Encyclopedia of Environmental Science and Engineering, Volume I and II

GROUNDWATER RESOURCES 443

their coordinate axes parallel as shown in Figure 4, the

values of W ( u ), u, s, and r^2 / t may be obtained for the center

of the matching segments. Then, T and S can be calculated

as follows:

T

Q

s

Wu S

uT

rt

4 

4

p () and ()^2

The Jacobs Method

This method involves a simplification of the well function.

Assume that a test well is being pumped at a rate Q with two

observation wells located at r 1 and r 2 in a radial direction

from the test well. As the test well is pumped, the drawdowns

and times of drawdowns are observed for the two observation

wells. The difference of drawdowns between the two obser-

vation wells can be obtained as follows:

ss

Q

T

Tt

rS

Tt

rS

Q

T

21 10

2

2

2 10

1

1

2

23

4

03 03

23

4


 

.

log

.

log

.

.

p

p

⎛

⎝⎜

⎞

⎠⎟

llog 10 1

2

1

2

2

2

rt

rt





(5)

where the subscripts 1 and 2 refer to the observation wells.

Now, if there exists only one observation well (i.e., r 1 
 r 2 ), the

subscripts now refer to the time at which observations are made

and recorded.^ Then the above equation can be reduced to:

ss

Q

T

t

(^2110) t
2
1
23
4


.
log
p
(6)
The above equation can be used to evaluate T from field data.
For a constant Q, drawdown versus time can be plotted on
a semi-log scale as shown in Figure 5. If t 2 / t 1 = 10, then the
transmissivity, T , can be calculated as follows:
T
Q
ss



23
(^421)
.
p()
(7)
If it is assumed the drawdown began at some time, say t 0 , the
storage coefficient, S, can be determined as
Center of Matching
Curve Segments
W(μ) ∼ μ
W(μ)
μ
r^2 /t
S ∼ r^2 /t
S
FIGURE 4 Application of Theis method.
t 0 0.01 0.10 1.00
Time of Drawdown in Days (Log Scale)
Drawdown S in Feet (Linear Scale)
s 2 –s 1 = ∆s
1 log cycle
FIGURE 5 Application of semi-log drawdown curve—Jacobs method.
C007_003_r03.indd 443C007_003_r03.indd 443 11/18/2005 10:28:24 AM11/18/2005 10:28:24 AM