The distribution uniformity DU[Eq. (5.119)] is determined by several design variables and can be expressed functionally by the following relation:
where P is the pressure (kPa) available at the sprinkler, A P is the variation of the pressure (kPa) in the operating set or along the moving lateral, S represents the spacings (m) of the sprinklers along the lateral and between laterals or spacings between travelers, dn is the nozzle diameter (mm), which influences the sprinkler discharge and the wetted diameter for a given P, WDP represents the water distribution pattern of the sprinkler, and WS is the average wind speed (ms-1).
All of the above variables are set at the design stage. The designer, together with the farmer, first has to select the system according to the field, farming characteristics, and crops to be irrigated. Then the designer selects the sprinkler characteristics and spacings. At this stage, variables S, dn, and WDP are set, and the average wind speed WS during operation has been forecasted. Then, the hydraulics calculations are performed to select pipe sizes, pump characteristics, and other system equipment. These computations produce values for the average pressure at the sprinklers and the respective variation along the laterals.
When sprinklers are selected, and thus the variables P, qs, Dw, and dn are defined, the main variables governing the distribution uniformity are the spacing S and the variation of pressure A P in the operating set. Excessive spacings often are observed as a cause of low system performance. Excessive spacing was observed in 65% of traveling guns and 70% of solid-set systems evaluated in France . The same problem was observed in California, where other causes for low uniformity also include inappropriate nozzles and high pressure variation within the system .
It is commonly accepted that the variation in sprinkler discharges in an operating system should not exceed 10%; thus the variation in pressure must be no more than 20%. Lower ranges are desirable when high-value crops are irrigated. To achieve this, the hydraulics design has to be performed carefully.
The application efficiency ea [Eqs. (5.115) and (5.117)] depends not only on the design but on management variables and it can be functionally described by
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