What is defined as the portion of the line under constant pressure between source and valves?

The mainline is defined as the portion of the irrigation system that maintains constant pressure between the water source and the valves controlling the flow to various zones or areas. This component is crucial for ensuring that water is delivered efficiently and uniformly to the system, as it acts as the primary conduit for carrying water under pressure. Properly sizing and installing the mainline helps to minimize water loss due to friction and pressure drops, which are critical for maintaining effective irrigation performance.

Understanding the role of the mainline is essential for an irrigation technician, as it influences the overall efficiency of the system and plays a significant part in how valves and laterals operate. The other options represent different components of an irrigation system, but they do not maintain constant pressure in the same way the mainline does.