JWSS - Journal of Water and Soil Science

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To maintain a high system-uniformity and also acceptable water use efficiency in a solid-set sprinkler irrigation system, the total committed pressure variation to subunits should not exceed 20% of the pressure head of the sprinkler which operates with the average pressure. Although some references often recommend giving the major part of this pressure variation to laterals, a scientific and precise criterion that allows designers to minimize the costs has not yet been developed. In this study, regarding the usual design criteria of this system in Iran and also respecting hydraulic rules, an economical analysis was conducted in order to optimize the system based on the appropriated permitted pressure head loss to each subunit. Then, the system irrigates the possible largest area by using minimum weight of pipe. The methodology consisted of 13 slope treatments for each subunit (0, ±0.1, ±0.5, ±1, ±2.5, ±5 and ±10%) and also the ratio of appropriated allowable head loss to the manifold (2.5, 5, 7.5, 10, 12.5, 15 and 17.5%). A simple software was developed to determine the size and the length of the manifold and laterals for each combination as well as their total weight and total irrigated area. Several criteria such as maximum and minimum velocity of water in the pipe, maximum head loss which occurs in 100 m of the manifold, maximum permitted head loss for each subunit and also maximum length of the laterals were considered here in order to derive practical design combinations. Because a constant inlet pressure for each subunit leads to a constant cost of energy, then the ratio of total weight of pipelines to the total irrigated area (Wtot /A0) was chosen as the standard, which helps to distinguish the best appropriation of allowable head loss to the manifold or laterals. Graphical diagrams were presented to help designers to know how to distribute the total permitted head loss between manifold and laterals. In general, results showed that total pressure head variation of each subunit greatly affects the system costs and also the total optimized appropriated pressure head loss to each subunit is greatly dependent on its own slope.

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Precise calculation of inlet pressure into sprinkler laterals is an important problem for proper distribution of uniformity. The adjusted average friction correction factor, FaAVG , provides the possibility of calculating the inlet pressure to mutli-outlet pressurized irrigation pipelines when the first outlet spacing from the pipe entrance is arbitrary. To investigate the effect of allowable head-loss in the lateral pipeline on inlet pressure, a new equation was developed for calculating this factor. A progression coefficient was assumed for variable discharge of the outlets. The results showed that though the inlet pressure of the lateral depends on the head loss between the outlets, it is negligible when more than 15 outlets are used. It was also concluded that when N is less than 15 and the ratio of distance between inlet and first outlet to outlet spacing is less than 1, the conventional approaches overestimate the inlet pressure. In this research, a new equation was also developed for Christiansen friction factor in which the first outlet is located at a fraction of outlet spacing. This new factor is dependent on the head loss between the first and last outlets, in addition to the number of outlets and the power of velocity equation. The results of applying this new factor showed good correlation with other researchers’ numerical results when a large number of outlets are coalesced.