4.4.1 Rectangular Spacing Example

If the container spacing in a typical nursery bed is one foot by 2 feet in an unjammed situation, and the container has a top diameter of 10 inches, what is the theoretical interception efficiency for the crop? A similar question could be asked for a greenhouse crop on a bench.

Using areas, instead of water volume, is a fairly easy method to make this calculation. A data file can be created for various containers and their typical spacing. The number calculated is a theoretical value but is the best estimate available quickly.

If the container spacing is 1 foot by 2 feet, then the centers of the containers will be the points at the corners of a rectangle 1 foot by 2 feet.

Draw this out on paper and you will see that each corner of the rectangle has one-quarter of a container in it. Putting these four corners together gives one container top; there is one container in each 1 foot by 2 foot rectangle drawn over the growing area.

Draw this out to prove it to yourself. Square Container Spacing illustrates the top container and ground areas used for this computation.

The most difficult calculation is determining the open area on the top of the container; the open area is calculated using the outside diameter of the top of the container. The area of a circle is the constant (Pi= 3.14) x (diameter / 2) 2. The (diameter divided by 2) is substituted for the radius of the circle.

This equation can also be expressed simply as:

Area of a circle = 0.785 x diameter (feet) x diameter (feet).

Convert the diameter, given in inches to feet, by dividing by 12. If using inches is easier for you, then divide the result by 144 to convert to square feet. Just be careful to watch the units! Thus, in our example,

Top container area = 0.785 x 0.83 (10/12) x 0.83 = 0.54 square feet

The area of the ground area (rectangle) is length times width.

Ground area = 1 foot x 2 feet = 2 square feet.

The Interception Efficiency equals the top container area divided by the total surface area times 100 to convert to percentage.

Interception Efficiency, % = (top container area divided by ground area) x 100.

                                            = (0.54 / 2)(100) = 0. 27 x 100 = 27 %

So, for this spacing, about 27% of the overhead-applied water will fall into the containers and 100 - 27 = 73% will fall onto the ground. Part of the 73 percent will evaporate, some may infiltrate, and the rest will runoff from the site.

Extra: Container volume calculation: This is useful information to calculate the number of containers that can be filled by a cubic yard of substrate.

  • Measure both the top and bottom diameters of the container, as many containers are tapered.
  • Calculate the both the top and bottom areas. Sum these two areas and divide by two to get an average.
  • Multiply the averaged area times the container height to get the container volume. The number of containers filled by a cubic yard of substrate can be calculated.
  • Divide the cubic yard by container volume to get number of containers filled. Remember to use conversion units: 27 cu ft = 1 cu yd and 1,728 cu in. = 1 cu ft.