# Evaporation-based irrigation scheduling

Page last updated: Friday, 15 July 2016 - 4:17pm

Please note: This content may be out of date and is currently under review.

## Calculating irrigation requirement

A crop’s water requirement is calculated in millimetres so it is convenient to replace it in millimetres. To schedule by evaporation, you need to know the application rate of your irrigation system. For information on calculating application rates of irrigation systems, see the following pages on this web site 'Measuring the delivery of drip irrigation systems' and 'Evaluating sprinkler systems'.

For this example, the application rate of an overhead system is 10mm per hour and the crop is at mid to late growth stage in Table 1. Evaporation for the previous 24 hours was 7.2mm and the soil has readily available water storage of 3mm/100mm depth.

Below is a worked example of how to use evaporation to schedule irrigation:

#### Step 1

Calculate daily water requirements by multiplying evaporation by the crop factor for the growth stage.

Daily water requirements = Evaporation x crop factor

7.2 x 1.1 = 7.9 mm

#### Step 2

Calculate run time in minutes by dividing the water required by the irrigation system application rate (mm per hour) and multiply by 60.

(Water required/application rate) x 60 = Run time

(7.9 /10) x 60 = 47 minutes

How the required depth of irrigation is applied is determined by the soil’s water-holding capacity, water quality and the crop growth stage. Irrigation should meet the water-holding capacity for the rooting depth.

Excess irrigation leads to water passing below the root zone lowering irrigation efficiency and  applied nutrients move down the profile beyond the reach of plant roots. For information on calculating the effective water holding capacity of your soil, see the following page on this website: 'Calculating readily available water'.

#### Step 3

Number of irrigations

Calculate soil water storage by multiplying effective root zone depth by the soil’s readily available water-holding capacity. In this case the readily available water is given as 3mm for 100mm depth of soil.

RAW storage = Root zone depth x Readily available water (RAW)

200mm root depth x 3mm/100mm depth

200 x 3/100 = 6mm RAW

For this example, daily water required is 7.9mm and the readily available water storage is 6mm. Irrigation will need to be split so the water applied does not exceed soil storage.

Confirming the irrigation effectiveness can be done using soil moisture equipment. Decision support systems such as the Vegetable Irrigation Scheduling System (VISS) can calculate Steps 1 and 2 for individual plantings or irrigation shifts and be viewed online. Some systems will email the requirements for the crops entered.

Alternatively, a simple table can be made as in Table 2. Look up the crop factor — shown along the top row — and daily evaporation — shown down the left-hand column — and the water requirement is shown where the two intersect.

Table 2 Example of a table to calculate daily water requirement (mm)
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
1 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
2 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
3 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 3.9 4.2 4.5
4 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8 5.2 5.6 6.0
5 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
6 3.0 3.6 4.2 4.8 5.4 6.0 6.6 7.2 7.8 8.4 9.0
7 3.5 4.2 4.9 5.6 6.3 7.0 7.7 8.4 9.1 9.8 10.5
8 4.0 4.8 5.6 6.4 7.2 8.0 8.8 9.6 10.4 11.2 12.0
9 4.5 5.4 6.3 7.2 8.1 9.0 9.9 10.8 11.7 12.6 13.5
10 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0

Reported crop factors are higher for sandy soils than for heavier soils because site inefficiencies such as low soil water-holding capacity of sands and irrigation efficiency have been taken into account. This simply reinforces the need to fine-tune application to match the specific farm situation. Beware of overseas information that is not geared to sandy soils.

Rohan Prince