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Water-Resources: Water Properties & Definitions

A Water Lily
Water Lily on a Lake (Photo: ref.)

This section is meant to provide some background information on Water Engineering and it's terminology.

Fluid Statics
  1. Useful Conversions
  2. Characteristics of a Fluid
  3. Definitions
  4. Water Properties

Useful Conversions

Characteristics of a fluid:

These characteristics can be used to distinguish between liquids and gases if necessary:


Bernoulli's equation
The Bernoulli's Equation describes the behavior of moving fluids along a streamline.
flow rate, usually in ft3/sec or gpm
Hydraulics is a branch of science and engineering concerned with the use of liquids to perform mechanical tasks
Ideal Gas
The Ideal Gas Law - For a perfect or ideal gas the change in density is directly related to the change in temperature and pressure as expressed in the Ideal Gas Law.
Reynolds number
The Reynolds number is used for determine whether a flow is laminar or turbulent.



Density (ρ) is mass per unit volume (lbm/ft3, kg/m3, slugs/ft3)


\rho = \frac{mass}{volume}

\rho_{slugs} = \frac{\rho_{lbm}}{g_c}

Using the ideal gas law, the density of gases can be found by:

\rho = \frac{p}{RT}


ρ = fluid density (lbm/ft3, kg/m3, slugs/ft3)
gc = 32.2 \frac{lbm-ft}{lbf-sec^2}
p = pressure ( From pV = nRT)
R = Specific Gas Constant
T = Absolute Temperature

Specific Weight (aka Unit Weight)

Specific Weight (γ) is weight per unit volume (lb/ft3) = ρg


\gamma = \frac{weight}{volume} = \rho g = \rho* \frac{g}{g_c}


ρ = fluid density (lbm/ft3, kg/m3, slugs/ft3)
g = gravitational acceleration (ft/sec2)
gc = 32.2 \frac{lbm-ft}{lbf-sec^2}

Note: In the United States specific weight is also known as density.

Specific Gravity

Specific Gravity (S) is the ratio of the weight (or mass) or a substance to the weight (or mass) of water.

S = \frac{\rho_s}{62.4} = \frac{\rho_s}{\gamma_w} = \frac{\rho_s}{\rho_w}

where the subscript s denotes of a given substance and the subscript w denotes of water.

Given S of a substance, you can easily calculate ρ and γ using the equations

\gamma_s = S*\gamma_w

\rho_s = S*\rho_w

Specific Volume

Specific Volume is the volume per unit mass (usually in ft3/lbm). Specific Volume is the reciprocal of density.

Specific Volume:

\nu = \frac{1}{\rho}


ρ = fluid density (lbm/ft3, kg/m3, slugs/ft3)


Viscosity (υ) is a measure of a fluid's ability to resist shearing force (usually in lbf·sec/ft2). υ is a proportionality constant and is also known as absolute viscosity or dynamic viscosity. Viscosity is often given in the SI unit of Poise. To convert to English units use the relationship: 1 \frac{lb*sec}{ft^2} = 479 Poise


\frac{F}{A} \propto \frac{dv}{dy}

\frac{F}{A} = \mu\frac{dv}{dy}


F = Force
A = Area
y = thickness
v = velocity
μ = coefficient of viscosity

Kinematic Viscosity

Kinematic Viscosity:

\begin{eqnarray} \nu &=& \frac{\mu}{\rho} \; \; \; \mbox{ (SI)} \\ &=& \frac{\mu g_c}{\rho} \; \mbox{ (US)} \end{eqnarray}


γ = kinematic viscosity (ft2/sec)
υ = dynamic viscosity (lbf·sec/ft2)
ρ = mass density (slugs/ft3)

Note: See CERM Appendix 14.A (page A-13) for a listing of water properties at various temperatures.

Absolute and Gauge Pressure

Absolute Pressure (psia) = Atmospheric Pressure + Gauge Pressure (psig)

Atmospheric Pressure changes with weather conditions and altitude though is usually assumed to be 14.7 psi at sea level. Pressures are usually given in psig, except for compressible flow.

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Page last modified on March 11, 2011