- Specific yield (Sy)
The specific yield of an unconfined aquifer is the ratio of volume of water which will flow under saturated condition due to gravity effect to the total volume of aquifer (v).
Sy = Vwy/V where, Vwy = Volume of water yielded under gravity effect and V = total volume of water.
- Specific retention: The specific retention of an unconfined aquifer is the ratio of volume of water retained against gravity effect to the total volume of aquifer (v).
SR = VWR/V where, VWR = Volume of water retained under gravity effect.
Coefficient of transmissibility
T = kH where, H = Thickness
k = Coefficient of permeability
- Theims Theory
h1 + s1 = h2 + s2
where, q = Rate of flow in m3/s
h1 = Height of water table of 1st observation well
h2 = Height of water table of 2nd observation well
s1 = Drawdown of 1st test well
s2 = Drawdown of 2nd test well.
r1 = and r2 are radius of 1st and 2nd observation wells respectively.
- Dupits Theory
R = 3000.S√K and S = H - h
Where, S = Drawdown in the well
k = Permeability coefficient in m/s.
R = Radius of influence in 'm'
150m ≤ R ≤ 300m
r = Radius of tes well in 'm'.
Results of dupits theory are not accurate because 'R' is based on empirical relation.
- Theims theory where, b = width
- Dupits theory
Spherical flow through well
qs = K.2πr.s where, r = Radius of well
S = Drawdown
qs = Rate of flow through spherical well in m3/s qs = 1/30.qradial flow
- Open end test
K = q/5.5rh where, r = Radius of pipe
h = Head of water above the base of pipe, it may include gravity head and pressure head.
- Tacker test
K = q/(2πLh)log10(L/r) ... when L > 10r
where, L = Length of perforated section of pipe
k = q/(2πLh)sin - 1(L/2r) ... when L < 10 r
r = Radius of pipe
h = Head of which water is added.
Open well (Recuperation test)
q = (C/A).Volume
Where, C/A = (2.303/T)log10(h1/h2)
C/A = Specific yield or specific capacity of an open well.
T = Time in 'sec'
h1 = Position of water table of t = 0
h2 = Position of water table of t = T
Value of Permeability