Soil-Limiting Flow from Subsurface Emitters.  I: Pressure Measurements

Subsurface drip irrigation has become a common method for the irrigation of field crops, trees, and landscaping. When the predetermined discharge of the emitter is larger than the soil infiltration capacity, water pressure at the dropper outlet increases and can become positive. This pressure buildup in the soil decreases the pressure difference across the dripper and, subsequently, decreases the trickle discharge in a manner that depends on the dripper characteristic curve. A device was developed to measure simultaneously the pressure difference at the inlet and outlet of a subsurface emitter and the emitter discharge. In a preliminary study, discharge rate declines of 10–50% were measured for unplugged subsurface emitters. The extent of the discharge decrease due to back pressure depends on: (1) the soil type (the lower the hydraulic properties the larger the decrease); (2) the dripper discharge (larger decreases occur for higher nominal discharge); (3) possible cavities near the dripper outlet (a larger cavity decreases the back pressure); and (4) the drip system hydraulic properties. The increase in back pressure is rapid at the beginning and then approaches a final value after several minutes, which allows the use of an analytical approximation that assumes steady-state conditions.