4.4 Interception Efficiency
Interception Efficiency (IE) is a theoretical measure of the amount of applied irrigation water that is captured by the containers in an area during a typical overhead irrigation event.

IE is expressed as a percentage of the applied water and can be calculated using areas. In terms of area, it is the "total container top area" divided by the "total ground area allotted to the container". The decimal value is converted to a percentage by multiplying by 100.

The Interception Efficiency Equation illustrates the relationship.

The type of irrigation system and the effects of container size and spacing are evaluated for efficiency by using the IE results. Overhead sprinklers will have higher risk, as the IE will be less than the 95 to 100 percent of drip/trickle systems.

Containers that are jammed together will give a higher IE (near 80 %) than those containers spaced apart. The maturity of the plant (canopy) and the overall size of the container in which it is planted is a factor in spacing containers for maximum light interception (growth). Large containers with large plants are usually spaced further apart.

Interception Efficiency is important because the fraction that misses the container (1.0 - IE) is the water (or water plus soluble nutrients) that theoretically falls onto the ground to create runoff. To repeat, the percentage of water falling onto the ground directly is (100 % - Interception Efficiency, in percent).

Interception Efficiency is not a perfect measure because water falling onto foliage may be deflected or shed outward to fall onto the ground or directed inward toward the plant. This is a crop specific factor that is unknown. The angle of the water application, and environmental factors such as wind, may also directly affect IE.

However, IE is a useful practical measurement to assess the efficiency of the various irrigation practices in the nursery or greenhouse, and to give an overall assessment of the risk of potential runoff (see section 2.6).

For drip irrigation the interception efficiency is usually near 100 percent because all the water is delivered to the container. Some microirrigation sprinklers may partially miss the container. Spray stakes are assumed to be placing the water into the container.

Containers may be spaced in a rectangular (square) pattern or in a diagonal (offset) pattern. The calculation procedurefor overhead irrigation is basically the same for both situations, but the explanation is given separately in the following sections.