Pressure varies along drip irrigation lines depending on elevation changes and slope. When lines are laid upslope, pressure decreases according to the formula PB=PA-∆H-∆S. When downslope, pressure increases as PB=PA-∆H+∆S. The loss or gain in pressure is proportional to the slope and length of the line. Design criteria for drip systems typically allow 10% variation in emitter flows for laterals and 10% for lateral flows in sub-mains, resulting in a 90% application efficiency. Nomographs can be used to design laterals based on length, operating pressure, slope, and desired pressure variation point.