Irrigating carrots for profit and environmental management

Page last updated: Tuesday, 7 May 2019 - 2:19pm

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In Western Australia, carrots are grown on sandy soils of low water-holding capacity. When evaporation exceeds rainfall, irrigation is important to ensure high yields and quality.

Efficient irrigation is the key to efficient nutrient use and environmental impact. Two key requirements are an efficient irrigation system and simple scheduling approach.

A good approach is to schedule using pan evaporation and adjust for the crop growth stage, then check with a soil moisture measuring device.

Irrigation balance

Under-irrigation will compromise yields while over-irrigation can affect root quality, promote excessive fertiliser use, and possibly result in pollution of groundwater and surface water bodies.

  • A 16-week summer carrot crop grown on sandy soil near Perth may require 1000mm of irrigation which is 10 000 kilolitres or 10 megalitres of water per hectare.
  • Carrots are relatively salt-sensitive, similar to lettuce, onions and strawberries.
  • Two key requirements are an efficient irrigation system and simple scheduling approach.
  • The irrigation needed depends on the crop growth stage, soil type, crop vigour and weather conditions.
  • Use a crop factor, which varies with crop growth stage, to determine the proportion of evaporation the crop should receive. For example, a crop approaching maturity with a crop factor of 1.3 should receive 1.3 times evaporation.
  • Evaporation figures can be supplied from direct measurement from a pan evaporimeter or calculated from weather station data.
  • Long-term pan evaporation values (see Figure 2, Tables 1 and 3) can be used to guide irrigation. However, using actual daily evaporation is more accurate.

Using actual daily evaporation figures and a crop factor (from Figure 1) you can calculate the daily water requirement and then the system run time to apply this amount as follows:

Water requirement [mm] = (Crop factor) x (daily evaporation)

Run time (minutes) = (Application rate [mins/mm]) x (Water requirement)

Use soil moisture probes to check irrigation adequacy.

Note that: 1000 litres = 1 kilolitre, 1000 kilolitres = 1 megalitre and 1000 megalitres = 1 gigalitre.