The answer to this question is becoming more critical as more low and ultra-low pressure sprinkler packages become available. One way to accurately determine where changes in elevation exist is to conduct a topographic survey of the field to be irrigationd. By setting out flags on a 500' by 500' grid across the area to be irrigationd, each flag will represent approximately six acres. Record field elevations at each flag. The difference in elevation between each flag and the pivot point can be calculated by subtracting the elevation at each flag from the pivot point elevation.
Negative values reflect uphill slopes, positive values reflect downhill slopes.
The flags can be grouped together in increments of 20 percent of the greatest elevation difference. For example, if there were 20 flags in the area to be irrigationd and the greatest elevation difference was 25 feet, the elevations should be grouped into blocks of five feet of elevation difference. The total number of acres in each block can be calculated by multiplying the number of flags in each five foot increment by six acres per flag. Due to the relatively low cost of regulators, if more than 20 percent of the total area has field elevations that cause the sprinkler flow rate to vary more than 10 percent according to Table I , pressure regulators should be installed. Flow rate out of a sprinkler or spray head is controlled by the size of the sprinkler nozzle and the pressure supplied to the sprinkler.