5.4 Nozzle or Emitter Discharge Rate

Total water application onto a management unit can be determined using:

Total water applied (gal) = nozzle discharge rate (gal per min) x time (min) water is applied x number of nozzles

Be careful of your units of measure.

The nozzle or emitter discharge can be measured for a minute to get the discharge rate in gallon(s) per minute. Better yet, collect the discharge from several nozzles for a minute each, record the values, sum them, and divide the total amount by the number of nozzles to get a representative average.

Measuring at the mid-point of a lateral will give a more representative or average value. It is difficult to be very precise in the field.

A container cut to slip over the nozzle to direct the water into a second larger container without splashing is recommended. A five-gallon container can be used to collect the discharge from a nozzle for one minute. Individual drip emitters will have a very low discharge rate and a small container (cup) will work well. In this method of checking application uniformity, the variation in nozzle/emitter discharge on a lateral should not exceed 10 percent from one end to the other end. This method checks for nozzle discharge uniformity rather than for on-the-ground application uniformity (the catch can method). Variations of more than 10 percent might suggest that there is more pressure loss than desirable resulting in more discharge variation. Pressure causes the amount of water discharge.

The discharge will vary from one end of a lateral to the other. It is good to check the discharge from nozzles at both ends of the lateral line to find the amount of variation. Also, check the pressure if possible and record both pressure and discharge rate data.

Charts are available to show the normal discharge of a nozzle at a given pressure. A test done in the field will not be comparable to catalog rates because the pressure at the base of the nozzle may not be known accurately. Also, the timing on the capture of water for just one minute may not be done very accurately. The bottom line is that a group of worn nozzles have larger diameters than new nozzles. More water can be pushed through the larger nozzles but this means the pump must supply more water and it may not be able to maintain the same pressure while supplying more water. The end result is a lower operating pressure, more water and poorer distribution pattern.

A drill bit of the appropriate size can be used to check nozzle wear. Be careful not to damage the nozzle in the process. This procedure might be used on an old system, particularly where pressure seems to be a problem. Nozzle wear should not be a problem in systems only a few years old.

If a drill bit of the same size will slide into the nozzle, wear has occurred. If the drill bit is loose in the nozzle, then wear is excessive.