HYDROGEOLOGY LECTURE NOTES
HYDROGEOLOGYLECTURENOTES(SPRING2004).DOC
PRINTED ON 8/2/2005
7
Chapter 2 – Fluid Potential and Kinetic Energy
Introduction
One of the fundamental equations that govern groundwater flow is called Darcy’s Law
(equation 21),
Q = K A
dh
/
dl
(Equation 21)
where:
Q = discharge [L
3
/T]
A = cross sectional area [L
2
]
K = hydraulic conductivity [L/t]
dh
/
dl
= hydraulic gradient or change in hydraulic head (h) per change in
distance (l) [ · ]
In plain English, this equation states:
Discharge (i.e., volumetric flow) through a cross sectional area is
proportional to the hydraulic gradient, and the constant of
proportionality is a quantity called the hydraulic conductivity
We will get into the details of the equation later in the course, but for now, let’s focus on the little “h” in the
hydraulic gradient term, i.e.,
hydraulic head
.
Fluid Energy
In simple, everyday terms, we think of hydraulic head as an elevation. More specifically, hydraulic head is the
elevation of water in a
manometer
in a pressurized water pipe (Figure 21), or in a
piezometer
(Figure 22).

A
manometer
is a vertical tube in a pressurized water pipe used to measure pressure in the pipe.

A
piezometer
is a vertical tube with an open or slotted interval (usually called the screened interval
or just the screen) inserted into the ground and used to measure hydraulic head in an aquifer; it is
basically a well constructed for the sole purpose of measuring groundwater levels.
Figure 21. Pipe of flowing water with manometers
showing the loss of head along the flow path
(© Uliana, 2001)
Figure 22. Cross section of aquifer showing
hydraulic heads in three wells (© Uliana, 2001)
In actuality, the water levels are a manifestation of the fluid energy in a groundwater system, and measuring a
water elevation in a well is really a measurement of the energy present in the fluid at a certain point in a
groundwater system (Figure 22).
An aquifer is a dynamic system of flowing water.
The fluid energy varies throughout the system and is
different from one point to the next.
The result is that we find different water levels throughout the aquifer.
These different water levels result in hydraulic gradients, and water movement is driven by those gradients
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HYDROGEOLOGYLECTURENOTES(SPRING2004).DOC
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What do we mean by “energy”?
Everything in the universe has some amount of energy associated with it, and that energy is present in various
forms. Some sort of energy drives every natural process, and the key to understanding physical processes is in
understanding the distribution of energy in a system.
Potential Energy
: energy stored in a piece of matter or at a point in a system; generally associated
with position or with thermodynamics of the system (elevation, pressure, chemical, thermal)
Kinetic
: energy associated with motion (velocity)
At every point in an aquifer, the fluid possesses some total amount of energy that is the sum
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 Fall '10
 ROOT
 Thermodynamics, Hydrogeology, Energy, Kinetic Energy, Hydraulic head, fluid energy, Hydraulic Potential

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