Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 23

Fig. 1.7 Schematic of fluid
flowing in a pipe

ρ Q e

ρ Q 1

This is the Continuity equation.

Bernoulli's Equation/Energy Equation (Conservation of Energy)

In reference to Fig. 1.7 consider that the fluid moves from the inlet to the outlet (over
the length L ) in time t .

Bernoulli's equation says that total energy per unit weight of flowing fluid is
constant over the flow section. That is,

δ

V 1 V 2 p 1

ρ

p 2
ρ g + 2 g +

z 1 =

g + 2 g +

z 2 = H = constant (1.15)

The dimension of each term of the equation is “meter” (m). Each term is called as
follows:

p
ρ g =

pressure head

V 2

2 g =

velocity head

=

z potential head or elevation head (with respect to a reference datum)

1.3.2 Water Pressure - Static and Dynamic Head

Water pressure at a certain depth of a water column is equal to the weight of water
above that point. Water pressure is normally expressed as “feet head” or “Ibs/in 2 ”
(PSI). Pressure,

P = ρ h (1.16),

where

h = height of water, ft

density of water, Ibs/ft 3 ρ =

pressure, Ibs/ft 2 (PSI) P =

Conversion

Head (ft) × 0.433 = PSI

PSI × 2.304 = head (ft) of water