|This photo on the left shows the wet spots on the surface that mislead the gardener that the wet spot bulbs (spreads) out underground (as seen in the color illustration).
The wet spot on the surface of the soil is misleading. The wet spots
are at certain intervals, but don't represent what is going on. The
photo on the left shows the misleading wet spots. The drawing on the
right (from my book Drip Irrigation for Every Landscape and All Climates) shows that the wet spots beneath each emitter merge to form one continuous zone of moisture. The soil for the entire length of the drip irrigation in-line tubing is moist some four to six inches beneath the surface, depending on the soil type. Sandier soils require tubing with the emitters pre-installed 12 inches apart. While heavier, clayey soils only need drip irrigation emitters every 18-24 inches.
To learn more about how this works, check out my Drip Irrigation book.