In the earlier part of this module we learned about creating pressure, learned that water flow in pipes was a factor of cross-sectional area of the pipe and the water velocity, and that a pressure loss occurred when water flowed through the pipe.
In the static situation with no water flow, pressure is created by a column of water, usually a water tower or a large difference in elevation (as a pipeline down a hill), and the pressure is the weight of the water on a surface. Since pressure is commonly expressed as pounds per square inch, we can imagine the column being one inch wide by one inch deep and many feet tall. The height of the water level in a water tower (elevated tank) above the ground is used to determine the pressure that can be created.
Pressure is commonly created by a pump but that is harder to visualize. The pump is designed to work in some range of water flow and pressure head. Work must be done by the pump to move some volume of water and be able to, in a sense, raise the water to some elevation (pressure head in psi or feet of head). Individual pumps are designed to work in some range of flows and some range of pressure heads.
In a static situation, the water flow carried by a pipeline can be calculated by multiplying the cross-sectional area by the planned water velocity and convert to gallons per minute or other units. Of course, friction loss charts that were discussed in an earlier section have the flow capacities for specific size and material pipelines at various water velocities.
In a dynamic situation of water flowing in a pipeline another factor comes into play and that is friction loss, a pressure loss (energy loss) caused by the water drag or turbulence against the pipe walls. Slow moving water can move over the roughness in the pipe walls without much drag. Increasing the water velocity increases the turbulence caused by the water flowing over the roughness of the pipe walls and thus increases the friction loss or pressure loss. At the end of the pipeline the operating pressure has been reduced by the friction loss to a lower pressure. Thus in a dynamic situation of a real irrigation system one can expect the pressure at any point in the pipeline to differ from other points. Pressure at sprinkler nozzles or drip emitters will also vary from one to another on a lateral line.