Geography 327      Hydrology

Groundwater

• saturated zone (phreatic) zone
• water under +ve (> atmospheric) pressure
  
• 30% of fresh water
• 99% of liquid fresh water
  
• average residence time of about 235 years, but varies from a few years to 1000s years for regional flow systems
• recharge - discharge = changes in storage (changes in the elevation of the water table)

recharge (input)

mostly percolation from the unsaturated zone, but also seepage from surface water bodies

discharge (output)

main source of water in rivers, and lakes; also diffusion into the capillary fringe

Groundwater flow: Darcy's law for saturated flow

• assumes laminar flow which is generally the case for porous media except marcopores, fractures or solution cavaties
  
  Vx = Q/Ax = K_sat * dh/dx
  
  • where, Vx = volume rate of flow (discharge) per unit area
  • K_sat = saturated hydraulic conductivity
  • h = total hydraulic head, mechanical energy per unit mass of fluid or the gradient of potential energy that induces flow
  • h = z + p/yw
  • z = gravitational head, elevation above and arbitrary datum
  • p/yw = pressure head
  • total head is measured with a piezometer, a tube extending between the water table and the surface

Distribution of groundwater

aquifer

a geologic unit with high porosity and permeability so that it stores and transmits water at a rate that is hydrologically significant

unconfined aquifer

• upper boundary is water table at atmospheric pressure
• thus total head is only elevation of water table above a datum, which is a function of recharge over a large part of the aquifer, discharge and variable rates of flow

confined aquifer

• bounded above and below by formations with significantly lower hydraulic conductivity (aquicludes)
• water is under pressure so in a well that penetrates the aquifer the water rises above the upper boundary to the potentiometric surface (if this above the ground surface, the flow is artesian)
• recharge is over a small area at the upflow end of the aquifer where it is exposed to the surface and there is a water table
• there are degrees of confinement, in the extreme case the boundaries of the flow do not change and the aquifer is analogous to a pipe

Patterns of groundwater flow

flow nets

• two-dimensional vertical slices (cross-sections) through unconfined aquifers
• constructed by solving for the hydraulic head at a number of points and then interpolation of equipotential lines for a set of boundary conditions (i.e., geologic and topographic conditions that govern the configuration of the water table)
• the flow lines or streamlines are perpendicular to the equipotentials
• the idealized pattern of regional flow assumes a static water table (no recharge or capillary rise), but is applicable if fluctuation in the water table are small compared to the size of the unconfined aquifer
• the water table is "a subdued replica of the ground surface", the higher the hydraulic conductivity the more subdued the replica
• the idealized flow net has a head gradient from surface divides towards streams with downward flow and upward/converging flow, respectively
• flow is away from the water table in recharge areas and towards the water table in discharge areas (analogous to accumulation and ablation zones of a glacier)

  topographic control

  • produces local flow systems, local recharge-discharge systems that circulate above an intermediate system where flow paths are longer but not as long as the regional system where discharge and recharge areas are the furthest distance apart
  • groundwater divides are closer to discharge areas such that these areas typically represent about 30% of a drainage basin

  geologic control

  • the lithology, stratigraphy and structure of rocks and sediments, that is the distribution of aquifers and confining beds, determines the directions and rates of groundwater flow
  • these geologic conditions combine in a multitude of ways which can be represented by spatial differences in the relative magnitudes of saturated hydraulic conductivity as a function of properties of the medium and the fluid
  • but the most significant factor is particle size (lithology), it accounts for a million-fold variation in conductivities, whereas as variations in the density and viscosity of water account for only a 100-fold variation in conductivity
  • in the saturated zone, the degree of saturation is equal to the porosity
    
    • primary porosity is the original proportion of void space
    • secondary porosity includes the void space that develops with fracturing or solution or that is filled by precipitation and mineralization

  Groundwater flow in southeastern Saskatchewan

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  Groundwater and surface water Links

  • groundwater is the source of nearly all baseflow except for water draining from lakes and wetlands and delayed interflow
  • contribution of baseflow is the basis for the common classifications of streams:
    
    • perennial: always baseflow
    • intermittent: seasonal baseflow (usually gaining)
    • ephemeral: event streams (usually losing)

    gaining (effluent)

    water surface slightly below water table, Q increases downstream
    

    losing (influent)

    water table slopes away from stream or the stream is perched, Q decreases downstream
    

    flow-through

    channel transects a sloping water table, so there is discharge on one bank and recharge on the other

  bank storage

  • during a flood wave, the stream surface rises above the water table reversing the local hydraulic head and inducing flow into the bank
  • after the flood wave has passes and with falling stage, the groundwater stored in the bank returns to the stream
  • this temporary storage subdues the hydrographic response to water input events

  lakes and wetlands

  • nearly always are sites of discharge
  • small lakes for local flow systems and large lakes for regional flow systems
  • although lakes can be any combination of discharge, recharge or flow-through with the complexity of geology and topography
  • prairie potholes capture snowmelt and then recharge the groundwater producing local water table mounds; in summer, trees growing on the perimeter of sloughs (willow rings) discharge groundwater by transpiration

  
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