The desired level of uniformity at the soil surface is difficult to determine, because of the effects of lateral water movement through the soil profile. Generally speaking, for a given sprinkler, nozzle size, pressure and wind pattern, there will be an optimum sprinkler spacing where the distribution of water between the sprinklers is most uniform. This spacing may then need to be modified to suit the equipment in use, standard pipe lengths, layout of the field and the pipe network.
Areas of die field which do not receive sufficient water will not reach their yield potential, and may ripen prematurely. Areas of die field which receive too much water will suffer yield loss due to temporary waterlogging. Consequently, there is an optimum depth of water to apply and it is desirable to minimise the variation from this.
The uniformity of distribution can be analysed by looking at sprinkler spray patterns, where a graph can be used to represent depth of water applied according to distance from die sprinkler.
Figure 6.20 Actual spray pattern -multiple sprinklers
If an ideal spray pattern was created, die spacing between sprinklers should be half of die wetted diameter; that is die next sprinkler should be located at the wetted perimeter of die first. This would result in wetting patterns overlapping, but the combined wetting pattern would be uniform.
However, in die field, irregularities and distortions occur, resulting in significant departure from an ideal wetting pattern.
It is also convenient to observe how depth of water application varies in body directions about die sprinkler, and depth charts can be constructed.
The position of a single sprinkler, and lines represent die contours of water depth. Simple measuring containers can be located in die field on a grid or radial pattern under the sprinkler to measure such a distribution in the field.
Adjacent spray patterns can then be combined to represent the distribution between adjacent sprinklers, as an aid to determine optimum sprinkler spacing.
Sprinkler manufacturers sometime publish coefficients of uniformity for the wetting pattern of their sprinklers. Check that the tests are conducted by an independent organization.