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 = 1TE = 0 Elastic BE = 0TE = 1

Specific storage (S s ) Aquifer porosity (n) Aquifer bulk modulus of elasticity (E s ) Specific weight of water (  w ) = 9.8 kPa/m Bulk modulus of water (E w ) = 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