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

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

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

Annual hydrographs - coast & interior Annual Peak Annual Peak

VFR Research - R. Hudson Seasonal distribution of precipitation T o t a l M o n t h l y P p t. ( m m ) T o t a l M o n t h l y P p t. ( m m ) J F M A MJJ A S O ND Upper Penticton Creek Russell Creek

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

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

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

VFR Research - R. Hudson Peak flow effects u Interior situation: peak flows driven by radiation snowmelt, all studies clearly show an increase and advancement in peak flows u 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 Changes in peak flows at H.J.A. u Under conditions of deep antecedent snowpacks and warm temperatures during rain-on-snow, increase in peak flow after logging 100% ranged from % –these changes mostly affected moderate peak flows u However, in some circumstances, peak flows were decreased due to logging

VFR Research - R. Hudson Changes in Peak Flow... u For larger events, the relative change in peak flows decreases u 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 Effects of roads u 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 Storm Hydrograph - Russell Creek Rising Limb Falling or Recession Limb

VFR Research - R. Hudson Components of streamflow u Baseflow –baseflow is flow supplied to a stream by groundwater flow through the porous matrix u 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 An example of baseflow separation Storm Runoff Baseflow

VFR Research - R. Hudson Significance of flow paths u 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, NO 3 ) 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 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 u 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 Significance –recovery of streamflow regimes to pre-harvest conditions must eventually consider the dominant hydrological flow paths operating in the watershed in question u 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 Significance u 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.