1. System Design Criteria

System Design Criteria Objectives:

• You will learn that designing an irrigation system means satisfying three criteria - water availability, irrigation system capacity and distribution uniformity.
• You will understand that a design is limited by operating time and water supply.
• You will learn that application uniformity is subject to keeping pressure losses low.

When building, modifying, maintaining, or operating any part of an irrigation system, one must always consider the effect of that part on the entire system. The original design must be kept in mind and the balance of flows and pressures must be maintained.

Three overall design criteria should be kept in mind.

Adequate availability of water. The water supply may be limited to some minimum flow rate for year round delivery. Wells may be fairly steady all year but surface water supplies may fluctuate. Determine the limits on the water supply and size primary and backup pumps to the available flow. Seasonal minimum flow rates will likely occur at time of maximum demand.

Irrigation system capacity. An irrigation system frequently can not everything at one time and must be divided into zones that do not exceed the water supply available in gallons per minute. An irrigation zone is the amount of the irrigation system that can be operated at one time without exceeding the water available. The total number of zones can not exceed the daily water supply. An irrigation zone must be able to supply the water needed by a mature crop on a hot, summer day.

An irrigation system should not be required to operate 24 hours a day to meet crop needs; some maintenance downtime should be included in the daily schedule. This extra capacity may be needed for extreme weather conditions.

Distribution Uniformity. Uniformity refers to the extent to which all plants in an irrigation zone receive the same amount of water. Although perfect uniformity is an ideal that can not be achieved in practice, a high level of uniformity is desired so that the crop is fairly uniform.

Pressure losses in the piping system, emitter or sprinkler variability, and variability in plant water demand all contribute to the non-uniformity. Uniformity can be improved by investing in more expensive irrigation equipment, such as larger diameter pipes and more sprinklers to achieve better overlap of watering patterns. This investment may not be justified by the slightly better uniformity. However, the more uniform the irrigation system, the less over-irrigation is required.