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  
Useful Conversions
 g_{c} = 32.2 ft/sec^{2} [lbmft/lbfsec^{2}]
 ρ_{water} = 1.96 slugs/ft^{3}
 γ_{water} = 62.4 lb/ft^{3}
 1 ft^{3}/sec = 449 gpm
 1 mgd = 1.547 ft^{3}/sec
 1 foot of water = 0.433 psi
 1 psi = 2.31 ft of water
 1 ft^{3} = 0.13368 gal
 101.33 kPa = 1 atm
Characteristics of a fluid:
These characteristics can be used to distinguish between liquids and gases if necessary:
 Compressibility : Fluids are very slightly compressible. For all intents and purposes they can be considered incompressible (fill up a small water balloon and squeeze it to test this)
 Shear Forces : Liquids (and gases) are unable to support any shear forces. They will constantly strive to find the path of least shear forces.
 Shape & Volume : Since fluids cannot transfer shear forces, they will form to the container they reside in. Although their volume during this will be fixed (unlike a gas which will expand to fill the container a fluid's volume cannot change).
 Pressure : the pressure in a fluid will be the same in all directions. One specific example is the pressure of a fluid transfered onto a surface will always be normal to that surface.
 Resistance to Motion : Due to a liquids viscosity, liquids will resist an instantaneous change in velocity.
Definitions
 Bernoulli's equation
 The Bernoulli's Equation describes the behavior of moving fluids along a streamline.
 Discharge
 flow rate, usually in ft^{3}/sec or gpm
 Hydraulics
 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.
Properties
Density
Density (ρ) is mass per unit volume (lbm/ft^{3}, kg/m^{3}, slugs/ft^{3})
Density: \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}

where:
g_{c} = 32.2 \frac{lbmft}{lbfsec^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/ft^{3}) = ρg
Density: \gamma = \frac{weight}{volume} = \rho g = \rho* \frac{g}{g_c}

where:
g = gravitational acceleration (ft/sec^{2})
g_{c} = 32.2 \frac{lbmft}{lbfsec^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.
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
Specific Volume
Specific Volume is the volume per unit mass (usually in ft^{3}/lbm). Specific Volume is the reciprocal of density.
Specific Volume: \nu = \frac{1}{\rho}

where:
Viscosity
Viscosity (υ) is a measure of a fluid's ability to resist shearing force (usually in lbf·sec/ft^{2}). 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
Viscosity: \frac{F}{A} \propto \frac{dv}{dy}
\frac{F}{A} = \mu\frac{dv}{dy}

where:
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}

where:
υ = dynamic viscosity (lbf·sec/ft^{2})
ρ = mass density (slugs/ft^{3})
Note: See CERM Appendix 14.A (page A13) 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.