system total dynamic head can be expressed as:
Ht = H. + H + H + H + H (4) When the water is to be pumped from the well it
where: is important to know the drawdown to account for
the additional lift. For a surface water source such
Ht = total dynamic head of the system (TDH) as lake or river this water level may drop during a
H = static head (static lift + static discharge) dry season. Any changes in static lift must be ac-
"Hd= drawdown counted for in the static head portion of the total
H. = operating head system head.
H, = friction loss head
H, = velocity head Operating head
Statc h d Some irrigation systems require pressure to
operate. The range of this pressure varies among
Static head is the vertical distance from the wa- systems. High pressure systems, such as traveling
ter level at the source to the highest point where guns and high pressure center pivots or sprinkler
the water must be delivered. It is the sum of static systems, may require large operating pressures (up
lift and static discharge. Static head is indepen- to 100 psi). Micro irrigation systems can operate at
dent of the system discharge (gpm) and is constant much lower pressures (8-30 psi). For gravity
for all discharge values. However, it is possible irrigation systems (furrow, flood or open ditch
that the static head may vary with time due to the subirrigation) the operating pressure can be close
changes in the system, to zero.
Static lift For most irrigation systems, the operating
The static lift is the vertical distance between pressure is constant. However, some systems may
the center line of the pump and the elevation of the have a variable operating pressure. A good
water source when the pump is not operating. If the example is a center pivot system with an end gun
water elevation of the source is below the pump el- for corner irrigation. Operating the gun requires
evation, the static lift is positive. If the pump is lo- additional pressure head for a relatively short time.
cated at the elevation below the water surface el-
evation, the static lift is negative.
Friction loss
Static discharge
Static discharge When water flows through a pipe there is a loss
The static discharge head is a measure of the el- of head due to friction. This loss can be calculated
evation difference between the center line of the using hydraulic formulas or can be evaluated using
pump or top of the discharge pipe and the final friction-loss tables, nomographs, or curves provided
point of use. When pumps discharge directly into by pipe manufacturers. The pump must add
canals a short distance from the pump at the same energy to the water to overcome the friction losses.
elevation, the static discharge head is zero. If, how- As the discharge of the system increases the
ever, a pump supplies water to some distant point velocity also increases. The friction loss increases
at another elevation, then it is necessary to com- as the square of the flow velocity. Due to the high
pute the static discharge head. To obtain this value, cost of energy, it is often recommended that a
subtract the elevation of the pump or discharge larger pipe size be used to decrease the velocity for
pipe from the elevation of the final point of deliv- the same discharge. This is usually economically
ery. feasible if the water velocity is more than 5 ft/sec.
Well drawdown
For a system having very long pipelines or
As a well is pumped the water level in the well undersized pipe for a given flow rate, the friction
declines. This phenomena is commonly called the loss can be very significant.
well drawdown. The amount of the drawdown is a
function of the pumping rate, the aquifer proper- Friction losses must be considered on both the
ties, well size, method of construction (well screen, intake and discharge sides of the pump. It is
etc.) and the time the pump is operated. The best especially necessary to compute or evaluate the
way to determine the well drawdown is to test friction loss on the suction-side of centrifugal
pump a well at various rates and observe the draw- pumps to assure enough net positive suction head
down. Testing of wells is described in detail in IFAS available (discussed below) to prevent pump cavita-
Extension Circular 803 "Water Wells for Florida tion.
Irrigation Systems."
