5. Water flows from area of high
energy to low energy
A unit of water has
energy due to 3
factors:
– Elevation
– Pressure
– Velocity (not important
for groundwater)
6. Total Energy is
expressed as feet of
“head”
Head is equivalent to a
column of water so
many feet high
(convertible to
pressure by
multiplying by the
weight of water)
7. Darcy’s Law
a) French engineer
experimented with water
flowing through sand in mid-
1800s
b) He found that the amount
of flow was
– i) proportional to the area
through which the flow
occurred, and was
– ii) proportional to the hydraulic
gradient (the change in head
per length of flow path), and
– iii) related to the nature of the
material through which the
flow was occurring
8. Hydraulic Conductivity
Darcy’s K, a velocity, is
known as Hydraulic
conductivity
Darcy’s K is actually a
function of
– the liquid as well as
– the porous media (sand,
gravel, clay, etc)
9. Darcy’s Law can be
used to predict total
flow if the hydraulic
gradient and other
factors are known
Transmissivity:
hydraulic
conductivity times
the thickness of the
aquifer (Kb)
11. Water content of an Unconfined
Aquifer
porosity sets the
maximum volume
available for water
below the water table
porosity (n) = Specific
yield (Sy - the water the
aquifer will release) +
Specific retention (Sr -
the water not released)
12. Surface water & Groundwater
Interactions
A “gaining” stream:
receives water from the
groundwater (the
groundwater discharges to
the surface)
– The water table is higher
than the stream bed
A “losing” stream
recharges the groundwater
– The water table is lower than
the stream bed
13. Effect on the Water Table of
Pumping from a Well
a “Cone of Depression” is created
in the water table (potentiometric
surface) when a well is pumped at
a sustained rate
This can result in a shift in
direction of flow as the
potentiometric surface is changed
sub-surface flow patterns are a
function of basin thickness, water
table gradient, and geologic
complexity ( mixed hydraulic
conductivities)
14. Darcy’s Law can be
used to predict the
drop in level of the
water table due to
pumping from a well,
or system of wells
15. Drop Down
Under steady state
conditions at a given
pumping rate, and if K is
known
Theis in the 1930s
predicted the drawdown
in the water table at any
given time while the
Cone of Depression is
expanding
Values of W(u) can be
looked up in Table 9.1 for
calculated values of u
16. Capture Zone of Wells
The zone of influence of
a well is determined by
the slope of the
potentiometric surface
(water flows down
“energy”)
Dimensions of the
capture zone can be
calculated if the
characteristics of the
aquifer are known
(thickness, K, hydraulic
gradient, etc.)
