R. Hudson

  2. Streamflow Hydrographs • A hydrograph is a graph of stream discharge over time • A hydrograph can cover any time period from an individual storm to a year or more • Hydrographs can be used to show: • peak flows • runoff volumes • runoff components and processes VFR Research - R. Hudson

  3. A storm hydrograph Peak Flow 30 ) s / 3 20 Volume of Flow m ( Q e g 10 r a h c s i D 0 Time (days) VFR Research - R. Hudson

  4. Annual hydrographs - coast & interior Annual Peak Annual Peak

  5. Seasonal distribution of precipitation 200 ) m m Upper Penticton Creek ( . t p P 100 y l h t n o M l a t o T 0 400 • Russell Creek ) m 300 m ( . t p P 200 y l h t n o 100 M l a t o T 0 J M M J J S N D F A A O VFR Research - R. Hudson

  6. What do studies show re: logging effects on streamflow hydrographs? • Volume of flow • Peak flow - increase or decrease • interior - radiation snowmelt • coast - rain on snow • Timing of peak flow • interior vs. coast • clear-cut logging vs. roads VFR Research - R. Hudson

  7. Results of case studies • Volume of flow • studies are almost unanimous in showing an increase in flow volume due to forest harvesting • Interior BC: Cheng found 26% increase in annual yield after logging 30% of area • Oregon: Hicks et al found 31% increase in annual yield after logging 100%, 9% increase from 25% logged • Why the difference between coast & interior? VFR Research - R. Hudson

  8. Other water yield studies (interior) Changes in annual yield, Rocky Mountain basins * effect of opening size VFR Research - R. Hudson

  9. Effect on peak flow • Interior: changes in magnitude & timing of annual peak flow: VFR Research - R. Hudson

  10. Peak flow effects • Interior situation: peak flows driven by radiation snowmelt, all studies clearly show an increase and advancement in peak flows • Coast: peak flows driven by rain or rain-on-snow, processes much more complex, studies are much less clear on the effects of logging on peak flows VFR Research - R. Hudson

  11. Changes in peak flows at H.J.A. • Under conditions of deep antecedent snowpacks and warm temperatures during rain-on-snow, increase in peak flow after logging 100% ranged from 25 - 45 % • these changes mostly affected moderate peak flows • However, in some circumstances, peak flows were decreased due to logging VFR Research - R. Hudson

  12. Changes in Peak Flow... • For larger events, the relative change in peak flows decreases • Jones and Grant analysis suggests a change in peak flow due to harvesting and/or roads, but the magnitude of the change is comparable to the magnitude of the variability • is it significant? • why is it so hard to detect peak flow changes for coastal watersheds? VFR Research - R. Hudson

  13. Effects of roads • Logging roads at H.J.A. caused an increase in peak flow and advancement of the peak • more efficient routing of water to stream channel through effective increase in channel network density • no change in flow volume • more direct link to peak flow increase than harvesting alone in coastal watersheds VFR Research - R. Hudson

  14. Storm Hydrograph - Russell Creek Falling or Recession Limb Rising Limb VFR Research - R. Hudson

  15. Components of streamflow • Baseflow • baseflow is flow supplied to a stream by groundwater flow through the porous matrix • Runoff • runoff is water that reaches the stream channel to quickly to arrive there by groundwater flow that is governed by Darcy’s Law • there are many mechanisms by which runoff can occur VFR Research - R. Hudson

  16. An example of baseflow separation Storm Runoff Baseflow VFR Research - R. Hudson

  17. Significance of flow paths • Water chemistry • different chemical components are associated with different flow paths • base cations (e.g., Ca, Na, Mg, K, …) are derived from weathering in mineral soil - are associated with matrix groundwater flow • acidic ions (e.g., Al, H, NO3) are derived partly from organic horizons • rain can be acidic, so flow that bypasses the soil matrix can contribute of acidification of sfc. water VFR Research - R. Hudson

  18. Significance • flows of unbuffered acidic water into lakes and streams can cause pH depressions that can be fatal to fish • there can be implications to domestic water quality • Assessment of hydrologic recovery • consideration of only snow or rain recovery as a function of regeneration growth is an oversimplification of true hydrologic recovery VFR Research - R. Hudson

  19. Significance • recovery of streamflow regimes to pre-harvest conditions must eventually consider the dominant hydrological flow paths operating in the watershed in question • Peak flow estimation • there are many formaulae to determine peak flows based on the Horton overland flow theory that may yield wildly erroneous results if used at face value VFR Research - R. Hudson

  20. Significance • Erosion and slope stability • since overland flow is of limited significance in B.C. forests, clear-cuts themselves tend not to be sources of increased sedimentation if yarding damage is minimized • instead, subsurface flow paths are of key concern in landslide initiation. • understanding of subsurface flows is necessary to understand how forest land management might influence slope stability. VFR Research - R. Hudson

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