Pressure variations occur in all systems. Two factors affect pressure: elevation and friction. Elevation can add to the pressure if the tubing is running downhill or reduce it, if the tubing is running uphill. If the highest point in the system is not more than 10 vertical feet above the control valve, and pressure compensating emitters are used, then the pressure difference is within the acceptable range. With friction, pressure is always lost as water travels through the tubing. Friction is greater at high flow rates and in smaller diameter tubing.
Tubing should be sized properly to keep pressure loss due to friction within acceptable limits, while keeping the cost of the system down. There are many different sizes of polyethylene hose. The most common, however, is 1/2” (15 mm) which is used mainly as the supply line and can handle flow rates up to 200 GPH (or less than 3.5 GPM). 1/4” (4 mm) tubing is usually used to branch off the supply hose to carry water to any area that is too hard to reach with the supply hose. Because of its small size, it should not be asked to handle more than 8 GPH (or (8) 1 GPH emitters).
Most drip irrigation systems require little in the way of design beyond the most practical or aesthetic considerations. Remember that you can add to a system if flow rates allow. If not, dividing the system into two or more sections is also relatively simple. No matter how large or small your system is, the scale drawing is crucial in determining your needs.