Converting Readily Available Water to litres for drip irrigation

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Drip irrigation is common in modern orchards. For this method of irrigation it is easier to use litres rather than the more traditional unit of millimetres when describing readily available water in the plant root zone. Using litres also allows simple calculation of irrigation time.

Several factors need to be considered when considering readily available water and drip irrigation and are explained.

Introduction

Where irrigation water and plant roots are evenly distributed over the whole planting area, water storage and plant water use can be measured in millimetres. Drip irrigation distributes water over a small part of the whole block and roots follow this water distribution. In these cases, it is often easier to use litres to describe both water use and storage in the plant root zone.

This also allows simple calculation of irrigation time as the discharge from drip systems is commonly reported in litres per hour.

Rule to remember

1mm depth of water = 1L applied to 1 square metre.

Water held in root zone

The volume of root zone that is wetted by the drip system will depend on the size and shape of the wetting pattern.

Figure 1 Overlapping wetting pattern from in-line drippers

Overlapping drippers

Where drip patterns overlap, assume that a wetted strip or sausage-shaped wetted pattern is produced (see Figure 1).

The volume of water held in the soil can be estimated from the width and length of the wetted strip and the Readily Available Water (RAW) in the root zone.

Volume stored (L) = wetted width (m) x wetted length (m) x root zone RAW (mm)

For example, for a width of 1.5m wetted, 3m tree spacing and root zone RAW of 14mm, the volume of Readily Available Water = 1.5 x 3 x 14 = 63 litres of RAW per tree.

If the crop root zone does not access the entire wetted strip, adjust the dimensions of the wetted area in your calculation. This is particularly important in young plantings where roots may have access to only a small portion of the wetted strip.

Non-overlapping drippers

Where wetting patterns do not overlap, calculate the wetted volume assuming a cylinder, sphere or cone-shaped wetting pattern (see Figure 2).

For example, if a root zone with a RAW of 14mm is wet by a dripper with a cylindrical wetting pattern and a radius of 0.2m, the volume of readily available water will be:

πr2 x root zone RAW (mm)

(πr2 is the area of a circle where pi (π) equals 3.14.)

3.14 x (0.2 x 0.2) x 14 = 1.8L/dripper.

If there is more than one dripper per plant, multiply this figure by the number of drippers to get the total litres available to each plant.

Figure 2 Non-overlapping wetting pattern

Calculating irrigation time

Irrigation time can be determined from the volume of water that can be held in the root zone wetted area and the discharge rate of the drippers.

Irrigation time (hours) = Volume RAW (L) ÷ dripper discharge rate (L/hr)

Examples

Example 1

Overlapping drippers with a RAW of 63 litres per tree, 2L/hr drippers spaced 0.5m apart. Each tree has access to the full 3m wetted length between trees.

• 3m wetted length ÷ 0.5m dripper spacing = 6 drippers per tree
• 6 drippers per tree x 2L/hr drippers = 12L/hr/tree
• 63L/RAW/tree ÷ 12L/hr/tree = 5.25 hours irrigation time.

Example 2

Non-overlapping drippers with a RAW of 1.8 litres per dripper and 8L/hr drippers.

• 1.8L/RAW/dripper ÷ 8L/hr =0.225 hours = 13.5 minutes

(Multiply time in hours by 60 to determine the number of minutes.)

Using RAW to determine irrigation time will give the maximum time you need to irrigate to refill the Readily Available Water. If your soil dries out beyond the moisture content that is considered readily available to your crop, irrigate for a longer period of time.

Measuring dripper discharge

Although manufacturers specify the expected output of drippers, check actual output rates as your system may be operating at a different pressure or be affected by blockages and wear. Check discharge by digging a hole under the dripper and use a container to measure the volume of water emitted over a known period. Randomly check drippers across the irrigation system, including drippers close to and furthest from the mainline.

Acknowledgement

This material was adapted from a similar publication produced by the Queensland Fruit and Vegetable Growers Water for Profit project.

It replaces Farmnote 542 'Converting Readily Available Water to litres for drip irrigation' by Helen Newman.