Pressure is what forces water out through the sprinklers or trickle emitters. Water pressure is created with the weight of the water itself or by mechanical means with a pump. The pressure at any point in a static column of water is simply the weight of the water above that point. For example, the column of water could be a water tower (discussed later) or tank of water on a hill. They create elevation above the point where water is used and in effect create a column of water that has weight and thus creates pressure. How this occurs will be covered next.
Pressure can be illustrated by using the common weight and volume relationships of water:
For example, using these relationships, stack twelve 1-inch cubes of water one on top of the other. These weigh 12 x 0.036 pounds or 0.433 pounds. The area under the column is one inch square (1 in. x 1 in.). The column exerts 0.433 pounds per square inch of pressure.
Because pressure changes with differences in elevation, two useful relationships to remember are:
A PSI to Feet Scale illustrates a scale for converting units of pressure and head.
Water Tower Example.
A municipal water system provides an illustration on maintaining water pressure in a large water system. If the top of a full water tank is 100 feet above the ground, the 100 ft. of water causes 100 ft. x 0.433 psi per ft. of column or 43.3 psi pressure at ground level. This is also expressed as 100 "feet of head." To fill the water tank, a pump must develop 43.3 psi or 100 ft. of head to lift water to the top of the water tank. Similarly, a water tower 150 ft. high would create 150 x 0.433 or 65 psi pressure at ground level. Look at Water Tower Pressure for a visual illustration of weight creating pressure.
If the water level drops slowly with demand, the height of the water column remains fairly constant to maintain the pressure. Also, a 500 gpm pump can be used to fill the tank and the large reservoir can service intermittent demands of 1 to 2,000 gpm.
A pump continually pushes water into one end of a pipe.
The small openings in sprinklers or emitters provide a release point for the water as more is pushed in. The release openings must be matched to the pumps capacity to push water into the pipe. Pressure is maintained while the restriction (sprinkler nozzle opening) allows the same amount of water to escape as is being pushed in at the beginning. If the pipe has a large opening, such as a break, water would flow through as fast as the pump pushed it into the pipe, but the pump would not create much pressure because there was no restriction to the flow.