Objective and scope of design

The surface irrigation system should replenish the root zone reservoir efficiently and uniformly so that crop stress is avoided, and resources like energy, water, nutrient, and labour are conserved. The irrigation system might also be used to cool the atmosphere around sensitive fruit and vegetable crops, or to heat the atmosphere to prevent their damage by frost. An irrigation system must always be capable of leaching salts accumulating in the root zone. It may also be used to soften the soil for better cultivation or even to fertilize the field and spread insecticides.

The design procedures outlined in the following sections are based on a target application, Z_req, which equals the soil moisture extracted by the crop. It is in the final analysis a trial and error procedure by which a selection of lengths, slopes, field inflow rates and cutoff times can be made that will maximize application efficiency. Considerations such as erosion and water supply limitations will act as constraints on the design procedures. Many fields will require a subdivision to utilize optimally the total flow available. This remains a judgement that the designer is left to make after weighing all other factors that he feels are relevant to the successful operation of the system. Maximum application efficiencies, the implicit goal of design, will occur when the least watered areas of the field are just refilled. Deep percolation will be minimized by minimizing differences in intake opportunity time, and then terminating the inflow on time. Surface runoff is controlled or reused.

An engineer may have an opportunity to design a surface irrigation system as part of a new irrigation project where surface methods have been selected or when the performance of an existing irrigation system requires improvement by redesign. In a new irrigation project, it is to be hoped that the surface irrigation system design is initiated after a great deal of irrigation engineering has already occurred. The selection of system configurations for the project is in fact an integral part of the project planning process. If a new or modified surface system is planned on lands already irrigated, the decision has presumably been based, at least partially, on the results of an evaluation at the existing site. In this case, the design is more easily accomplished because of the higher level of experience and data available.

In either case, the data required fall into six general categories :

a) the nature of irrigation water supply in terms of the annual allotment, method of delivery and charge, discharge and duration, frequency of use and the quality of the water;

b) the topography of the land with particular emphasis on major slopes, undulations, locations of water delivery and surface drainage outlets;

c) the physical and chemical characteristics of the soil, especially the infiltration characteristics, moisture-holding capacities, salinity and internal drainage;

d) the cropping pattern, its water requirements, and special considerations given to assure that the irrigation system is workable within the harvesting and cultivation schedule, germination period and the critical growth periods;

e) the marketing conditions in the area as well as the availability and skill of labour, maintenance and replacement services, funding for construction and operation, and energy, fertilizers, seeds, pesticides, etc.; and

f) the cultural practices employed in the farming region especially where they may prohibit a specific element of the design or operation of the system.