Geography 327      Hydrology

Runoff

• water flux over the earth's surface
• mostly streamflow since overland flow is very restricted in time and space
• much less variable in time and space than precipitation, because of the large storage capacity of vegetation, soil, rock and lakes (top 30 cm of soil holds more water than the overlying atmosphere)

Regional Runoff in Canada

• regional precipitation versus runoff is analogous to total water input versus effective precipitation

W_eff = W - losses

• W_eff : effective water input or effective precipitation
• W: total water input (measured precipitation)
• the losses are storage of precipitation in vegetation (interception), depression storage and the evaporation of this water

Sources of runoff

1. channel precipitation
   
   • direct interception of precipitation by streams
   • much less than 5% and usually less than 1% of runoff reflecting the area of surface water as a proportion of basin area, i.e., only important where there is a large area of wetlands or lakes or during floods
   • but it always occurs
2. overland flow
   
   • unconfined runoff, expressed as discharge per unit width of slope

   Hortonian

   • saturation from above
   • described by Horton in a series of papers in the '30s and '40s, where he suggested it was the principal, if not only, mechanism of overland flow and occurred on virtually all slopes
   • sheet flow or anastomosing rivulets move as a kinematic wave (independent of the force, e.g., like traffic jams moving at a different rate than the traffic)
   • it occurs when precipitation intensity exceeds infiltration capacity, i.e. only with
     
     • soil that is saturated with antecedent water or after a long duration rain
     • frozen ground
     • steep slopes and thin soil
     • hydrophobic or compacted soil
     • bare soil, because on vegetated surfaces, organic matter and biological activity contribute to higher hydraulic conductivity and thus infiltration capacity
       
   • in the 1960s, the partial-area concept was introduced, whereby Hortonian overland flow occurs in a limited contributing area that varies from basin to basin but is fairly constant within a basin, except for extreme events
   • then field studies demonstrated that Hortonian overland flow was far from ubiquitous and that more often saturation occurs from below

   saturation overland flow

   • saturation from below
   • water is returned to the surface by exfiltration
   • first identified for recharge of shallow groundwater, i.e., adjacent to humid region streams
   • it also occurs:
     
     • in hillslope hollows where groundwater flow lines converge
     • at slope concavities (i.e., base of many slopes); with decreasing hydraulic gradient, flow decelerates and therefore the depth of subsurface flow increases
     • in thin soils over an impermeable surface, e.g., rock, frozen ground, heavy soil horizon
     • where hydraulic conductivity decreases with depth and thus soil water becomes perched

   variable source area concept

   the area of saturation from below varies greatly with time, reflecting the overall wetness of the watershed

    

3. interflow
   
   • lateral movement of soil water between the ground surface and a regional or perched water table
   • can account for a significant proportion of the runoff in humid regions
   • in sloping landscapes, interflow may exfiltrate at the surface producing saturation overland flow

     matrix flow

     • "water-content-dependent conductivity anisotropy:" (Dingman), i.e., depends on antecedent soil water conditions and flow is preferentially lateral given greater lateral hydraulic conductivity than vertical
     • a common condition since the porosity and permeability of soil tend to decrease with depth given the weight of overlying soil and the translocation of material in percolating water

     macropore flow (rapid interflow)

     • soil water passing quickly to a stream through root canals, animal tunnels or pipes produced by subsurface erosion
4. groundwater flow
   
   • in the zone of saturation below a perched or regional water table
   • days, weeks or even years may pass before water that seeps to the water table eventually reaches a stream
   • in dry soil, infiltrating water largely replenishes soil water storage and thus does not contribute to groundwater recharge
   • however, some groundwater can reach a stream during or shortly after an input event via
     
     • translatory flow, when a belt of antecedent water is forced by new seeping water
     • a rapid local rise in the water table producing a groundwater mound or ridge near streams; the higher local hydraulic gradient produces a rapid flux of water from the streamward side of the mound; it may also cause saturation overland flow on the floodplains
     • a perched groundwater below a slope; these sloping slabs are not connected to the regional groundwater circulation; they thicken downslope (again producing saturation overland flow) and can contribute baseflow within days or less with intense rain and/or permeable sediments

Flood (event, storm, response) hydrographs

• measurement of stream discharge
• parameters to describe and quantify storm hydrographs: peak discharge, lag time, baseflow, storm flow, rising limb, receding limb (recession), groundwater recession
  

  unit hydrograph

  • a basic tool for evaluating the response of a watershed (area above a hydrometric station) to precipitation or snow melt
  • constructing by removing the baseflow and scaling the discharge data to represent the response to 25 mm of water input
  • for example, for the stormflow from 75 mm of rain, all the discharge data would be divided by three (75/25) and plotted as a unit hydrograph
  • these standardized hydrographs are then directly comparable among hydrometric stations and runoff events
• stream response to hydroclimatic events reflects the relative contributions of surface and subsurface runoff and thus all aspects of the physcial and cultural geography of drainage basins
• these controls on the flood hydrograph range from transient (storm characteristics) to permanent (basin physiography) with land use in between since it can be relatively stable (e.g., forest) or change annually (crop rotation)
• the influence of individual variables is fairly direct but then variables interact to create very complex relationships between controlling factors and stream response

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