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

• 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

• 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

• 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

Effects of a barometric pressure step increase in an unconfined aquifer

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

b) pressure head

c) total head

d) water levels