The old gold miners back in the 1870's figured it out. The upper portion of the line needs to be large diameter to ensure enough feed. The lower portion needs to restrict the volume so you always have at least that known pressure point above where the two diameters join. If it's single diameter all the way down there's no telling where the head is. You'll likely have great pressure at the start but then the penstock can't keep up with the discharge and the pressure drops off.