تخصیص بهینه افت مجاز در زیرواحدهای سیستم آبیاری بارانی کلاسیک ثابت- آبپاش متحرک

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.