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Barometric Efficiency. The observed influence of barometric pressure (BP) on water levels (WL) Defined using: BE = - WL / BP This method looks at long-term changes of BP on WL Another method accounts for short-term changes BE = - ∆WL/ ∆BP These assume that the response is instantaneous

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barometric efficiency
Barometric Efficiency
  • The observed influence of barometric pressure (BP) on water levels (WL)
  • Defined using:

BE = - WL / BP

    • This method looks at long-term changes of BP on WL
  • Another method accounts for short-term changes

BE = - ∆WL/ ∆BP

  • These assume that the response is instantaneous
  • Normally applied to confined aquifers
    • Because surface loading causes aquifer compression
    • Part of the load borne by water, rest by mineral skeleton
  • Also see a response due to trains, tides, precipitation
slide2
Tidal Efficiency, TE

TE = ∆TH / ∆BP

= ∆ (WL + BP) / ∆BP

= - BE + 1

  • so that

TE + BE = 1

  • Inelastic

BE = 1 TE = 0

  • Elastic

BE = 0 TE = 1

slide3
Specific storage (Ss)
  • Aquifer porosity (n)
  • Aquifer bulk modulus of elasticity (Es)
  • Specific weight of water (w) = 9.8 kPa/m
  • Bulk modulus of water (Ew) = 2.2 GPa
  • Bulk modulus is reciprocal of the compressibility
delayed response
Delayed Response
  • For situations where the response is not instantaneous, we must use convolution:

∆TH(t) = ∑i u(i) ∆BP(t-i)

= u(0) ∆BP(t) + u(1) ∆BP(t-1) + u(2) ∆BP(t-2) +

where u(i) is the unit response function

  • Delayed responses occur for various reasons
    • water table (unconfined)
    • wellbore storage
    • dual porosity
slide5

Effects of a barometric pressure step increase in an unconfined aquifer

Diagram of locations of measurement points 1 and 2 within the well and aquifer, respectively

pressure head

total head

water levels